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• Review Article
• Published: 18 November 2020

Dietary fibre in gastrointestinal health and disease

• Samantha K. Gill, 
• Megan Rossi, 
• Balazs Bajka & 
• Kevin Whelan 

Nature Reviews Gastroenterology & Hepatology volume 18, pages101–116 (2021)Cite this article

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Abstract

Epidemiological studies have consistently demonstrated the benefits of dietary fibre on gastrointestinal health through consumption of unrefined whole foods, such as wholegrains, legumes, vegetables and fruits. Mechanistic studies and clinical trials on isolated and extracted fibres have demonstrated promising regulatory effects on the gut (for example, digestion and absorption, transit time, stool formation) and microbial effects (changes in gut microbiota composition and fermentation metabolites) that have important implications for gastrointestinal disorders.

In this Review, we detail the major physicochemical properties and functional characteristics of dietary fibres, the importance of dietary fibres and current evidence for their use in the management of gastrointestinal disorders. It is now well-established that the physicochemical properties of different dietary fibres (such as solubility, viscosity and fermentability) vary greatly depending on their origin and processing and are important determinants of their functional characteristics and clinical utility. Although progress in understanding these relationships has uncovered potential therapeutic opportunities for dietary fibres, many clinical questions remain unanswered such as clarity on the optimal dose, type and source of fibre required in both the management of clinical symptoms and the prevention of gastrointestinal disorders. The use of novel fibres and/or the co-administration of fibres is an additional therapeutic approach yet to be extensively investigated.

요약

역학 연구에 따르면, 

통곡물, 콩류, 채소, 과일과 같은 정제되지 않은 통곡물을 섭취할 때 

식이섬유가 위장 건강에 유익하다는 사실이 지속적으로 입증되고 있습니다. 

분리 및 추출된 섬유에 대한 메커니즘 연구와 임상 시험을 통해, 

장(예: 소화 및 흡수, 통과 시간, 대변 형성)과 

미생물(장내 미생물 구성 및 발효 대사 산물의 변화)에 대한 유망한 조절 효과가 입증되었으며, 

이는 위장 장애에 중요한 영향을 미칩니다. 

이 리뷰에서는 

식이섬유의 주요 물리화학적 특성과 기능적 특성, 

식이섬유의 중요성, 

그리고 위장 장애 관리에 식이섬유를 사용하는 것에 대한 현재의 증거를 자세히 설명합니다. 

이제 다양한 식이섬유의 물리화학적 특성(예: 용해도, 점도, 발효성)이 

원산지와 가공 방법에 따라 크게 달라지며, 

식이섬유의 기능적 특성과 임상적 유용성을 결정하는 

중요한 요소라는 사실이 잘 알려져 있습니다. 

이러한 관계에 대한 이해가 진전되면서 

식이섬유에 대한 잠재적인 치료 기회가 밝혀졌지만, 

임상 증상 관리와 위장 장애 예방에 필요한 

최적의 섬유 섭취량, 종류, 공급원에 대한 명확성과 같은 

많은 임상적 질문은 아직 답이 나오지 않고 있습니다. 

새로운 섬유 사용 및/또는 섬유 병용 투여는 

아직 광범위하게 연구되지 않은 추가적인 치료 방법입니다.

Key points

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요점

• 식이섬유는 여러 가지 질병의 발생과 관리, 그리고 사망률과 관련된 여러 가지 중요한 연관성이 있는 것으로 역학 및 중재 연구에서 밝혀졌습니다.
• 식이섬유는 위장관에서 그 기능을 결정하는 물리화학적 특성(예: 용해성, 점도, 발효성)을 가지고 있으며, 예를 들어 미량 영양소의 이용 가능성, 장 통과 시간, 대변 형성 및 미생물 특이성에 영향을 