Re: fasting - refeeding 탐구. 인체건강과 장수 촉진 2025 endocrine reviews

[문형철]

1. 배경 및 목적

• 인류는 기아(starvation) 기간을 생존하기 위해 포도당 → 지질 대사 전환 중심의 적응 반응을 진화시켰음.
• 현대는 비만/칼로리 과잉 시대지만, 단식 생리 메커니즘은 여전히 중요:
  1. 세계 일부 지역 여전히 기아 문제.
  2. 비만 유전적 소인과 단식 적응 메커니즘의 maladaptive 측면.
  3. 모델 생물에서 칼로리 제한(CR)과 간헐적 단식(IF)이 수명 연장.

논문 목적: 단식의 이점 기전 + 위험(특히 뼈 취약성) + 인간 대사 연구 결과 종합 평가.

2. 단식의 주요 적응 메커니즘

• 초기: 글리코겐 분해(glycogenolysis) → 글루코네오제네시스 (gluconeogenesis, 글리세롤·아미노산 이용).
• 지속 시: 케토제네시스 + 지질 동원 (ATGL, HSL, lysosomal lipolysis).
• 호르몬 변화: Leptin ↓ → 에너지 소비 감소 (갑상선 호르몬 저하, GH 저항성, 성선기능저하).
• 기타: FGF21 ↑ (지질 산화·케톤 생성 촉진), 미토콘드리아 생합성, autophagy, ROS 감소, 염증 조절.

장기 단식(prolonged fast)은

급성 말초 인슐린 저항성(acute peripheral insulin resistance)을 유발할 수 있음.

이는 적응 반응으로 해석되며,

포도당을 뇌·필수 조직으로 제한하고 지질/케톤 이용을 촉진하기 위한 전략으로 봅니다.

https://pmc.ncbi.nlm.nih.gov/articles/PMC12126231/

1. 핵심 메시지

간(glycogen) 저장량이 직접 AMPK를 통해 CRTC2의 안정성을 조절하여,
공복 시 gluconeogenesis(포도당 신생합성)를 '적절한 크기'로 제어한다.

기존에는 glycogen이 단순히 에너지 저장소로만 생각되었는데,
신호전달 물질로서 gluconeogenesis의 transcriptional regulator를 직접 통제한다는
새로운 기전입니다.

2. 배경 지식 (복습)

• Glycogenolysis: 간에 저장된 glycogen을 분해해 glucose를 만드는 과정.
• Gluconeogenesis: lactate, glycerol, amino acid 등 비탄수화물 전구체로부터 새로 glucose를 합성하는 과정 (주로 공복 시 중요).
• 이 두 과정은 hepatic glucose output을 유지해 혈당을 안정시키는 핵심.

기존 모델: 공복 → glucagon/epinephrine ↑ → cAMP/PKA → CREB 활성화 → gluconeogenic genes (PEPCK, G6Pase 등) 발현. 이번 연구는 glycogen 자체가 이 pathway의 amplitude(크기)를 직접 결정한다는 점을 밝힘.

좌측: Fast 상태 (Low Glycogen)

• Glucagon이 Glucagon receptor를 활성화 → cAMP ↑ → Protein kinase A (PKA) 활성화.
• PKA는 SIK2 (Salt-inducible kinase 2)를 억제.
• 동시에 Low glycogen이 AMPK를 활성화.
• AMPK 활성화 → CRTC2를 phosphorylation & stabilization (안정화) → 핵으로 이동.
• 안정화된 CRTC2 + CREB → CREB phosphorylation → Gluconeogenic genes 전사 활성화 → Hepatic glucose production ↑.

우측: Fed 상태 (High Glycogen)

• Insulin이 Insulin receptor를 활성화 → AKT 활성화 → SIK2 활성화.
• High glycogen이 AMPK를 억제 (allosteric inhibition).
• SIK2 활성화 + AMPK 억제 → CRTC2가 인산화되어 불안정화 → degradation (분해) 또는 세포질에 머무름.
• 결과적으로 CREB transcriptional activity ↓ → Gluconeogenesis 억제

Glycogen의 Sensing 역할 (가장 중요한 신규 발견)

• Low glycogen: AMPK 활성화 (직접적)
• High glycogen: AMPK 억제 (직접적)
• 이는 glycogen이 단순 저장소가 아니라 대사 센서(metabolic sensor)로서 작동한다는 증거.

3. 주요 실험 결과 및 기전 (단계별 설명)

① Glycogen 수준과 gluconeogenesis의 역상관 관계

• Fasting → 간 glycogen ↓ → gluconeogenic gene expression‎ 증가.
• Feeding → 간 glycogen ↑ → gluconeogenic gene expression‎ 억제.

② Genetic 모델로 증명

• PTG (Protein Targeting to Glycogen) liver-specific knockout:
  • PTG는 glycogen synthase를 glycogen particle에 targeting하는 scaffolding protein.
  • Knockout → glycogen 수준 감소 → gluconeogenic genes ↑ + hepatocyte glucose production ↑.
• PYGL (Glycogen Phosphorylase) knockdown or inhibition:
  • PYGL은 glycogen을 분해하는 효소.
  • Knockdown → glycogen 수준 증가 → gluconeogenic genes ↓.

→ Glycogen 양 자체가 gluconeogenesis의 스위치 역할을 한다는 직접적 증거.

③ Sensing mechanism: AMPK

• Glycogen 수준 변화는 AMPK (AMP-activated protein kinase)가 감지.
• Low glycogen → AMPK 활성 증가.
• High glycogen → AMPK 활성 억제 (allosteric inhibition).

④ Downstream effector: CRTC2

• AMPK가 활성화되면 CRTC2 (CREB-regulated transcriptional coactivator 2)를 phosphorylation + stabilization 시킴.
• 안정화된 CRTC2는 핵으로 이동해 CREB와 결합 → cAMP-responsive genes (gluconeogenic program) 강력 활성화.
• High glycogen 상태에서는 AMPK 억제 → CRTC2 degradation ↑ → CREB transcriptional activity ↓.

⑤ Hepatocyte 수준 확인

• Low glycogen or High AMPK 조건의 hepatocyte → CRTC2 protein level ↑ → gluconeogenesis priming 상태.

https://www.annualreviews.org/content/journals/10.1146/annurev-nutr-120524-013857

Abstract (전체 요약 번역)

공복(fasting)을

치료적·예방적 수단으로 활용하려는 관심이

전 세계적으로 높아지고 있다.

이 리뷰는

공복이 지방조직(adipose tissue) 대사에 미치는 영향을 종합적으로 정리하며,

특히 성별 차이(sexually dimorphic regulation)에 주목한다.

공복 시 지방조직에서 LPL (Lipoprotein Lipase) 활성 조절의 핵심 메커니즘

좌측: Fed state (섭식 상태)

• LPL 활성 ↑ (파란 상자에 3·8 표시)
• ANGPTL8 (주로 간에서 유도됨)이 ANGPTL3와 heterodimer를 형성하여 LPL 활성 억제를 해제하거나, 특정 조직(지방)에서 LPL을 활성화.
• 결과: 순환 중 chylomicron/VLDL의 TAG가 지방조직으로 효율적으로 uptake → 지방 저장 우세.
• NEFA는 상대적으로 적게 방출됨.

우측: Fasted state (공복 상태)

• LPL 활성 ↓ (파란 상자에 4 표시)
• ANGPTL4가 강력 유도되어 LPL을 직접 억제 (inhibitory complex 형성).
• ANGPTL8은 공복 시 감소하거나 역할이 바뀜.
• 결과: 지방조직으로의 TAG uptake 차단 + intracellular lipolysis (ATGL/HSL) 활성화로 NEFA 대량 방출 → 혈액으로 나감 (간·근육으로 이동).

하단 심장: 순환 혈액의 흐름을 상징. Fasted 상태에서는 NEFA가 심장을 통해 전신으로 공급됨.
범례:

• 3 = ANGPTL3, 4 = ANGPTL4, 8 = ANGPTL8
• LPL = Lipoprotein Lipase (지방조직 내피세포에 존재)
• 검은 화살표 = Low LPL activity / High LPL activity

공복 시 지방조직에서는:

Triacylglycerol (TAG) 저장이 억제된다

  → 세포외 지방분해 효소 Lipoprotein Lipase (LPL)가 억제됨. 

저장된 TAG 분해와 NEFA(비에스테르화 지방산) + glycerol 방출이 증가된다

  → 세포내 지방분해 효소 Adipose Triglyceride Lipase (ATGL)와

       Hormone-Sensitive Lipase (HSL)가 활성화됨.

이러한 변화는

인슐린, 글루카곤, 코르티코스테로이드, 성장호르몬, 카테콜아민 등의 체액성 인자와

신경계 경로에 의해 조절되며,

전사 및 전사후 수준에서 핵심 효소들이 조절된다.

결과적으로 공복은

지방조직 대사를 근본적으로 재프로그래밍하여 저장된 TAG를 동원하고,

순환 중인 TAG를 비지방조직(간, 근육 등)으로 재분배한다.

• Feeding (좌): 인슐린 ↑ → LPL 활성화 → chylomicron/VLDL로부터 TAG uptake ↑ → 지방 저장 우세. ATGL은 억제 상태.
• Fasting (우): 인슐린 ↓ + 성장호르몬·코르티솔·카테콜아민 ↑ → LPL 억제 + ATGL/HSL 강력 활성화 → NEFA + glycerol 대량 방출 → 간으로 이동하여 β-oxidation, ketogenesis, gluconeogenesis에 사용

(1) 저장 경로 억제 (LPL 억제)

• 공복 시 ANGPTL4 (Fasting-Induced Adipose Factor)가 강력 유도됨.
• ANGPTL4는 LPL을 억제 → 순환 중 지질이 지방조직에 저장되지 않고 간·근육 등으로 향하게 함.
• 유도 인자: 글루코코르티코이드, PPARα, 저인슐린 상태.

(2) 분해 경로 활성화 (Intracellular Lipolysis)

ATGL (TAG → DAG + FA, rate-limiting) HSL (DAG → MAG + FA) MGL (MAG → glycerol + FA)

활성화 기전:

• cAMP-PKA 경로 (가장 중요): 카테콜아민(노르에피네프린) + 글루카곤 → β-adrenergic receptor → adenylate cyclase → cAMP ↑ → PKA 활성화.
  • PKA는 Perilipin-1 (PLIN1)을 인산화 → 지질 droplet 표면 구조 변화 → lipase 접근 용이.
  • PKA는 HSL 직접 인산화 + translocation 촉진.
  • PKA는 PEX5 인산화 → ATGL을 세포질에서 지질 droplet으로 이동.
• 인슐린 저하: PDE3B 억제 해제 → cAMP 분해 감소 → PKA 지속 활성화.
• G0S2 (ATGL 억제인자) ↓, CGI-58 (ATGL co-activator) 활성화.
• 추가로 AMPK, autophagy (lipophagy)도 관여 (일부 논란 있음).

성별 차이 (Sexually Dimorphic Regulation)

리뷰에서 특히 강조하는 부분입니다.

• 여성은 카테콜아민에 대한 전신 lipolysis 반응이 더 크다.
• 지방 분포(피하지방 vs 내장지방), 호르몬(에스트로겐) 영향으로 ketogenesis로의 partitioning이 다를 수 있음.
• 유전자 발현 패턴도 fasting-refeeding 시 성별 차이가 관찰됨.

1. 좌측: Fed State (섭식 상태) — 지방 저장 모드 주요 흐름:

• Glucose uptake: Glucose transporter 4 (GLUT4)가 인슐린에 의해 활성화되어 세포 내 glucose 증가.
• De novo lipogenesis (DNL):
  • Glycolysis → TCA cycle → Citrate → ACLY → Acetyl-CoA
  • ACACA (Acetyl-CoA carboxylase) → Malonyl-CoA
  • FASN (Fatty acid synthase) → Palmitate (지방산 합성)
  • Elongation/desaturation → 다양한 지방산
• TAG 합성 (Lipid droplet로 저장):
  • Glycerol-3P + Acyl-CoA → GPAT → AGPAT → LPA → PA → LPIN → DAG → DGAT → TAG
  • 지방산 재에스터화(re-esterification)도 활발.
• LPL (이전 그림 참조): 활성 ↑ → 혈중 TAG uptake.

결과: 에너지 잉여를 지방으로 저장. Insulin signaling이 모든 과정을 강력히 촉진.

2. 우측: Fasted State (공복 상태) — 지방 동원 모드 주요 변화:

• 인슐린 ↓ → PI3K-Akt signaling ↓ → PDE3B 활성 감소 → cAMP 분해 ↓
• Norepinephrine (교감신경) ↑ → β3-adrenergic receptor → Gs protein → Adenylate cyclase → cAMP ↑ → Protein Kinase A (PKA) 활성화
• PKA의 다중 작용:
  • Perilipin, HSL, ATGL 관련 단백질 인산화 (P) → intracellular lipolysis 강력 활성화 (이전 그림 참조)
  • ChREBP (Carbohydrate response element-binding protein) 인산화 → 핵으로 이동 억제 → lipogenesis gene 억제
  • SREBP1 등 lipogenic transcription factor 억제 → De novo lipogenesis genes ↓, (Re-)esterification genes ↓

결과:

• TAG 합성 거의 정지.
• 기존 lipid droplet의 TAG가 NEFA + glycerol로 분해되어 방출.
• NEFA는 혈액으로 → 간(β-oxidation, ketogenesis), 근육(에너지)으로 공급.

3. 전체적인 Switch Mechanism 요약

Fed state에서는
인슐린이 lipolysis를 강력히 억제하고,

Fasted state에서는
Growth Hormone + Norepinephrine + PKA가 lipolysis를 다단계로 활성화하면서
transcriptional level까지 연결하는 전체 그림을 보여줍니다.

1. 좌측: Fed State (인슐린 중심 억제 모드)

• Insulin receptor → PI3K-Akt signaling → PDE3B 활성화
  • PDE3B: cAMP를 ATP로 분해 → cAMP ↓ → PKA inactive
• Lipid droplet 주변:
  • G0S2, HILPDA: ATGL 강력 억제
  • PLIN1 (비인산화 상태): lipase 접근 차단
  • CGI58: ATGL co-activator지만 억제된 상태
• 결과: TAG 저장 유지, lipolysis 거의 정지

2. 우측: Fasted State (다중 호르몬 활성화 모드)

주요 호르몬 신호:

1. Norepinephrine (β3-adrenergic receptor) → Gs → Adenylate cyclase → cAMP ↑ → PKA 활성화 (가장 강력하고 빠른 경로)
2. Growth Hormone → JAK2 → STAT5 & FOXO1 인산화 → lipolysis gene expression‎ ↑ (장기적 적응)
3. MAPK/ERK signaling (GH와 교차) → 추가 transcriptional 지원

PKA의 핵심 작용 (Protein phosphorylation):

• PLIN1 인산화 → lipid droplet 구조 변화 → lipase 접근 허용
• HSL 직접 인산화 → DAG → MAG + NEFA
• ATGL 관련:
  • PEX5 인산화 → ATGL을 세포질에서 lipid droplet으로 translocation
  • FABP4: NEFA 운반 보조
• MGL: MAG → Glycerol + NEFA

Transcriptional level (핵):

• FOXO1, STAT5 활성화 → lipolysis 관련 유전자 (ATGL, HSL 등) 발현 증가 → 지속적인 lipolysis capacity 강화

3. Lipolysis Cascade 상세 (Lipid Droplet 내부) TAG → DAG → MAG → Glycerol + 3 NEFA

• ATGL (rate-limiting): CGI58 co-activator 필요, G0S2/HILPDA에 의해 억제 해제
• HSL: PKA에 의해 가장 강력히 활성화
• MGLL (MGL): 최종 단계

4. 전체 논문 그림 시리즈 통합 이해

1. LPL/ANGPTL 그림 → Extracellular TAG uptake 차단 (저장 입력 차단)
2. Fed/Fasted 전체 대사 그림 → Lipogenesis 완전 억제 + hormonal switch
3. 이번 그림 → Intracellular lipolysis의 multi-layered activation (rapid PKA + transcriptional GH/STAT5/FOXO1)

공복 시 지방세포는
단순히 “분해”하는 것이 아니라,
입력 차단 + 합성 억제 + 분해 가속 + 장기 적응 유전자 발현까지 동시에 진행합니다.

이는
에너지 위기 상황에서
효율적이고 지속 가능한 NEFA 공급을 보장합니다.

https://www.nature.com/articles/s41467-025-56613-3

이 논문은
금식(특히 장기 금식) 시
지방 조직(지방세포)에서 에너지 저장소를 동원하는
'비정형(Non-canonical) 리소좀 리폴리시스(lysosomal lipolysis)' 경로를 새롭게 밝힌 중요한 연구입니다.


배경: 기존 지식 vs. 새로운 발견

• 기존 canonical(정형) 경로: 지방세포의 중성 지방(triglyceride)을 분해하는 주요 효소는 ATGL (Pnpla2) → HSL → MGL입니다. 이는 특히 단기 금식이나 아드레날린(adrenergic) 자극(예: isoproterenol) 시 빠른 지방산(NEFA)과 글리세롤 방출에 핵심적입니다.
• 하지만 ATGL을 완전히 제거해도 잔여 리폴리시스가 관찰되고, canonical lipase 유전자 발현이 금식 시 일관되게 증가하지 않는 점, 그리고 인간 지방 조직 전사체 분석에서 lysosomal 관련 유전자(특히 LIPA/LAL)가 증가하는 관찰이 기존 모델의 한계를 지적했습니다.

장기 금식에서는
리소좀(lysosome) 기반의 대체 경로가 지배적으로 작동하며,
이는 Lysosomal Acid Lipase (LAL/LIPA)와 MiT/TFE transcription factor family (TFEB, TFE3, MITF)에
의존합니다.

주요 결과 요약 (Mouse & Human 증거)

1. 금식 시 리소좀 프로그램 활성화:
   • 24시간 금식 후 지방 조직(특히 inguinal WAT)에서 LAL, TFEB/TFE3/MITF 단백질 증가, ATGL은 감소.
   • 지방세포에서 lysosomal genes (Lipa, Lamp1, Mcoln1 등) ↑, canonical lipolysis genes는 ↓ 또는不变.
   • LAMP1+ 리소좀 puncta 증가, 리소좀 분획에서 perilipin (lipid droplet marker) 검출 → 리소좀이 lipid droplet와 상호작용.
2. 기능적 증명 (Pharmacological & Genetic):
   • 리소좀 억제제(예: lysosomal inhibitors) 투여 → 금식 유발 NEFA/glycerol surge 감소.
   • Adipocyte-specific Lipa (LAL) knockout → 장기 금식(24h) 시 lipolysis 크게 감소 (특히 female mouse에서 더 뚜렷).
   • MiT/TFE (특히 TFEB) adipocyte-specific KO → LIPA 발현 ↓ + lipolysis 감소.
3. 시간 의존적 전환 (Temporal Transition):
   • 단기/급성: Canonical (ATGL-dependent) 경로가 아드레날린 자극이나 초기 금식에 중요.
   • 장기 금식: Lysosomal (LAL-dependent) 경로가 지배적. ATGL KO에서는 lysosomal 경로가 compensatory하게 더 빨리 활성화될 수 있음.
4. 인간 관련성:
   • 인간 지방 조직 explant 및 adipocyte culture에서 nutrient restriction 시 lysosomal lipolysis 증거.
   • 이전 인간 inpatient 10-day fasting transcriptomics 데이터와 일치 (LIPA 등 lysosomal genes ↑).
   • MiT/TFE-LIPA axis가 인간에서도 작동할 가능성 높음.

3. 이점 (Benefits)

모델 생물:

• CR/IF → 수명 연장 (선충, 초파리, 생쥐, 원숭이).
• 기전: 인슐린/IGF-1 신호 저하, 미토콘드리아 기능 향상, ROS ↓, autophagy ↑, epigenomic remodeling (β-hydroxybutyrate로 HDAC 억제), 병적 biomass 분해.

인간:

• 체중 감소 + 지질 대사 재프로그래밍.
• 심혈관 대사 지표 개선 (인슐린 저항성 ↓, TG ↓, LDL 개선) — 특히 과체중/비만군에서 강함.
• 종양 위험 감소 가능 (인슐린/IGF-1 저하 + 케톤 효과).
• 일부 종교적 단식 연구: 염증 바이오마커 감소.

4. 위험 및 한계 (Risks & Caveats) — 논문의 가장 강조하는 부분

• 뼈 건강 (가장 중요한 잠재적 위해):
  • 단식 관련 체중 감소 → 뼈 대사 음성 조절 (cortisol ↑, IGF-1 ↓, methionine 변화 등).
  • CALERIE trial (24개월 CR): 척추·고관절 BMD 감소.
  • 신경성 식욕부진(anorexia nervosa) 모델: 여성 85% BMD 저하, 35% 골다공증 기준 충족.
  • 급성 단식(젊은 여성)에서도 뼈 형성 ↓ + 미네랄 용해 ↑.
  • 저체중 여성, 골다공증 위험군에서는 이득보다 해가 클 수 있음.
• 기타 위험:
  • 장기 단식 후 재급식 시 염증 증가 (지방조직 macrophage influx, CRP ↑).
  • 정상 체중군이나 장기 연구 부족.
  • 이득이 체중 감소 자체 때문인지, 단식 특유의 효과인지 불명확.

5. 인간 임상 연구 요약

• RCT 79편 분석 (대부분 3주~6개월).
• 대부분 과체중/비만 (BMI >25) 대상.
• 이점 신호: 체중 ↓, HOMA-IR 개선, 지질 프로파일 개선.
• 한계: 운동 병행 시 추가 이득 미미, 정상 체중군 데이터 부족, 장기 지속성 불확실.
• 결론: 과체중·비만군에서 심혈관대사 위험 감소에 유용할 가능성이 가장 강함. 하지만 뼈 건강 등 위해를 상쇄하는지 여부는 미지수.

이 그림은
단식 시 지방 조직에서 트리글리세라이드(지방)가 어떻게 분해되고
유리지방산(Free Fatty Acid)으로 방출되는지를 보여주는 핵심 메커니즘 도식.

특히
장기 단식(prolonged fasting)에서
중요한 비전형(non-canonical) 경로를 강조합니다.


(A) Canonical (전형적) Adipocyte Lipolysis

• 주요 효소:
  • ATGL (Adipose Triglyceride Lipase): Triglyceride → Diacylglycerol (첫 단계).
  • HSL (Hormone-Sensitive Lipase): Diacylglycerol → Monoacylglycerol.
  • MGL (Monoacylglycerol Lipase): Monoacylglycerol → Glycerol + Free Fatty Acid (FFA).
• 특징: 호르몬(카테콜아민, 글루카곤) 자극에 의해 활성화되는 고전적 경로. 단식 초기~중기 주요 기전.
• 결과: FFA가 혈액으로 방출되어 간(케톤 생성), 근육·심장(에너지원)으로 이용.

(B) Non-canonical (비전형) Adipocyte Lipolysis

• Lysosomal Acid Lipase (라이소좀 산성 리파아제)가 핵심.
• 라이소좀(Lysosome)이 lipid droplet를 직접 분해.
• 특징: 장기 단식이나 특정 스트레스 상황에서 활성화되는 대체 경로. Canonical 경로가 포화되었을 때 보완.
• 의의: 단식 지속 시 더 효율적인 지방 동원 가능. Autophagy/lysosomal pathway와 연계 (형철님 관심사: autophagy 강화).

(C) Non-canonical Release of Triglyceride-containing Exosomes

• 지방 droplet에서 Triglyceride를 포함한 Exosome이 형성되어 세포 외로 방출.
• Exosome가 대식세포(Macrophage)로 전달 → Macrophage에서 추가 분해 → FFA 방출.
• 특징: 완전히 새로운 비전형 경로. 지방세포가 직접 FFA를 방출하지 않고, exosome를 통해 이웃 세포(대식세포)에게 “지방 짐”을 넘기는 방식.
• 단식에서의 역할: 장기 단식 시 지방 조직 염증/재구성 과정에서 중요. 지방 조직 macrophage infiltration과 연계.

이 그림은
단식(특히 장기 단식) 시
시상하부-뇌하수체 축과 말초 기관의 호르몬 변화를 종합적으로 보여줍니다.

Leptin 감소가
핵심 trigger로 작용합니다.

주요 변화 요약 (그림 기준)

1. Leptin ↓ (지방 조직)
   • 지방 축적 감소 → Leptin 분비 급감.
   • 전체 적응의 마스터 regulator: 에너지 부족 신호 전달.
2. 시상하부 (Hypothalamus) & 뇌하수체 (Pituitary)
   • GnRH, TRH, TSH, LH/FSH ↓ (억제).
   • GH (성장호르몬) ↑ (활성화).
   • 결과:
     • 갑상선 (Thyroid): T3/T4 ↓ (에너지 소비 감소, basal metabolic rate ↓).
     • 성선 (Gonads): Estradiol ↓ (여성), Testosterone ↓ (남성) → 생식 기능 억제 (에너지 절약).
3. 간 (Liver)
   • GH ↑에도 불구하고 IGF-1 ↓ (GH resistance 상태).
   • FGF21 ↑ (강력한 대사 조절 인자): 지질 산화·케톤 생성 촉진, 식욕 억제.
   • GH-IGF-1 uncoupling: 성장 대신 지방 동원 우선.

전체적인 생리적 의미

• 에너지 보존 모드 전환:
  • 불필요한 에너지 소비 (갑상선, 생식, 성장) 억제.
  • 지방 분해·케톤 이용 극대화 (GH ↑, FGF21 ↑).
  • 포도당 절약: 뇌·적혈구 제외 조직에서 지질/케톤 우선 이용.
• 장기 단식에서의 적응:
  • 초기: GH ↑로 lipolysis 촉진.
  • 지속: IGF-1 ↓ + leptin ↓로 추가 에너지 절약.
  • FGF21: “starvation hormone”으로서 autophagy, mito protection 강화.

이 Figure는
Time-Restricted Eating (TRE)과
Intermittent Fasting (IF) RCT들을 종합 분석한 핵심 데이터입니다.

체중 변화와 대사 지표 개선의 상관관계를 보여줍니다.

(A) 연구 대상자 특성

• X축: Mean Age (20~80세).
• Y축: Mean BMI (20~45).
• TRE (빨강 점): 상대적으로 젊거나 BMI 다양한 연구.
• IF (보라 점): 다양한 연령·BMI.
• 점 크기: 연구 규모 (N). 큰 점 = 대규모 연구 (예: N=95).

→ 대부분 과체중/비만 (BMI 25~35) 대상 연구들로, 정상 체중군 데이터는 제한적.

(B) 절대 체중 변화 (kg)와 대사 지표 상관

• HOMA-IR (인슐린 저항성): 체중 ↓와 강한 양의 상관 (R²=0.43, p=0.0013). 체중 4~6kg 감소 시 HOMA-IR 크게 개선.
• Triglycerides (중성지방): 체중 ↓와 상관 (R²=0.22, p=0.007).
• LDL: 체중 ↓와 상관 (R²=0.41, p=0.0002).
• HDL: 거의 상관 없음 (R²=0.007, p=n.s.).

(C) 체중 변화 %와 대사 지표

• HOMA-IR: 더 강한 상관 (R²=0.54, p=0.0011).
• Triglycerides, LDL: 유의한 개선.
• HDL: 여전히 유의미하지 않음.

결론 (논문 해석):

• TRE/IF의 대사 이점 (인슐린 민감성 ↑, TG ↓, LDL ↓)은 대부분 체중 감소에 의존적입니다.
• 체중 변화가 클수록 효과가 뚜렷함.
• HDL 개선은 약하고 일관성 없음.
• 한계: 체중 감소 독립적 효과인지, 정상 체중군·장기(>6개월) 데이터 부족

이 그림은
논문 Fazeli & Steinhauser (2025)의 balanced assessment를 시각적으로 가장 잘 보여주는
마무리 Figure입니다.

(A) 균형 저울 (Balance Scale)

• 왼쪽 (긍정적 효과, 녹색):
  • Weight loss
  • Cardiometabolic effect (인슐린 민감성 ↑, 지질 개선 등)
  • Malignancy prevention (암 예방 가능성)
• 오른쪽 (위험, 빨강):
  • Bone fragility (뼈 취약성)
• 중앙 물음표: “이득 vs 위험, 어느 쪽이 더 클까?”
  → 논문의 핵심 테마. 
  체중 감소가 매개하는 이점이 강하지만, 뼈 건강이 주요 trade-off.

(B) Fasting Reprogramming Wheel (재프로그래밍 바퀴) 

중앙 “Fasting Reprogramming”을 중심으로 단식이 영향을 미치는 다양한 생물학적 영역을 보여줍니다:

• Body weight (체중)
• Cellular metabolism (세포 대사)
• Insulin sensitivity (인슐린 민감성)
• Inflammation (염증)
• Carcinogenesis (암 발생)
• Lipids (지질)
• Metabolome / Proteome / Transcriptome / Epigenome (오믹스 변화)
• Microbiome (장내 미생물)
• Biomass turnover (생체 물질 turnover)
• Bone turnover (뼈 turnover)

→ 단식은 단순한 칼로리 제한이 아니라, 전신적·다층적 재프로그래밍을 일으킨다는 점을 강조.

논문 전체를 관통하는 메시지

• 이점: 모델 생물 수명 연장, 인간에서 체중·대사 개선, 잠재적 항암·항염증 효과 (IGF-1 ↓, autophagy ↑, ketone 등).
• 위험: 특히 체중 감소 동반 뼈 손실 (BMD ↓, 골다공증 위험). 저체중·여성·노인에서 더 주의

참고) cold induced lipolysis - 냉욕탐구

지방세포(adipocyte)에서

lysosomal acid lipase (LIPA)가 매개하는 lysosomal lipolysis가

기존에 알려진 cytosolic lipolysis(ATGL/HSL 중심)와는 독립적인 비전형(noncanonical) 경로로,

금식(fasting)과 한랭노출(cold exposure) 시 지방 분해(lipolysis)와 thermogenesis(열생성)에 필수적이라는 발견입니다.

이 경로는

기존 cytosolic lipolysis를 보완/독립적으로 작동하며,

LIPA 결핍 시 지방 분해가 제대로 안 되고,

체온 유지 실패,

산소 소비 저하,

결국 비만 및 glucose metabolism 장애로 이어집니다.

주요 Figure별 핵심 내용

Figure 1: LIPA가 fasting, cold exposure, β-agonist에 의해 활성화됨

• Fasting, cold exposure (CE), CL316,243 (β-agonist) 모두 LIPA mRNA/protein을 강하게 유도.
• 특히 iWAT, eWAT, BAT에서 현저.
• Cytosolic lipolysis (ATGL, HSL 등)와는 별개로 lysosomal pathway가 활성화됨.

상단: 정상적인 Lysosomal Lipolysis 과정

• 자극: Fasting(금식) 또는 Cold exposure(한랭 노출) → 지방세포 활성화.
• 주요 구조:
  • Lipid Droplet (LD): 지방세포 내 저장된 중성지방(트라이글리세라이드) 방울.
  • Lysosome: 라이소좀(황색 원) 안에 Lysosomal acid lipase (Lipa) 존재.
• 작용 과정:
  1. Lipid droplet가 라이소좀과 결합(autophagy-like 과정, lipophagy).
  2. Lipa가 LD를 분해 → Fatty acid(지방산) + Glycerol(글리세롤) 생성.
  3. 방출된 지방산은 thermogenesis(열 발생) 로 이어져 에너지 소비 증가 (갈색지방 또는 beige fat 활성화).

이 과정은 기존의 cytosolic lipolysis(세포질 지질분해, HSL/ATGL 경로)와는 별도의 lysosomal 경로로, 특히 스트레스(금식, 추위) 상황에서 중요한 역할을 합니다

Figure 2 & 3: LIPA knockout 또는 inhibitor (Lalistat-2) 투여 시 lipolysis가 억제됨

• Adipocyte-specific LIPA KO 마우스에서:
  • Fasting, Cold exposure, β-agonist(CL) 유도 lipolysis (free fatty acid + glycerol 방출)가 강하게 억제.
• Pharmacological inhibition (Lalistat-2)에서도 동일한 효과.
• Cytosolic lipolysis는 억제되지 않음 → LIPA 경로는 독립적임을 증명.

Figure 4: LIPA 결핍 → 체온 유지 및 energy expenditure 장애

• Cold exposure 시 LIPA KO 또는 Lalistat-2 투여 마우스는 체온 급강하 (hypothermia).
• CL316,243 투여 시에도 체온 상승과 oxygen consumption(VO₂)이 현저히 저하. → Thermogenesis에 LIPA-mediated lysosomal lipolysis가 핵심.

Figure 5: LIPA 발현은 비만과 역상관, 결핍 시 adipose tissue morphology 변화

• 고지방식이(HFD)나 ob/ob 모델에서 LIPA 발현 ↓.
• LIPA KO 마우스: iWAT, eWAT, BAT에서 adipocyte hypertrophy (지방세포 비대), lipid droplet 크기 증가.

Figure 6: LIPA 결핍은 diet-induced obesity를 악화시킴

• 16주 HFD 후 LIPA KO 마우스:
  • 더 많은 체중 증가, 지방량 증가.
  • Glucose intolerance 악화.
  • Energy expenditure (VO₂) 감소, RER 변화 등 대사 장애.

Figure 7: LIPA-mediated lipolysis는 ATGL과 독립적

• LIPA KO에서도 cytosolic lipases (PNPLA2/ATGL, HSL, ABHD5 등) 발현/단백질은 정상 또는 보상성 증가.
• ATGL inhibitor (Atglistatin) 처리에도 LIPA KO의 lipolysis 억제 효과가 추가로 나타남 → 두 경로가 병렬(parallel)로 작동.

공복후 refeeding study

장기 단식(Prolonged Fasting, PF: 에너지 섭취를 4일 이상 금하는 것)은

최근 건강·노화·대사 질환 개선을 위한 잠재적 개입으로 관심을 받고 있지만,

혈장 단백체(plasma proteome)에 미치는 생물학적 영향은 잘 알려지지 않았습니다.

연구진은

의학적으로 감독된 물만 단식(평균 9.8일) + 재급식(평균 5.3일)을

20명의 중년 참가자(평균 연령 52세, BMI 28.8)에게 적용해

단백질체 변화와 임상 지표를 분석했습니다.

주요 결과는

단식 중 급성 염증 반응과 혈소판 활성화가 유발된다는 점입니다

(긍정적 적응 vs. 잠재적 위험).

주요 발견 (Figures 기반)

1. 전체 단백체 변화 (Figure 1A Volcano Plot):
   • 단식 중(up-regulated, 빨간 점): SERPINA5, KLK8, LEPR(렙틴 수용체), NAGK, MMP9 등 급성기 반응·염증 관련 단백질 증가.
   • Down-regulated (파란 점): INHBA, FETUB, MSTN(미오스타틴, 근육 성장 억제) 등 일부 근육·대사 관련 감소.
   • Canonical pathway 분석(Figure 1D): STAT3, IGF 조절, IL-15, 아밀로이드 섬유 형성 감소, PPARα/RXRα 활성화 등.
2. 특정 단백질 변화:
   • Figure 1B/C: INHBA, FETUB, Myostatin, PCSK9, GDF15, Leptin receptor, FGF19 등이 단식 중 변화 후 재급식에서 회복 경향.
   • Adiponectin, β-amyloid 40/42: Aβ40/42(뇌 아밀로이드 플라크 주요 성분) 유의하게 감소 — 신경보호 잠재력 시사.
3. 염증 지표 상승 (Figure 2 등):
   • CRP(고감도 C-반응성 단백), Hepcidin(헤프시딘, 철 대사·염증), Midkine, IL-8, MMP9, IL-8 등이 단식 중 유의하게 증가.
   • 대규모 후향적 코호트(1,422명)에서도 CRP 상승 확인(2.8 → 4.3 mg/L).
   • TGF-β signaling, complement/coagulation cascade 활성화.
4. 혈소판 활성화와 응고 관련 (Figure 3 등):
   • vWF(von Willebrand Factor), GP1BA, Fibrinogen, 11-dehydro-TXB2(혈소판 활성화 마커), Platelets count 변화.
   • 혈소판 degranulation 증가, complement/coagulation cascade upregulation — 혈전 위험 관련 우려 제기.
5. 기타:
   • 체중 감소(~7.7%), BHB(케톤체) 상승 → 단식 순응 확인.
   • 근육/뼈 보존, lysosomal biogenesis 증가, PPARα-mediated lipid metabolism 향상, 아밀로이드 섬유 형성 감소 등 긍정적 변화도 관찰.

결론과 함의 (논문 Abstract/Conclusion)

• 긍정적 측면: 아밀로이드 β 감소, 지방 대사 개선, 일부 근육 보존 등.
• 부정적/주의 측면: 급성 전신 염증 + 혈소판 활성화가 유발되며, 이는 TGF-β signaling과 연관. 기존 심혈관 질환이 있는 사람에게는 혈전/심혈관 위험을 높일 수 있음. 재급식 후 일부 회복되지만, 장기 영향은 더 연구 필요.

A Critical Assessment of Fasting to Promote Metabolic Health and Longevity 

Pouneh K Fazeli , 

Matthew L Steinhauser

Endocrine Reviews, Volume 46, Issue 6, December 2025, Pages 856–876, https://doi.org/10.1210/endrev/bnaf021

Published:

23 July 2025

 Article history

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Abstract

The adaptive starvation response allows us to survive periods of starvation—a characteristic of the environment in which humans evolved. We are now in an evolutionary transition from a global environment that was characterized by periods of famine to a world where obesity and caloric excess have become a new reality, but the mechanisms of fasting physiology remain relevant.

First, many parts of the world are still plagued by famine with insufficient food resources and therefore the adaptive mechanisms required for survival during periods of decreased caloric intake are not simply relevant to our evolutionary past. Second, the obesity epidemic provides strong rationale for understanding the biology of fasting, as the same efficiencies that have evolved to allow us to survive periods of starvation also likely drive a genetic predisposition to obesity, and therefore some of the adaptive mechanisms may be maladaptive in the setting of food excess. A third compelling reason to explore the biology of fasting is that in model organisms, caloric restriction, without overt starvation, is an intervention that prolongs lifespan. The purpose of this review is to provide an overview of the biology of fasting. We will highlight potential mechanisms of benefit from fasting as well as examine data from model organisms and humans that indicate potential health risks of fasting, particularly related to bone fragility. Finally, we will review clinical studies to date that have investigated the effects of fasting on metabolic outcomes and suggest signals of benefit.

적응형 기아 반응(adaptive starvation response)은

인간이 진화한 환경의 특징인 기아 기간을 생존할 수 있게 해줍니다.

우리는

지금 전 세계적으로 기근이 만연했던 환경에서

비만과 과잉 칼로리가 새로운 현실이 된 진화적 전환기(evolutionary transition)에 있습니다.

하지만

금식 생리학(fasting physiology)의 기전은 여전히 중요합니다.

첫째, 세계 많은 지역은 여전히 식량 부족과 기근에 시달리고 있어, 칼로리 섭취가 감소하는 기간 동안 생존에 필요한 적응 기전이 단순히 우리의 진화적 과거에만 관련된 것이 아닙니다.

둘째, 비만 유행(obesity epidemic)은 금식의 생물학을 이해해야 할 강력한 근거를 제공합니다. 기아 기간을 생존하기 위해 진화한 동일한 효율성(efficiencies)이 비만에 대한 유전적 소인(genetic predisposition)을 유발할 가능성이 높기 때문입니다. 따라서 일부 적응 기전은 음식 과잉 상황에서는 부적응적(maladaptive)일 수 있습니다.

셋째, 금식 생물학을 탐구해야 하는 세 번째 강력한 이유는, 모델 생물에서 명백한 기아 상태가 아닌 칼로리 제한(caloric restriction)이 수명을 연장하는 개입(intervention)이라는 점입니다.

이 리뷰의 목적은

금식의 생물학(biology of fasting)에 대한 개요를 제공하는 것입니다.

우리는 금식으로부터 얻을 수 있는 잠재적 이점의 기전을 강조하고,

모델 생물과 인간 연구 데이터를 검토하여

금식의 잠재적 건강 위험(특히 골격 취약성(bone fragility)과 관련된 위험)을 살펴볼 것입니다.

마지막으로,

금식이 대사 결과(metabolic outcomes)에 미치는 영향을 조사한 임상 연구들을 검토하고,

이익의 신호(signals of benefit)를 제시하겠습니다.

intermittent fasting, alternate day fasting, weight loss, longevity, bone metabolism, caloric restriction, time-restricted eating

Issue Section:

 Review

Collection: Endocrine Society Journals

ESSENTIAL POINTS

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Relevance of Fasting Physiology

The adaptive starvation response enables humans to survive for prolonged periods (days to months) without energy intake (1). Although we are in an evolutionary transition from a global environment characterized by cyclic famine to a new reality of food excess and obesity, the mechanisms of fasting physiology remain relevant. First, many parts of the world still have insufficient food resources, and therefore understanding the adaptive response to caloric restriction remains relevant in today's world to understand both survival mechanisms and the pathology associated with prolonged starvation. Second, the obesity epidemic itself has been cited as rationale for understanding the biology of fasting, as the same efficiencies that we evolved to survive periods of starvation may potentially drive a genetic predisposition to obesity, the so-called “thrifty gene hypothesis” (2). Third, and perhaps the more compelling reason to examine the biology of fasting, is that restriction of calories is a conserved behavioral intervention that prolongs lifespan in model organisms (3-7).

The dietary restriction of calories can take different forms. In this review, we will focus solely on total caloric intake rather than restriction of specific dietary components, as there is a separate body of literature examining restriction of specific nutrients such as methionine (8). The term “caloric restriction” usually describes a sustained reduction in caloric intake, entailing an approximate 10% to 30% reduction in daily calories. Another approach involves intermittent periods of dramatically reduced calories or complete fasting, alternating with periods of “normal” caloric intake, so-called intermittent fasting. A hybrid of the 2 is time-restricted feeding, where food intake is restricted to a narrow daily time window, thereby incorporating a partial-day fast into each day. The lines between these 3 modes are often blurred and within each category there is substantial variability in approach. Despite the general lack of historical standardization and that the 3 categories share a tendency to achieve net restriction of calories (9), in this review, we will restrict the term caloric restriction to denote protocols of sustained daily calorie reduction and the term intermittent fasting to protocols involving a zero or near-zero calorie fast. Although the primary subject of this article is fasting, we will also include the concept of caloric restriction because there is undoubtedly shared biology between these 2 paths to negative energy balance.

금식 생리학의 관련성 (Relevance of Fasting Physiology)

적응형 기아 반응(adaptive starvation response)은

인간이 에너지 섭취 없이 수일에서 수개월 동안 생존할 수 있게 해줍니다(1).

우리는

순환적 기근(cyclic famine)이 특징이었던 과거의 전 지구적 환경에서

식량 과잉과 비만이 새로운 현실이 된 진화적 전환기에 놓여 있지만,

금식 생리학의 기전은 여전히 중요합니다.

첫째, 세계 많은 지역에서 여전히 식량 자원이 부족하기 때문에, 칼로리 제한에 대한 적응 반응을 이해하는 것은 오늘날에도 생존 기전과 장기 기아와 관련된 병리를 이해하는 데 중요합니다.

둘째, 비만 유행 자체가 금식 생물학을 이해해야 할 근거로 인용되고 있습니다. 기아 기간을 생존하기 위해 진화한 동일한 효율성이 비만에 대한 유전적 소인(genetic predisposition)을 유발할 수 있기 때문이며, 이를 이른바 “절약 유전자 가설(thrifty gene hypothesis)”(2)이라고 합니다.

셋째, 그리고 아마도 금식 생물학을 연구해야 하는 가장 설득력 있는 이유는, 칼로리 제한이 모델 생물에서 수명을 연장하는 보존된 행동적 개입(conserved behavioral intervention)이라는 점입니다(3-7).

칼로리 제한(dietary restriction of calories)은

다양한 형태로 나타날 수 있습니다.

본 리뷰에서는

특정 영양소(예: 메티오닌)의 제한에 관한 별도의 연구 문헌이 존재하므로(8),

특정 식이 성분의 제한이 아닌 총 칼로리 섭취량에만 초점을 맞추겠습니다.

“칼로리 제한(caloric restriction)”이라는 용어는

일반적으로 일일 칼로리 섭취를 지속적으로 약 10~30% 줄이는 것을 의미합니다.

또 다른 접근법은

간헐적 금식(intermittent fasting)으로,

극적으로 칼로리를 줄이거나 완전 금식을 하는 기간과 “정상” 칼로리 섭취 기간을 번갈아 가지는 방식입니다.

이 둘의 하이브리드 형태가

시간 제한 식사(time-restricted feeding)로,

하루 중 음식 섭취를 좁은 시간대에 제한하여 매일 부분 금식을 포함하는 것입니다.

이 세 가지 방식 사이의 경계는 종종 모호하며,

각 범주 내에서도 접근법의 변동성이 상당합니다.

역사적으로 표준화가 부족하고 세 가지 모두 결국 순(net) 칼로리 제한을 달성하는 경향이 있음에도(9),

본 리뷰에서는 칼로리 제한이라는 용어를

지속적인 일일 칼로리 감소 프로토콜에 한정하고,

간헐적 금식은 0 또는 거의 0에 가까운 칼로리 금식 프로토콜을 의미하는 것으로 제한하겠습니다.

이 글의 주요 주제는 금식이지만,

두 가지 경로(칼로리 제한과 금식)가

음 에너지 균형(negative energy balance)에 이르는 공통 생물학을 공유하고 있으므로

칼로리 제한 개념도 포함하겠습니다.

The purpose of this review is to provide an overview of the biology of fasting. The adaptive fasting response has been extensively studied for decades and therefore this review cannot do justice to the complexity and nuance of the operative hormonal and molecular mechanisms. Instead, we focus on central themes and mechanisms that are crucial to fasting and that may be relevant to metabolic health. Indeed, intermittent fasting is increasingly practiced in one form or another by patients, often without prescription or supervision by a medical practitioner. Yet, the burgeoning interest in intermittent fasting is shared by many in the biomedical research community because of theoretical intrinsic salutary properties of fasting and signals of benefit from early translational human studies. As a lead-in to the human clinical studies, we will first provide an overview of the adaptive fasting response. We will highlight potential mechanisms of benefit from fasting; however, we will also examine data from model organisms and humans that indicate potential health risks to fasting, particularly related to bone fragility. Although the science and clinical evidence base is not yet sufficient to widely recommend fasting, the signals of benefit provide rationale for further study, and we will conclude by articulating our view of the key unanswered questions.

본 리뷰의 목적

본 리뷰의 목적은

금식의 생물학에 대한 개요를 제공하는 것입니다.

적응형 금식 반응은

수십 년 동안 광범위하게 연구되어 왔기 때문에,

본 리뷰에서 작동하는 호르몬 및 분자 기전의 복잡성과 미묘함을 모두 다루기는 어렵습니다.

대신,

금식에 핵심적이며 대사 건강(metabolic health)과 관련될 수 있는 중심 주제와 기전에 초점을 맞춥니다.

실제로 간헐적 금식은

환자들 사이에서 처방이나 의료진의 감독 없이도

다양한 형태로 점점 더 실천되고 있습니다.

그러나

금식의 이론적 내재적 이익(salutary properties)과

초기 전임상·임상 인간 연구에서 나타난 이익의 신호 때문에,

생의학 연구 커뮤니티에서도 간헐적 금식에 대한 관심이 급증하고 있습니다.

인간 임상 연구를 소개하기에 앞서,

먼저 적응형 금식 반응(adaptive fasting response)에 대한 개요를 제공하겠습니다.

금식으로부터의 잠재적 이점 기전을 강조하는 한편,

모델 생물과 인간 연구에서 나타난 금식의 잠재적 건강 위험(특히 골격 취약성(bone fragility) 관련 위험)도 검토할 것입니다.

현재 과학적·임상적 근거가 아직 충분하지 않아 금식을 광범위하게 권고하기는 어렵지만,

관찰되는 이익의 신호는 추가 연구의 근거를 제공합니다.

마지막으로,

아직 풀리지 않은 주요 미해결 질문들에 대한 우리의 관점을 제시하겠습니다.

The Adaptive Fasting Response (Fig. 1)Endogenous Glucose Production During Early FastingGlycogenolysis

The onset of fasting can be defined as the point at which calories are no longer ingested. However, the physiological adaptation to fasting commences when absorption of digested food is completed.

A classical view of the adaptive fasting response describes at least three adaptive post-absorptive phases:

(1) release of glucose from glycogen stores;

(2) synthesis of glucose by gluconeogenesis; and

(3) lipid catabolism and onset of ketogenesis.

Progression to the third phase marks the shift from glucose to lipid metabolism, enabling survival during prolonged periods of fasting by preventing preterminal catabolic sacrifice of the crucial protein backbone. In humans, the third phase, during which lipids serve as the principal fuel source, can last for months (10). Although rodent model organisms also exhibit a similar adaptive fasting response, their faster relative metabolic rate compresses the timescale, resulting in death if fasting is extended beyond a few days (11).

적응형 금식 반응 (The Adaptive Fasting Response, Fig. 1)

조기 금식 기간 동안의 내인성 포도당 생산 (Endogenous Glucose Production During Early Fasting)

글리코겐 분해 (Glycogenolysis)

금식의 시작은 칼

로리 섭취가 중단되는 시점으로 정의할 수 있습니다.

그러나

금식에 대한 생리학적 적응은

소화된 음식의 흡수가 완료되는 시점부터 시작됩니다.

적응형 금식 반응에 대한 고전적 관점은 적어도 세 가지 적응성 흡수 후(post-absorptive) 단계를 기술합니다:

1. 글리코겐 저장고로부터 포도당 방출
2. 포도당 신생합성(gluconeogenesis)에 의한 포도당 합성
3. 지질 분해(lipid catabolism)와 케톤 생성(ketogenesis)의 시작

세 번째 단계로 진행되는 것은

포도당 대사에서 지질 대사로의 전환을 의미하며,

중요한 단백질 골격(protein backbone)을 말기적 파괴(preterminal catabolic sacrifice)로부터 보호함으로써

장기 금식 기간 동안 생존을 가능하게 합니다.

인간에서는

지질이 주요 연료원이 되는 이 세 번째 단계가 수개월 동안 지속될 수 있습니다(10).

설치류 모델 생물 역시 유사한 적응형 금식 반응을 보이지만,

상대적으로 빠른 대사율 때문에 시간 척도가 압축되어

며칠 이상 금식을 지속하면 사망에 이릅니다(11).

Figure 1.

The metabolic adaptation to fasting. (A) The 3 classical phases of the postabsorptive response to fasting overlap in time and include (1) glycogenolysis, which is the release of glucose from glycogen stores; (2) gluconeogenesis; and (3) ketogenesis, which marks the onset of lipid catabolism. Note that time = 0 days marks the onset of fasting. A lag between time = 0 and the onset of glycogenolysis accounts for the absorption of the last meal. The timing of these phases is approximate, as the temporal dynamics may be affected by factors such as basal metabolic rate, composition of the last meal, and degree of glycogen stores. (B) Adaptation to fasting in critical tissues is shown to contrast prolonged from early fasting. Early: glucose metabolism dominates. Glycogen stores in the liver and skeletal muscle supply glucose in the early postabsorptive period. Glucogenic amino acids, for which skeletal muscle is an important source, are used as substrate for gluconeogenesis by the liver and kidney (not shown). Prolonged: lipid stores, predominantly in adipose tissue, are mobilized by lipolysis, releasing fatty acids and glycerol. Glycerol serves as substrate for gluconeogenesis, which must persist to supply minimal glucose needs, especially for glucose obligate red blood cells. Tissues with the machinery to directly oxidize fatty acids (eg, skeletal muscle) do so, whereas the liver uses fatty acids as substrate for ketone production. Ketones serves as a critical lipid-derived fuel source for the central nervous system.

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The first phase, glycogenolysis, involves the release of glucose molecules from glycogen chains, which accumulate in the liver and skeletal muscle in the fed state (12, 13). The breakdown of glycogen is enzymatically driven by glycogen phosphorylase, yielding individual molecules of glucose-1 phosphate, which in turn are converted to glucose-6 phosphate by the action of phosphoglucomutase. In skeletal muscle, where the glycogen stores provide a rapidly accessible energy source during periods of intense muscle contraction, glucose-6 phosphate is directly catabolized by glycolysis. In the liver, by contrast, the phosphate moiety is removed by glucose-6 phosphatase, yielding glucose, which in turn can exit the cell via glucose transporters to maintain systemic glucose levels and supply distant tissues with fuel for glycolytic metabolism. As such, the liver glycogen stores are of greater relevance to the systemic adaptive response to fasting.

Gluconeogenesis

Induction of gluconeogenesis maintains glucose homeostasis with depletion of glycogen stores. The liver and kidney are the principal sites of gluconeogenesis (14). The small intestine also expresses the requisite enzymatic machinery for gluconeogenesis and may also support glucose homeostasis with fasting (15). Gluconeogenesis largely operates as the enzymatic steps of glycolysis in reverse except for the actions of phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase, which, respectively, catalyze conversion of oxaloacetate to phosphoenolpyruvate, conversion of fructose-1,6-bisphosphate to fructose 6-phosphate, and the hydrolysis of glucose 6-phosphate to yield free glucose.

The activity of key enzymatic control nodes in glycogenolysis and gluconeogenesis are regulated at multiple levels, including by the action of canonical hormonal systems, and via transcriptional control. The combination of hypoinsulinemia and glucagon surge, for example, impairs glycolysis and drives glycogen catabolism and gluconeogenesis (16). Signal transduction pathways, including canonical cyclic-AMP signaling, operate dynamically to transmit hormonal signals to phosphorylate key enzymes driving both glycogenolysis and gluconeogenesis (17). Signal transduction cascades may ultimately also end in the nucleus with transcription factor dependent transcriptional programs, which can reinforce transcription of genes encoding the core enzymatic machinery that may already be expressed at some constitutive level or facilitate de novo expression‎ of fasting-specific regulators. One canonical example is the cyclic AMP response-element binding protein (CREB), a transcription factor translocated to the nucleus in response to glucagon and cAMP signaling and which in turn activates genes, including those coding key enzymes such as glycogen phosphorylase (PYGL, glycogenolysis) and phosphoenolpyruvate carboxykinase (PCK1, gluconeogenesis) (18-21). Although genetic loss of function of CREB results in a phenotype of severe hypoglycemia in mice, consistent with its critical role in fasting glucose metabolism, coactivators such as PGC1α and TORC2 and multiple collaborating transcription factors such as FOXO1, NR4A1, and peroxisome proliferator-activated receptor α (PPARA), amongst others, also play regulatory roles with fasting (22-27).

Gluconeogenesis ultimately also depends on substrate availability. In the fasted state, glycerol released from adipose tissue and glucogenic amino acids derived from skeletal muscle serve as critical substrate for gluconeogenesis (28). Alanine is a particularly important amino acid substrate for gluconeogenesis, the dynamics of which also illustrate the complexity of systemic metabolism during fasting. Studies involving arterial-venous sampling reveal a net release of amino acids, including alanine from skeletal muscle, yet circulating alanine levels drop with fasting because of the greater uptake of alanine by the liver for utilization as gluconeogenic substrate (14). Even though there is net release of amino acids from skeletal muscle in the early phases of fasting, the extent to which this flux is attributable to export of free cytosolic amino acids (29), decreased protein synthesis (30), and/or increased protein catabolism (31) is not definitively resolved. Nonetheless, the high cost of producing gluconeogenic substrate from protein catabolism underscores the importance of the ensuing shift away from glucose-centric metabolism as reflected by a decline in the Respiratory Quotient (an indicator of macronutrient catabolism on indirect calorimetry), and accounting for the relatively short duration of the phase in which glucose is a dominant systemic fuel source (32).

The Shift to Lipids as Dominant Fuel Source

The shift to lipid utilization is the key adaptation that allow humans to survive prolonged periods of fasting. To accomplish this shift, 3 key events must occur: (1) cells containing the requisite mitochondrial machinery must transition to oxidation of fatty acids as their principle energy source; (2) lipid stores, largely contained in adipocytes, must be mobilized into systemic circulation; and (3) the liver must use circulating fatty acids to produce ketone bodies which can cross the blood-brain barrier and supply the sizeable energy needs of the brain and central nervous system (33). Apart from red blood cells, which lack mitochondria, most tissues can transition to use of lipid-derived substrate as fuel, either through the direct oxidation of fatty acids or by the utilization of lipid-derived ketones.

Mobilization of adipocyte lipid stores (Fig. 2)

As the central energy reserve, adipose tissue plays a crucial role during fasting. Energy is stored in adipocytes in the form of neutral lipid droplets—ie, triglycerides. Mobilization of this energy depot requires transfer from intracellular lipid droplets out of the adipocyte and across the capillary endothelial barrier. Triglycerides cannot directly cross the plasma membrane and therefore the canonical model of adipocyte lipid mobilization involves lipolytic release of 3 fatty acid chains capable of crossing the plasma membrane by diffusion or fatty acid transporters. Adipose triglyceride lipase (ATGL/PNPLA2), hormone-sensitive lipase (HSL/LIPE), and monoacylglycerol lipase (MGL/MGLL) catalyze the sequential release of 3 fatty acid chains from each triglyceride molecule (34-38) (Fig. 2A). As such, genetic loss of function of ATGL/Pnpla2 in mice results in triglyceride accumulation in adipose tissue (39). The high specificity of HSL for diacylglycerols relative to triglycerides accounts for the accumulation of diacylglycerols with targeted disruption of HSL/Lipe (35). In turn, monoacylglycerols accumulate with genetic loss of function of Mgll, the gene encoding MGL (40).

Figure 2.

Mechanisms of triglyceride mobilization in adipocytes. (A) Canonical lipolysis pathway operative in adipocytes. ATGL liberates a fatty acid chain from triglycerides contained in adipocyte lipid droplets resulting in a diacylglycerol (DAG). The action of HSL on DAG releases a second fatty acid chain and results in monoacylglycerol. The final fatty acid chain is released by the action of MGL. (B) Recently described mechanism of adipocyte lipolysis with fasting in which lysosomal acid lipase (LAL), the only lipase known to operate in the low pH environment of the lysosome, liberates free fatty acids (45). (C) Recently described mechanism of lipolysis that depends on resident adipose tissue macrophages (46). Triglycerides contained in adipocyte-derived exosomes are released into the extracellular space where they are taken up by macrophages, which have lipolytic capacity.

Abbreviations: ATGL, adipose triglyceride lipase; HSL, hormone-sensitive lipase; MGL, monoacylglycerol lipase.

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Lipolysis is generally attributed to hormonal signaling and signal transduction pathways that drive activity of canonical lipases (41). Activation of g-protein coupled β-adrenergic receptors by catecholamines activates adenylate cyclase, which generates cyclic AMP. cAMP-dependent protein kinase A activity phosphorylates—and thus activates—HSL and α/β hydrolase domain-containing protein 5, a key regulator of ATGL. Insulin signaling is also important. In the fed state, insulin engagement of the insulin receptor results in its phosphorylation and activation of IRS1/2, the phosphoinositide 3-kinase (PI3K)/AKT signaling pathway, and effectors that dampen cAMP and its stimulatory actions on ATGL and HSL. In the hypoinsulinemic state of fasting, the potent inhibitory effects of insulin on lipase activity are attenuated. Although the exact intracellular signaling path to lipase activation may differ depending on the physiological and hormonal inputs, these signaling pathways converge on lipolytic release of fatty acids at the interface between lipid droplets and the adipocyte cytosol.

Although murine genetic loss of function studies point to important roles for the 3 canonical lipases in adipocyte lipid metabolism, the fasting mediated transcriptional induction of canonical lipase genes is either undetectable or modest relative to any observed changes in lipolytic activity, underscoring the importance of posttranscriptional regulatory functions and/or complementary roles for noncanonical lipases (42-44). Indeed, the canonical view of adipocyte lipolysis has recently been complicated by the identification of alternative paths to adipocyte lipid release. In a recent longitudinal fat transcriptomics study of fasting humans, we found induction of genes associated with noncanonical lysosomal lipolysis (44). Moreover, in a murine model of inducible adipocyte-specific loss of function of lysosomal acid lipase (LAL/Lipa), the lipase operative in the low pH state of the lysosome, we discovered that lysosome-mediated lipolysis becomes dominant as fasting is prolonged beyond a few hours (Fig. 2B) (45). Another alternative mechanism of lipid mobilization involves nonlipolytic release of adipocyte triglycerides via exosomes (46). Triglyceride-rich exosomes may then be engulfed by resident macrophages, capable of digesting lipid and releasing the catabolic byproducts, including fatty acids (Fig. 2C). Regardless of the relative contributions from canonical and noncanonical mechanisms, however, the mobilization of lipid stores with a prolonged fast are sufficiently effective that they may even exceed the substrate requirements of the liver and lead to acute steatosis (47).

Switch to fatty acid oxidation

Mitochondrial-rich skeletal muscle is 1 of the largest fuel-consuming tissues in the body and therefore is a relevant case study for the fuel switch that occurs with fasting. In the fed state, insulin signaling drives vesicles containing type 4 glucose transporters (GLUT4) to the plasma membrane and subsequent transport of glucose into insulin responsive cells, such as adipocytes and skeletal myocytes (48-51). Like the liver, skeletal muscle stores excess glucose as glycogen (52), whereas adipocytes convert excess glucose into triglycerides by supplying substrate for the glycerol backbone and fatty acids by de novo lipogenesis (53). Glucose is also used to fuel glycolysis, which supplies substrate to the mitochondria for the tricarboxylic acid (TCA) cycle and oxidative phosphorylation. With fasting, these processes are attenuated. Circulating glucose and insulin levels drop, and with prolonged fasting, the effects of hypoinsulinemia may be enhanced further in peripheral tissues by relative insulin resistance (54, 55). The fed-fasting dynamics of GLUT4 levels and activity present an extreme dichotomy in adipocytes, where the insulin deficient state of fasting results in dramatic declines in GLUT4 mRNA, protein, and vesicular trafficking (48-51). Muscle is distinct from fat tissue in that there are insulin-independent mechanisms of GLUT4 vesicular trafficking to meet energy import requirements with exercise (56, 57), which likely accounts for the lack of consistent correlation between insulin levels or activity and GLUT4 mRNA and protein levels (58). Nonetheless, the hypoinsulinemia of fasting reduces GLUT4 in myocyte plasma membranes of resting skeletal muscle (59), effectively driving a reduction in glucose uptake. Fasting, particularly when prolonged also downregulates expression‎ of the glycolytic gene program (54). Therefore, the collective effects of fasting on resting skeletal muscle reduce glucose uptake and utilization (60).

Concomitant to the reduction in glucose utilization is an increase in fatty acid oxidation. The mechanisms by which fatty acid uptake and utilization are achieved in the fasted state are not as well elucidated relative to regulation of glucose metabolism during fasting. Lipids and fatty acids are a heterogeneous array of molecules. In addition, the immiscibility of neutral lipids in water and the poor solubility of free fatty acids necessitates a variety of specialized protein factors to facilitate trafficking and metabolism, both systemically and at the individual cellular level (61). Indeed, although it was once thought that fatty acids could freely diffuse across plasma membranes—which is supported by flux analyses across synthetic protein-free lipid bilayers—achievement of sufficiently dynamic fatty acid flux to meet the oxidative fuel needs during fasting requires the additional action of protein chaperones (62). Gain and loss of function studies targeting membrane proteins, such as CD36 or members of the fatty acid-binding protein (FABP) family, demonstrate the sensitivity of physiological fatty acid flux to modulation of these critical plasma membrane proteins (63-65). Once fatty acids gain access to the cytosol, they are bound by fatty acid binding proteins (62). Activation of fatty acids by acyl-coenzyme A (CoA) enzymes precedes their transport into mitochondria by the carnitine-palmitoyltransferase system, where fatty acids are catabolized by β-oxidation, producing acetyl-CoA molecules to supply the TCA cycle (66). Analogous to the insulin-stimulated translocation of GLUT4 to the plasma membrane in the fed state, similar mechanisms may be at play in the regulation of CD36 and FABPs, as both have been shown to be dynamically recruited by fasting (67, 68). Mitochondrial fatty acid oxidation gene programs are also induced by fasting (69). These collective actions during fasting result in the augmentation of fatty acid importation and oxidative utilization.

In the preceding 2 paragraphs, we have described the attenuation of glucose utilization and the augmentation of fatty acid utilization as distinct; however, these processes are not independent of one another. Randle first postulated that substrate availability—ie, competition between glucose and fatty acids—is a determinant of fuel utilization (70). Although specific mechanistic details of Randle's hypothesis have been questioned, particularly with respect to the underlying role of lipids in the pathophysiology of insulin resistance and type 2 diabetes (71), human and mouse genetic models support the general concept that fatty acid metabolism can regulate glucose utilization with fasting. For example, homozygous loss of function of many of the genes involved in fatty acid oxidation predisposes to hypoglycemia with fasting (66). In mice, genetic knock out of long-chain acyl-CoA dehydrogenase, a key enzymatic mediator of mitochondrial fatty acid oxidation, phenocopies fasting mediated hypoglycemia in part by augmenting glucose uptake and utilization (72). These genetic models suggest that attenuation of inhibitory mechanisms operative during fasting that are dependent on fatty acid catabolism augments glucose utilization; conversely, intact fatty acid oxidation during fasting reinforces the switch from glucose to lipid metabolism by actively suppressing key nodes in glucose uptake and/or catabolism.

Ketogenesis

Ketones are produced from ketogenic amino acids and from the products of fatty acid catabolism. The production of ketones by the liver requires many of the same steps of fatty acid uptake and oxidation in mitochondria that have already been discussed. The key branch point occurs after generation of acetyl-CoA (73). Instead of supplying the TCA cycle as occurs in skeletal muscle, 3 acetyl-CoA molecules are converted to acetoacetate by the sequential actions of acetyl coenzyme A acetyltransferase, 3-hydroxy-3-methylglutaryl (HMG)-CoA synthase and HMG-CoA lyase. β-hydroxybutyrate is generated from acetoacetate by β-hydroxybutyrate dehydrogenase. Acetoacetate and β-hydroxybutyrate are 2 ketones used by extrahepatic tissues for energy.

The rate-limiting enzyme for ketogenesis is HMG-CoA synthetase. Nutrient sensitive pathways regulate transcription of the HMG-CoA synthetase gene (HMGCS2), including by the fasting-mediated induction of the nuclear receptor, PPARα (74). There are PPAR binding motifs at HMGCS2 cis-regulatory domains and HMGCS2 is a PPARα-responsive gene (75); however, there may be additional indirect PPARα-dependent mechanisms that account for its fasting effects. For example, PPARα transcriptionally regulates additional enzymes involved in fatty acid oxidation and the PPARα-regulated secretion of fibroblast growth factor (FGF)21 may in turn regulate lipid metabolism and ketogenesis by indirect autocrine and paracrine mechanisms (76). The role of PPARα as a central node in the regulation of lipid metabolism with fasting is exemplified by the phenotype exhibited by the global genetic loss of function murine model, which has impaired fatty acid oxidation and ketogenesis with fasting (77, 78).

FOXA2 is another transcription factor that binds the promoter for HMGCS2 and activates its transcription. In the fed state, insulin signaling leads to phosphorylation and nuclear export of FOXA2, thereby inhibiting HMGCS2 transcription (79, 80). With the reduced insulin action during fasting and the additional counterregulatory action of glucagon signaling, which promotes FOXA2 acetylation and transcriptional regulatory function, transcription of HMGCS2 is augmented (81). Therefore, FOXA2 represents an important node in the insulin/glucagon regulatory axis that is central to cycles of feeding and fasting.

The result of these fasting-responsive regulatory networks in the liver is that ketones are released into systemic circulation, where they serve as a critical energetic substrate for the central nervous system, which is insulated from circulating lipids because of the blood-brain barrier. In neuronal mitochondria, ketones are converted back to acetyl-CoA to supply the TCA cycle for ATP production. These mechanisms also underscore the central role of the liver in the adaptation to fasting: the liver maintains circulating glucose, first by mobilization of glycogen and then by gluconeogenesis in early fasting; then in the later phase where lipid metabolism is essential to survival, the liver supplies energy in the form of ketones to fulfill the energy demands that cannot be met directly by fatty acids.

Reduction in Energy Expenditure

Superimposed on the 3 classical phases of adaptive starvation is a reduction in energy expenditure. The classic Minnesota 6-month semi-starvation experiments on conscientious objectors during World War II demonstrated sustained reductions in both physical activity and resting energy expenditure (82). Reductions in energy expenditure can also be detected in individuals with anorexia nervosa, a primary psychiatric disorder with inappropriately low caloric intake that serves as a representative cohort for studying chronic caloric deprivation (83), and with less severe forms of long-term caloric restriction or with acute zero-calorie fasting (84, 85).

Neuroendocrine Adaptations to Negative Energy Balance (Fig. 3)Hypothalamic-pituitary-thyroid axis

The hormonal and adaptive effects of reduced energy intake are well characterized in anorexia nervosa. In this disorder, adaptive hormonal responses result in a reduction in energy expenditure. A critical determinant of basal metabolic rate is the hypothalamic-pituitary-thyroid axis, long recognized to be sensitive to calorie deprivation. A characteristic finding in anorexia nervosa is thyroid laboratory studies consistent with a state of nonthyroidal illness syndrome—depressed T3 and T4 levels coupled with a low or normal TSH level and an increased T4 to T3 ratio suggesting decreased peripheral conversion of T4 to T3 (86-89). Resting energy expenditure is decreased in individuals with anorexia nervosa (83) and this decreased resting energy expenditure is associated with low T3 levels (90). With weight gain, T3 levels rise (88, 89, 91, 92) and, importantly, this increase is associated with an increase in resting energy expenditure (90), suggesting that the low T3 levels in anorexia nervosa, a model of chronic starvation, are an adaptive response that results in decreased energy expenditure.

Figure 3.

Neuroendocrine adaptations to chronic negative energy balance in humans. Hormonal adaptations to states of chronic caloric deficit include GH resistance, hypogonadotropic hypogonadism, and a decrease in thyroid hormone (T3 and T4) levels that are likely in part due to decreased T4 and T3 secretion as well as decreased peripheral conversion of T4 to T3. In murine models, FGF21 is a mediator of GH resistance (101) and may be in humans as well (102). In animal models, a decrease in leptin secretion may mediate decreased TRH secretion (115).

Abbreviations: FGF, fibroblast growth factor; TRH, thyrotropin-releasing hormone.

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GH resistance and hypogonadotropic hypogonadism

Two additional neuroendocrine adaptations that help reduce energy expenditure in anorexia nervosa include GH resistance and hypogonadotropic hypogonadism (93). These hormonal adaptations likely decrease overall energy expenditure in states of chronic energy restriction by shunting calories away from processes that are not critical for survival. GH resistance, in states of starvation, allows for the exploitation of the beneficial effects of GH while minimizing energy expenditure on growth. GH is a hormone secreted by the pituitary gland and signals the liver to produce IGF-1. As its name implies, GH is an essential hormone for growth, maintenance of bone density, and muscle mass; these processes are predominantly mediated by IGF-1. Yet GH also has IGF-1-independent lipolytic actions and insulin resistance properties (94). These counterregulatory actions are beneficial or advantageous during periods of caloric deprivation (95). Therefore, a state in which GH levels are normal or elevated but IGF-1 levels are low, to minimize energy expenditure on growth, is adaptive in the short term. This state of GH resistance has been demonstrated with acute fasting (96) and in anorexia nervosa (97-99). Notably, even supraphysiologic recombinant GH supplementation cannot overcome GH resistance in states of chronic starvation (100). What causes GH resistance and therefore the block in GH's ability to signal the liver to produce IGF-1 in states of undernutrition? The exact mechanisms are not known but potential hormonal mediators include FGF-21 (101, 102), low insulin levels (103), low leptin levels (104), and ghrelin (105).

Hypogonadotropic hypogonadism is another key neuroendocrine adaptation to starvation. Hypogonadotropic hypogonadism results in a decrease in expenditure of energy on reproduction and the reproductive axis. Most women with anorexia nervosa, for example, are amenorrheic, the consequence of hypogonadotropic hypogonadism, and males with anorexia nervosa have low testosterone levels compared to normal-weight controls (106). Hypogonadotropic hypogonadism occurs due to disruption of GnRH pulsatility in the hypothalamus, which results in decreased pulsatility and amplitude of LH secretion from the pituitary (107-111). This disruption is likely mediated by a reduction in leptin—a hormone secreted by adipose tissue—the level of which drops during fasting or when adipose tissue stores are reduced. Indeed, treatment with recombinant human leptin normalizes LH pulse frequency and restores ovulatory cycles in women with hypogondotropic hypogonadism due to a state of negative energy balance (112). Similarly, in men, short-term fasting (72 hours) results in a decrease in LH pulsatility and a decrease in testosterone levels; these changes are prevented with treatment with recombinant human leptin during the fast (113). Therefore, both GH resistance and hypogonadotropic hypogonadism are hormonal responses to decreased caloric intake that minimize energy expenditure on processes that are not acutely critical for survival.

Additional hormonal mediatorsLeptin

Soon after its discovery as an adipocyte-derived signal of obesity, leptin was demonstrated to be dynamically repressed during acute fasting and regulate neurohormonal adaptations to fasting in mice (114, 115). Although leptin is secreted by adipose tissue, levels of leptin drop by approximately 50% within 24 hours of a fast and before any significant change in body weight (116). In mice, the drop in leptin levels with fasting has been shown to mediate hypogonadotropic hypogonadism and reduced thyroid hormone (T4) levels (115), thereby reducing energy expenditure. By contrast, treatment of fasted mice with recombinant leptin attenuates the fasting-mediated drop in testosterone in male mice, prevents the estrus delay in female mice, and blunts the reduction in T4 levels (115).

In humans, hypoleptinemia has most clearly been implicated as a mediator of hypogonadotropic hypogonadism. In women in states of negative energy balance with hypogonadotropic hypogonadism from disruption of normal LH pulsatility—so-called functional hypothalamic amenorrhea—treatment with recombinant human leptin improves LH pulsatility and restores ovulatory cycles (112). The improvement in LH pulsatility occurs despite concomitant weight loss, exemplifying that it is not due to an improvement in energy balance (112). Leptin has also been implicated as a mediator of GH resistance in humans, as treatment with recombinant human leptin for 3 months in women with functional hypothalamic amenorrhea and low IGF-1 levels results in a significant rise in circulating IGF-1 (104). Short-term (72 hours) fasting studies in men show leptin to be a possible mediator of decreased TSH secretion (113); therefore, leptin may help mediate the down-regulation of the hypothalamic-pituitary-thyroid axis in humans during fasting as well (117).

In rodent models, the acute drop in leptin with fasting has been implicated in the critical shift from carbohydrate to lipid metabolism, mediated by activation of the hypothalamic-pituitary-adrenal axis (118). We have shown that in normal and overweight humans undergoing a 10-day zero-calorie fast, there is also an inverse association between the drop in leptin levels and the rise in cortisol levels during the early fasting response (116). Therefore, the drop in leptin levels with starvation may be an important hormonal mediator of both the acute response to decreased caloric intake as well as an adaptation to chronic states of starvation.

FGF21

FGF21 is a hormone secreted predominantly by the liver with fasting (76, 85, 119). In mice, FGF21 increases within hours of fasting onset and before ketogenesis (76, 119). Experiments with genetic mouse models demonstrate that FGF21 executes PPARα-dependent functions, including promotion of ketone generation by the liver. FGF21 is also attributed roles in fasting-associated hypogonadism (120) and fasting-mediated torpor (119), an extreme manifestation of the drive to reduce energy expenditure. These findings led to the view of FGF21 as the prototypical fasting hormone and generated excitement in the use of FGF21 or FGF21 analogues to pharmacologically drive beneficial metabolic responses. In humans, however, the fasting-mediated FGF21 surge is not seen until 7 to 10 days into a fast (85, 121)—and well beyond the induction of ketogenesis—suggesting that circulating FGF21 levels may play a role in the late adaptive response to fasting, rather than the acute response. FGF21 may play a role in mediating GH-resistance during starvation in both humans (102) and murine models (101). Interspecies differences in FGF21 response and functionality may explain why FGF21 pharmacological efforts have not replicated the full breadth of beneficial metabolic properties observed in mice, although FGF21 effects on lipid metabolism may be sufficient to improve systemic triglyceride levels and metrics of metabolic dysfunction-associated steatotic liver disease (122-128).

Potential Mechanisms of Benefit From Fasting

Many theories have been proposed to explain why we age, often attributing causality to specific processes known to be deranged with aging. Examples include the mitochondrial theory of aging (129), the DNA damage theory of aging (130), and the inflamm-aging theory (131), which assign causal roles in aging biology to mitochondrial dysfunction and reactive oxygen species, cumulative DNA damage, and low-grade inflammation, respectively. A singular unifying theory of aging may not be realistic, however, as there is substantial overlap in pathophenotypes at the cellular, tissue, and organismal levels between these putative processes. Many of the proposed mechanisms of benefit of fasting involve the intersection between the adaptive mechanisms described in prior sections of this manuscript and putative pathological pathways implicated in aging biology. As such, our discussion of the potential clinically measurable benefits of fasting reflects how the molecular and metabolic adaptations to fasting may plausibly modulate aspects of aging and or diseases of aging.

Weight Reduction

Epidemiological data demonstrate a range of negative consequences associated with overweight or obesity, including increased risk of diabetes (132), heart disease (133), many cancers (134), and dementia (135), among other diseases. Sustained reduction of caloric intake results in reduced adiposity and weight, even though adaptive reduction in energy expenditure partially attenuates the pace of weight loss (136). In overweight or obese individuals who have already developed impaired glucose homeostasis or frank type 2 diabetes, weight loss improves and, in some cases, reverses diabetes (137). Therefore, patients most likely to derive benefit from fasting-mediated weight loss are those with unambiguously elevated body mass index (BMI) (obese). It is important to recognize, however, that the use of BMI to define overweight or obese thresholds is crude. Individuals with low muscle mass may have relatively high degrees of adiposity and still fall within the normal BMI range. In addition, genetic differences in body frame influence the relationship between BMI and adiposity. Individuals of Asian descent, for example, may have unhealthy adiposity at the upper limit of “normal” BMI because of a genetic predilection for smaller frame size (138). A more nuanced view of body weight, incorporating ethnicity and clinical observations of adiposity may identify individuals who would benefit from fasting-mediated weight loss even if their BMI measurement does not place them in a high-risk category. Importantly, however, the weight loss associated with fasting and caloric restriction protocols may represent a direct benefit, but also a confounder, as it is difficult to definitively determine the degree to which observed benefits are mediated by changes in weight. Understanding the positive as well as potential negative effects of fasting independent of weight loss is currently an active area of investigation (139, 140).

Insulin Resistance and Glucose Metabolism

Insulin signaling is a critical regulator not just in response to the postprandial carbohydrate surge, but also in the fasted state, where the absence of insulin signaling is a precondition for aspects of the adaptive fasting response as described in this manuscript. A prolonged fast can lead to acute peripheral insulin resistance (55), which may be an adaptive response to limit utilization of glucose by tissues capable of shifting to fatty acid metabolism to support energetic needs. In contrast to this acute response, however, studies in rodents and nonhuman primates demonstrate long-term improvements in glucose homeostasis with caloric restriction or intermittent fasting (141-143). A range of possible cellular mechanisms may account for improved glucose homeostasis with fasting or caloric restriction, including improved β-cell survival or function (144, 145), improved skeletal muscle GLUT4 cell surface occupancy and responsiveness to insulin (55), and augmented mammalian target of rapamycin (mTor) signaling in adipocytes (146). However, improved insulin resistance with caloric restriction or fasting is undoubtedly in part due to reduction in weight, a potential mechanistic confounder that is difficult to fully account for.

Reprogramming of Cellular Metabolism

Energy balance directly impacts cellular metabolism as the requisite shift to lipid metabolism that accompanies caloric restriction or fasting involves activation of catabolic pathways in mitochondria and peroxisomes. Mitochondria catabolize fatty acids to produce acetyl CoA to supply the TCA cycle and produce substrate for ketogenesis. Peroxisomes catabolize long- and very-long-chain fatty acids. To accommodate the shift to fatty acid oxidation, achievement of negative energy balance may promote mitochondrial fission, mitochondrial biogenesis, and improve overall mitochondrial function in part through nutrient-sensitive PGC1α dependent transcriptional programs (147-149). Fasting may also improve coordination of mitochondrial and peroxisome metabolic processes (150). Importantly, augmentation of mitochondrial biogenesis has not been uniformly observed across studies and therefore the degree to which such adaptive mechanisms drive protective effects of dietary restriction are not fully known.

Cellular respiration results in the production of reactive oxygen species (ROS) as an obligate byproduct. The overall reduction in energy expenditure that occurs with fasting reduces energy production and therefore may also reduce cumulative exposure to ROS. Fasting may also directly suppress mitochondrial ROS via induction of transcriptional regulators of the antioxidant response, including Nuclear factor erythroid 2-related factor 2, and downstream antioxidant enzymes including heme oxygenase and glutathione peroxidase 4 (151, 152). Such mechanisms converge to reduce ROS generation as suggested by studies of caloric restriction in aging mice and the demonstration of reduced oxidative damage in the heart of rats subjected to intermittent fasting (153, 154).

One potential mechanism by which metabolic reprogramming effects could be sustained is through remodeling of the epigenome through chemical modifications to either DNA or DNA-associated histones. Indeed, links between aging, exposures that accelerate age-related pathobiology (eg, tobacco use), and epigenomic remodeling provide the conceptual basis for the development of so-called DNA methyl clocks as novel readouts for biological age (155-158). Importantly, some of the same metabolic pathways that provide substrate for epigenetic chemical modifications are also sensitive to energy balance and fasting, including methionine metabolism and the supply of DNA methyl donors, catabolic production of acetyl-CoA from glucose and lipids to supply substrate for histone acetylation, and ketogenesis with ketones such as β-hydroxybutyrate exhibiting direct inhibitory properties on the action of histone deacetylases, while also providing substrate for β-hydroxybutyrylation of histone lysines (159, 160). There are multiple potential points of intersection between fasting-sensitive metabolic processes and epigenomic remodeling that in theory could restore a more youthful epigenomic landscape, with the caveat that genome-scale changes to transcriptional regulation may not be uniformly beneficial. Although fasting mediated changes to the epigenomic landscape provide a potential mechanism for sustained metabolic reprogramming, the exact nature and temporal dynamics of epigenetic remodeling with fasting and refeeding—ie, the durability of any putative reprogramming effect, remains incompletely defined.

Inflammation

Low-grade sterile inflammation is associated with aging and in turn inflammation is a potential driver of aging biology and diseases of aging, including diabetes and cardiovascular disease (131, 161). Chronic caloric restriction in nonhuman primates is associated with reduction in inflammatory biomarkers and intermittent fasting or caloric restriction in rodents reduces inflammatory biomarkers and inflammatory transcriptional programs (153, 162, 163). Humans who engage in religious fasting also demonstrate a reduction in inflammatory biomarkers (164).

Attenuation of leptin and leptin signaling as occurs with acute fasting may directly regulate immunity. Although obesity is generally a pro-inflammatory state, the obesity associated with genetic loss of leptin function is paradoxically associated with immunosuppression and increased risk of severe infection (165). At the other end of the weight spectrum, the leptin deficiency associated with failure of fat development or acquired loss of adipocytes in lipodystrophic syndromes is also associated with increased risk of severe infection (166). The role of leptin as an immunomodulator is further supported by the expression‎ of the leptin receptor on immune cells and reductionist studies demonstrating that signaling through the leptin receptor in many of these is functionally active (167-169). Collectively, these examples point to a potential intrinsic anti-inflammatory or immunomodulatory effect of caloric restriction or fasting and offer one plausible contributing mechanism of benefit for dietary restriction.

mTor signaling is another example of a nutrient-sensitive pathway that regulates the immune system, including through immune cell proliferation and differentiation, cytokine production, antigen presentation, and immune cell trafficking, which are attributes that form the basis for the use of the mTor inhibitor rapamycin as an immunosuppressant medication (170). It is therefore tempting to speculate that the life-prolonging effect of targeting mTor with rapamycin, as observed in model organisms (171), is attributable at least in part to a reduction in inflammation. Indeed, while targeting mTor signaling may reduce some aspects of sterile inflammation and “inflammaging” in model organisms (172), contradictory evidence suggests that benefits of mTor targeting are uncoupled from the innate inflammatory response (173). Moreover, rapamycin in low doses may paradoxically promote and rejuvenate immune responsiveness (174, 175). The collective data on mTor signaling illustrates how signaling pathways activated by fasting or caloric restriction play complex regulatory roles in immunity and inflammatory responses, while also underscoring the reality of the competing adaptive and maladaptive roles for the immune system in survival. It is also critical to recognize that fasting may have distinct effects on different immune cell types, modulating the immune cell composition of circulating or tissue reservoirs and modulating immune cell phenotypes.

Cancer

Intermittent fasting and caloric restriction attenuate cancer development in preclinical models of diverse cancer types—including melanoma, breast, bladder, colon, and pancreatic cancer—and many of the metabolic and hormonal adaptions involved in the fasting response are relevant to cancer biology (176-180). Fasting mediated changes in hormonal signaling, such as the insulin pathway and the GH/IGF1 axis, and/or fasting-mediated shifts in the metabolic milieu may account for attenuation of malignant transformation and/or cancer progression.

A reduction in calories results in a cumulative decrease in insulin release if carbohydrates are proportionally reduced. Aside from regulating glucose homeostasis, insulin signaling through the PI3K signaling pathway promotes growth (181). The mitogenic effects of insulin-PI3K signaling are well documented in cancer biology, providing the conceptual basis for anticancer drugs targeting the pathway. Therefore, lower cumulative exposure to insulin and in turn insulin stimulated PI3K/AKT signaling over long periods may reduce cancer initiation and/or progression.

IGF-1 can also bind to the insulin receptor, albeit with reduced affinity, and therefore may replicate some aspects of insulin action; however, the more important pro-growth properties are attributable to signaling through the IGF-specific receptor. IGFR1 is a tyrosine kinase receptor that when engaged by IGF-1—or insulin with lesser affinity—leads to signaling cascades that are mitogenic (182, 183). The pro-growth effects of GH are primarily mediated through the actions of IGF-1. Therefore, the GH resistance that is characteristic of the fasting response, which results in reduced circulating IGF-1, may in turn reduce cumulative mitogenic exposure. The potential role of decreased IGF-1 signaling as a mediator of benefit from fasting or caloric restriction is conceptually supported by increased longevity in mice in which the GH/IGF-1 axis is genetically targeted via disruption of the GH receptor and the observation that reduced IGF-1 expression‎ is associated with longevity both within and across diverse species (184, 185).

Last, fasting metabolites themselves may play a role in cancer biology. The most notable example may be ketones, which may inhibit cancer cell growth (186). Such observations underpin a current interest in dietary interventions that augment ketone production as an adjunct intervention for cancer therapeutics (187). Even though fasting leads to ketogenesis within approximately 1 day, diets that are ketogenic simply through manipulation of macronutrient composition may be a more efficacious approach to achieve sustained elevation in ketone bodies.

Lipid Metabolism

Circulating bioactive lipids can drive an array of pathophenotypes linked to diseases of aging including atherosclerosis and insulin resistance and therefore any potential reprogramming of lipid metabolism with fasting could be disease modifying. Acute fasting does not have a consistent effect on traditional components of a clinical lipid panel, but when granular metabolomics methods are applied, dramatic changes in triglyceride composition are evident. The evolution of triglycerides with a 10-day fast in humans involves a shift in favor of high carbon content, unsaturated triglycerides and a reduction in low carbon content, saturated triglycerides (116). Importantly, this shift is already evident in the first day of fasting, persists in the day after refeeding, and is a pattern that in epidemiological studies predicts protection from diabetes and age-related frailty (188-193). Transcriptomics data from fasting and refed mouse liver also suggest the possibility that fasting reprograms lipid metabolism, as the upregulation of some genes involved in lipid metabolism persist after refeeding (194). Such data raise the question of whether fasting can reprogram lipid metabolism such that the benefits persist beyond the period of actual fasting.

Catabolic Dissolution of Pathological Biomass

We introduce the term “pathological biomass” to describe the cellular or extracellular molecules that promote disease or aging pathology either because their accumulation in excess drives tissue dysfunction or because the molecules themselves, particularly when damaged, have pathological signaling properties. The term “biomass” is more commonly applied in cancer biology to describe how tumor material expands because of anabolic growth and cellular proliferation (195). Outside of cancer, in states where growth is not unrestrained to the point of tissue invasion and metastasis, less extreme imbalances in the turnover of biomolecules can also lead to pathological accumulation. Certain biomolecular classes turn over at lower rates with aging, including, for example, neutral lipid in adipocytes and specific proteins in the brain (196, 197). In addition, subsets of proteins in a variety of tissues have been shown to exhibit extremely long half-lives, which may therefore render them sensitive to cumulative damage even without an age-dependent change in turnover rates (198-201). Similarly, exposure of lipids to oxidative stress drives formation of oxidized species including lipid peroxides, which may accumulate in some tissues with aging and have been associated with diseases of aging, in part from damaging effects to cells and proinflammatory properties (202-204). Several diseases of aging are linked to accumulation of specific pathological protein aggregates, such as Alzheimer’s disease, where extracellular amyloid plaques and intracellular tau are disease drivers, or senile amyloidosis, where polymerization of transthyretin proteins causes tissue dysfunction in the heart and nervous system among other tissues (197, 205, 206).

Because fasting is an inherently catabolic state, a theoretical mechanism of benefit of fasting is through the catabolic dissolution of pathological biomass. We have advanced the concept that fasting drives a transition from glucose to lipid metabolism and therefore fasting that is of sufficient duration to activate lipid catabolic programs will lead to increased lipid turnover. Indeed, there is precedent for a prolonged acute fast being sufficient to lower circulating biomarkers of lipid peroxidation (207). We have also acknowledged that the shift to lipid metabolism is a survival mechanism to prevent catabolic consumption of the protein backbone of the body. However, even with the reduction in glucose utilization coincident with the adaptive transition to lipid metabolism, there is ongoing gluconeogenesis to support minimal glucose requirements, including those of the pool of red blood cells, for which glucose is the obligate energy source. As such, catabolic breakdown of macromolecules during fasting extends beyond glycogen and lipids to include proteolytic release and utilization of amino acids.

Catabolism of proteins and other biomolecules can proceed via multiple mechanisms. Analogous to the canonical lipolysis pathway, specialized enzymes contained in proteosomes can catabolize intracellular proteins that have been targeted for degradation by ubiquitination (208). Autophagy is a process that can be activated by fasting in which cells consume intracellular components up to entire organelles (209, 210). When mitochondria are damaged, for example, the process of mitophagy entails encompassing of entire mitochondrion by a membrane formed from the endoplasmic reticulum into a vesicle called an autophagosome, which is in turn trafficked to lysosomes, specialized organelles that are acidified and contain degradative enzymes that operate at low pH and serve as the ultimate catabolic effectors (211). Extracellular biomass can be subject to enzymatic degradation, for example by the action of metalloproteinases on extracellular matrix; however, more complex and higher volume states of localized biomass may require the action of professional phagocytic cells. In the case of atherosclerotic vasculopathies, for example, which can be viewed as states of pathological biomass because of the obstruction of blood flow from cholesterol accumulation and the cells and debris that comprise an advanced atherosclerotic lesion, it is increasingly recognized that the failure of professional phagocytic cells to consume debris and dead/dying cells in atherosclerotic plaques is a pathological driver (212) (213). Fasting has been linked to many of the catabolic pathways that could be operative in the dissolution of pathological biomass including autophagy, proteosome activity, and activation of professional phagocytes (44, 214, 215). In mice, the potential benefits of activating catabolic pathways, such as autophagy, are demonstrated by enhanced lifespan when the aging decline in autophagy is rescued by transgenic overexpression‎ of an autophagy program (216). While augmented turnover of biomolecules that are dysfunctional and/or present in excess is a theoretical benefit of fasting, 1 caveat is that catabolic mechanisms may not preferentially target biomolecules that are dysfunctional or present in excess.

Potential Mechanisms of Harm From Fasting

A long-established evidence base suggests life-prolonging benefits of caloric restriction in model organisms, which have been extended to intermittent fasting and time-restricted feeding protocols (217, 218). Yet, the narrow and controlled conditions in which model organisms are studied undoubtedly do not recapitulate the full breadth of stressors that impact human health. Murine studies have historically been biased toward studies in male mice and with much of the evidence base established in a narrow set of inbred mouse strains. In 1 study in which caloric restriction was eval‎uated across 41 mouse strains, only 5% and 21% of strains in males and females, respectively, exhibited increased lifespan in response to caloric restriction (219). Even if salutary benefits of fasting are conserved in humans, however, it is possible that negative consequences of fasting may be more relevant to free-living human populations (ie, individuals who are exposed to the external stressors of daily life). Epidemiological studies demonstrate a J-shaped curve with respect to the relationship between BMI and mortality, as mortality increases when BMI is low (220, 221). It is also well known that patients with anorexia nervosa have a high mortality rate, that is independent of associated psychiatric disease and suicide (222). These data raise the question of whether achievement of a low BMI by caloric restriction or fasting would be harmful, providing added rationale to consider possible mechanisms of harm.

Inflammation

We discussed improvement in low-grade systemic inflammation as a potential benefit of caloric restriction or intermittent fasting. Surprisingly, when we performed longitudinal transcriptomics analysis of human adipose tissue with prolonged fasting, we discovered an increase in transcripts associated with macrophages and inflammation (44). This translated into a corresponding increase in histological evidence of macrophages and an increase in the circulating inflammatory biomarker, C-reactive protein. The fasting-mediated C-reactive protein surge with prolonged fasting has been confirmed in another human study (223). When the contraction of fat mass and weight loss is protracted over several weeks in a model of chronic caloric restriction in obese mice, an influx of macrophages is detectable (224). Similarly, intermittent fasting in obese mice can drive accumulation of lipid-associated macrophages that may contribute to metabolic inflammation and pathological adipose tissue phenotypes (225). However, even in instances when fasting stimulates inflammatory pathways or inflammation, it may not result in improved immune function. In mice, acute fasting for 24 hours followed by refeeding increases trafficking of old monocytes from the bone marrow to peripheral tissues, such as the lung, and augments pro-inflammatory cytokine responses, but also reduces host defense to clinically relevant bacterial infections of the lung (226). The extreme catabolic state associated with more prolonged fasting periods may partially account for the observed proinflammatory effects; however, these data suggest that fasting and/or caloric restriction may promote potentially maladaptive inflammation in some contexts.

Loss of Bone Mineral Density

In individuals with anorexia nervosa, a model of chronic starvation, the most common medical complication is low bone mineral density (227). Approximately 85% of women with anorexia nervosa have a bone mineral density value at least 1 SD below the mean of women of similar age and nearly 35% have a bone mineral density value that is at least 2.5 SDs below the mean—the definition of osteoporosis in postmenopausal women (227). Males with anorexia nervosa also have low bone mineral density (228, 229) and, as is seen in other populations with low bone mineral density, individuals with anorexia nervosa also experience a higher rate of fractures (230-234). Similarly, in a randomized controlled trial of 24 months of calorie restriction (CALERIE), significant decreases in bone mineral density at the spine and hip were observed in the calorie restriction group (mean weight loss of 7.5 ± 0.4 kg) compared to the control (ad libitum feeding) group (mean weight change of +0.1 ± 0.5 kg) after both 12 and 24 months of calorie restriction (235). In fasting individuals, bone formation markers decrease within the first 4 days of fasting initiation (236, 237). With recent advances in bone imaging, even short-term fasts result in changes in bone microarchitecture. Trabecular bone parameters including trabecular number and bone volume fraction in the radius were shown to decrease significantly after a 10-day zero-calorie fast (237). Potential mechanisms for this loss of bone mineral density include changes in methionine levels with fasting and hormonal adaptations to fasting including increases in the counter-regulatory hormone cortisol and decreases in IGF-1 levels due to GH resistance (see section Growth Hormone Resistance and Hypogonadotropic Hypogonadism), which have negative effects on bone mass (93).

Translational and Human Fasting Studies With Clinical EndpointsData From Model Organisms

In model organisms, the lifespan prolonging effects associated with sustained caloric restriction have also been observed with dietary restriction protocols that enforce periods of zero-calorie fasting (141, 217, 238, 239). In addition, periodic fasting protocols in mice have also improved pathological metrics in experimental models of diseases of aging, such as Alzheimer’s disease (240, 241). The timescale of the adaptive fasting response between rodent models and humans challenges easy translation of results from rodent studies to humans. This includes the different timescales of the adaptive fasting response, such that the fasting period in murine “time-restricted feeding” studies (eg, often 18-21 hours) is effectively a much more profound fasting period than a similar fasting duration in humans. The much longer human lifespan sets practical limits on testing the effect of dietary restriction protocols on longevity. Chronic diseases of aging often also transpire over much longer time periods and therefore require long, large-scale studies to assess efficacy of dietary interventions for disease prevention. Consequently, the evidence base supporting fasting interventions to promote metabolic health in humans is limited.

Human Studies

Studies of human populations that engage in fasting for religious or other reasons demonstrate intriguing signals of potential benefit (242, 243). However, the potential benefits of fasting have only recently begun to be explored in a prospective fashion in humans. We undertook a survey of the existing literature and eval‎uated studies identified via a PubMed search using the following criteria: randomized, clinical trials published in English that included intermittent fasting, time-restricted feeding, time-restricted eating, or alternate-day fasting in the title or abstract through October 14, 2024. We included studies with a control group and did not include studies that were focused on a specific disease population other than metabolic syndrome, fatty liver disease, or diabetes. Therefore, we excluded studies focused on outcomes in patients with rheumatoid arthritis or chronic kidney disease. We also only included studies that were greater than 2 weeks in duration. Our list included 79 publications, with the earliest study being published in 2011 (244).

The review of the literature demonstrates that the studies are predominantly short term. All but 5 studies ranged in length from 3 weeks to 6 months with the remaining studies being 1-year studies. There was great variance and overlap in definitions of time restricted eating and intermittent fasting. With respect to time restricted eating manuscripts (n = 38), hours of eating ranged from 4 to 12 hours daily with the majority (n = 25) indicating 8-hour eating windows. Similarly, intermittent fasting definitions varied greatly with respect to number of calories that could be consumed during a fasting day and few studies prescribed zero calories during fasting periods. Additionally, 1 study looked at the combination of intermittent fasting with time-restricted eating on metabolic outcomes in individuals at risk for type 2 diabetes mellitus (245), whereas others combined intermittent or alternate-day fasting with exercise, with the majority of these short-term studies showing no added benefit of exercise with respect to cardiometabolic endpoints (246-248).

As changes in dietary composition can also have potential cardiometabolic effects, we looked at whether dietary composition was altered or controlled in any of the reported studies. Nearly half of the 79 studies did not require participants to follow a specific macronutrient composition, although 9 of these studies did provide participants with dietary or nutritional counseling. Eleven additional studies made specific recommendations regarding macronutrient content, with the majority recommending balanced macronutrient intake by either following a Mediterranean diet or a diet of similar macronutrient composition (∼25%-30% fat, ∼50%-55% carbohydrates, and ∼15%-20% protein). An additional 22 studies provided meals to the participants for at least some portion of the study, with the majority providing meals with a balanced macronutrient breakdown. Therefore, while some studies prescribed changes to dietary macronutrient content as an additional and potential confounding factor to any fasting effect on cardiometabolic outcomes, nearly half of the studies did not have participants alter their dietary composition.

In the majority of manuscripts (n = 69), mean or median BMI of the participants randomized to time restricted feeding or intermittent fasting was >25 kg/m2. Overall, studies of intermittent fasting demonstrated metabolic benefits associated with weight loss (Fig. 4). Whether fasting has metabolic benefit independent of weight loss has not been well studied. A 3-week study of lean (BMI < 24.9 kg/m2) individuals investigated the effects of alternate day fasting with compensatory increases in caloric intake on nonfasting days as compared to 2 other groups: (1) daily caloric restriction and (2) alternate-day fasting with energy intake matched to the caloric restriction group (139). The calorically neutral alternate day fasting group lost significantly less body mass than the calorically decreased alternate-day fasting group (−0.52 kg vs −1.60 kg; P = .04). Despite these differences in weight loss, there were no significant differences in homeostatic model assessment of insulin resistance, total cholesterol, low-density lipoprotein (LDL), or high-density lipoprotein (HDL) between the groups and levels did not significantly change compared to baseline in either of these alternate-day fasting groups (139). Importantly though, this study was of short duration (3 weeks in length), which may in part explain these findings. Collectively, prospective studies of fasting interventions in humans demonstrate signals of metabolic benefit; however, associated weight loss may be an important mediator. Understanding the potential effects of fasting independent of weight loss remains an active area of investigation (140).

Figure 4.

Weight loss and cardiometabolic metrics in human trials of intermittent fasting or time-restricted eating. (A) Time-restricted eating (TRE) and intermittent fasting (IF) studies are plotted as a function of the number of participants in the intervention arm (Study “N”), the mean age, and the mean body mass index (BMI). Dotted lines indicate overweight (BMI of 25 kg/m2) and obese (BMI of 30 kg/m2) thresholds, underscoring that the majority of studies have been conducted in participants with excess body weight. Studies conducted in normal weight participants were generally in young, healthy populations, often with sports physiology outcomes rather than cardiometabolic health outcomes. (B) Decrease in absolute weight (kg) is associated with positive changes in insulin resistance (homeostatic model assessment of insulin resistance [HOMA-IR]), triglycerides (mg/dL), and LDL (mg/dL) across studies; change in HDL (mg/dL) was not significantly associated with change in absolute weight (kg) across studies. Mean changes are plotted on both axes. (C) Decrease in weight (% change from baseline) is associated with positive changes in insulin resistance (HOMA-IR), triglycerides (mg/dL), and LDL (mg/dL) across studies; change in HDL (mg/dL) was not significantly associated with percent change in weight across studies. Mean changes are plotted on both axes.

Abbreviations: HDL, high-density lipoprotein; LDL, low-density lipoprotein.

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Conclusions, Unanswered Questions, and Future Directions

In this manuscript, we have focused on adaptive mechanisms to fasting and their potential relevance to health and disease (Fig. 5). We conclude that there are clear signals of potential health benefits to fasting that are strongest in the context of overweight and obesity, where fasting protocols may promote weight loss and mitigate associated risks of cardiometabolic disease, reproductive dysfunction, and cancer. There may be other patient populations that also derive more pronounced health benefits from fasting interventions, for example, those with autoimmune diseases such as rheumatoid arthritis where fasting has been shown to have at least short-term benefits because of a putative anti-inflammatory effect (249). However, we also acknowledge that there are potential harms from fasting, most notably through effects on bone metabolism. Therefore, it is likely that there are at least some contexts in which the potential harm from fasting exceeds the benefits, for example individuals who are prone to osteoporosis including low-weight women. Despite more than a century of intense scientific study of the biology of fasting, however, much remains unknown. In the ensuing final paragraphs, we contextualize what we view as key unanswered questions.

Figure 5.

Molecular reprogramming and candidate mechanisms mediating benefits and potential harms of fasting. (A) There are clear signals of potential health benefits from fasting including weight loss, improvement in cardiometabolic metrics, and prevention of malignancy amongst others. The degree to which such benefits of fasting are potentially offset by harm to bone is not known. (B) Multi-omics readouts demonstrate large-scale changes to the microbiome, epigenome, transcriptome, proteome, and metabolome, with functional implications at the cellular, tissue, and systemic levels, including key parameters listed at the perimeter of this wheel and discussed in this manuscript.

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Can Unbiased Multi-omics Analyses Redefine the Stages of Starvation?

It is impossible in such a review to ignore the centrality of canonical hormonal systems—eg, insulin and glucagon signaling—which have a large evidence base to support their involvement across multiple cell types and tissues. However, when unbiased-omics methods are applied to fasting humans or model organisms, the sheer number of bioactive molecules that are dynamically modulated in circulation or at the tissue level with fasting is substantial. In fasting rats, greater than one third of measured serum metabolites and approximately 5% to 10% of transcripts were statistically modulated in metabolically relevant tissues (liver, muscle, intestine) with fasting (250). We conducted a longitudinal analysis of humans subjected to a 10-day zero calorie fast, with repeated sampling of blood and subcutaneous fat for metabolomics and transcriptomics. Of 544 measured plasma metabolites, 65% changed in a statistically meaningful way (116), whereas 20% of the nearly 25 000 detectable adipose tissue transcripts were significantly modulated (44). The dramatic transcriptional response to fasting is also reflected by the circulating proteome, with a recent study demonstrating significant modulation of approximately one third of ∼3000 measured plasma proteins over a 7-day fast (251). Prolonged fasting may also remodel the microbiome, changing the composition of microbial populations to which immune cells are exposed. Beyond local effects attributable to the direct interfacing of the microbiome with immune cells, increased microbial diversity is generally viewed as healthy and accounts for some of the observed improvements in systemic metabolic function with fasting protocols (252-256). Additional operative mechanisms of benefit may include modulation of immune cell trafficking and levels of bioactive microbial metabolites in systemic circulation, including short-chained fatty acids (253). These examples demonstrate that there remains much yet to be learned about the bioactivity of many of these molecules in fasting, not to mention how these responses are modulated by disease states. Indeed, other studies involving different -omics time-series analyses challenge the concept of discrete and sequential phases of adaptive fasting, with evidence of transcriptional responses associated with putative late adaptive stages (eg, lipid catabolism) already evident at early timepoints (257, 258), also in line with evidence in mice that nervous system control of adipocyte lipolysis is triggered at an early fasting stage by depletion of liver glycogen (259). This underscores that the current view of adaptive fasting is undoubtedly oversimplified. It is likely that increasingly granular multi-omics time-series analyses integrating all aspects of the regulatory hierarchy including epigenomic, transcriptomic, proteomic, with metabolite fluxes will not only reveal important new regulatory nodes, but also demonstrate that adaptive responses operate at multiple different time scales. The application of single cell methods in human studies will also be critical, in particular to define the nuanced effects of fasting on the immune system, as it is clear from studies in model organisms that fasting not only affects the cellular phenotypes of immune cells in distinct ways (eg, promoting M1 to M2 phenotypic switching of macrophages), but may also modulate cell trafficking and in turn the cellular composition of both circulating and tissue immune cell reservoirs (226, 260, 261). Multi-omics analyses may also reveal the temporal dynamics and durability of putative molecular reprogramming effects after completion of fasting.

How Does Aging Modify the Fasting Response?

Intermittent fasting and caloric restriction are considered potential antidotes to aging, but advancing age may itself modulate the response to these interventions. Epidemiological studies demonstrate that weight loss in the elderly often precedes a functional decline and death. Such associations likely implicate some degree of reverse causation-- that the weight loss is a biomarker of illness rather than a causal mediator. Yet, this signal may not be fully accounted for by occult illness. Indeed, recent murine studies in which caloric restriction or intermittent fasting are initiated in older animals in some instances appears to accelerate death (262). It will be important to definitively determine if the weight loss that often accompanies initiation of caloric restriction or intermittent fasting is harmful in the elderly, and if so, what are the underlying mechanisms.

Is There an Optimal Fasting Protocol With Respect to Duration and Frequency?

As described at the outset of this review, intermittent fasting is not well-defined and encompasses protocols with varying durations, frequencies, and degrees of fasting. The duration of each fasting dose may be critical because the adaptive phases to fasting occur at different time scales. If the transition to lipid metabolism is beneficial, for example, then a day or longer of fasting may be required to maximize benefit. It will also be important to determine the optimal frequency of fasting to achieve and maintain any beneficial reprogramming effect. Although it is not unusual for studies to include at least 1 analytical time-point after some period of refeeding, the duration of any putative molecular or metabolic reprogramming effects due to fasting have not been defined to the granular degree applied to the response to fasting itself. Last, it is conceivable that individual factors such as age, sex, and the degree of baseline adiposity may influence the temporal dynamics of both the fasting response and durability of any reprogramming effect, an understanding of which could inform “precision” fasting prescriptions.

Are There Benefits of Fasting Independent of Weight Loss?

Intermittent fasting (and caloric restriction) are both associated with weight loss, particularly in human studies conducted to date, which have largely been in overweight and obese individuals (see section Human Studies). The potential confounding effects of alleviation of excess adiposity may also be relevant to studies in model organisms. For example, fasting or caloric restriction studies are often conducted in obesogenic mouse strains that may exhibit pathological adiposity even on a standard laboratory “control” diet. It will be important to determine the degree to which benefits of fasting can be extended to normal weight individuals and the degree to which any observed benefits are independent of weight loss.

Are Benefits of Caloric Restriction or Fasting Fully Conserved in Humans?

Definitively testing whether caloric restriction or intermittent fasting extends life in humans is challenging due to the length of the human lifespan relative to model organisms, not to mention the notorious difficulty achieving long-term compliance with any experimental dietary intervention. This reality underscores a more general unmet need in the aging field for more sophisticated aging biomarkers. Major efforts to define and map senescent cells with more sophisticated methods and additional discovery and validation of novel circulating biomarkers such as epigenetic DNA “clocks” could greatly accelerate the testing of dietary—or pharmacological—interventions to promote lifespan.

Are There Sex Differences in the Fasting Response?

Given the differences in body composition and hormonal composition between males and females, it is not surprising that there are differences observed in metabolic responses to various processes including fasting (263). Differences between males and females have been studied predominantly and most rigorously in response to fasting for 72 hours or less. Consistently, glucose levels are significantly lower in females compared to males and plasma free fatty acid levels are higher in response to these shorter term, prolonged fasts (263, 264). These differences in the fasting response may be due in part to estradiol, given estradiol's effects on insulin action. For example, when postmenopausal women with baseline serum estradiol levels of <15 pg/mL are administered intravenous conjugated estrogens, there appears to be improved insulin action in in the setting of a hyperinsulinemic-euglycemic clamp (265). Given differences in response to short-term fasting and hormonal differences, it will be important for future studies investigating the fasting response to adequately delineate outcomes in males vs females and also to differentiate between premenopausal and postmenopausal females.

Can the Beneficial Effects of Fasting be Recapitulated by Pharmacologically Targeting Key Regulatory Nodes?

Even if some version of the dietary interventions described in this article are proven to have robust health benefits in humans, broad implementation may not be practical. In model organisms, caloric restriction or intermittent fasting protocols associated with extension of lifespan are typically implemented early in life. In humans, there is generally poor long-term compliance with dietary prescriptions, even those with proven benefit. Investigation of the cellular and molecular mechanisms underpinning the salutary benefits of fasting could in theory provide a pharmacological target. Canonical signaling pathways that are modulated by fasting are also the targets of drugs that are already in clinical practice. AMP kinase is induced by fasting, is 1 of the pathways targeted by the diabetes drug metformin, and therefore metformin is being repurposed as a potential antiaging therapy (266). mTor is another nutrient sensing regulatory node. Rapamycin increases life and health span in model organisms, which explains ongoing interest in developing the drug for treatment of aging and related diseases (171). The ideal fasting mimetic would activate beneficial fasting pathways in key tissues without recapitulating negative consequences of fasting or producing off target effects, underscoring the importance of incorporating careful consideration of drug toxicity into implementation of mimetic strategies. In the case of rapamycin, for example, potentially harmful metabolic (insulin resistance) and immunological effects may be at least partially mitigated by using lower doses than commonly used for immunosuppression of transplant patients (267). Ultimately, it may be through such repurposing of existing drugs or development of new fasting mimetics that the promise of the health benefits of fasting are realized beyond the rare individuals who have the ability to implement challenging, life-long dietary prescriptions.

How Can the Potential Life- and Health-span Benefits of Fasting or Fasting Mimetics be Assessed in Human Trials?

The collective clinical trials of fasting interventions to date have generally been modest in size, of duration ranging from weeks to months, and focused on easily quantified cardiometabolic endpoints that tend to be sensitive to weight loss. Because longevity is not a practical endpoint for human trials, short-term randomized studies have looked at potential biomarkers associated with aging (268). An important question, however, is what endpoints would be relevant to assess efficacy of interventions to improve healthspan or augment lifespan in normal weight populations or populations with low cardiometabolic risk? Recognizing that a single biomarker may not capture the aging process, an expert group advanced a multimarker panel inclusive of inflammatory, metabolic, and tissue specific markers of aging phenotypes (269). There is also great interest in validating epigenetic clocks, which quantify patterns of epigenetic modifications purported to indicate biological age, thereby identifying individuals that are aging faster or slower than predicted by chronological age (155). Such ongoing efforts to validate a consensus biomarker predictive of healthspan or lifespan is a critical barrier to advancing antiaging interventions, including fasting or fasting mimetics (270-272).

Funding

This work is supported by the National Institutes of Health (NIH)/NIDDK grant numbers R01 DK133578 and R01 DK137913.

Disclosures

P.K.F. is a consultant for Regeneron. She has also served on advisory boards for Camurus, Crintetics, and Chiesi. She receives research support from Quest, Corcept, and Crinetics.

References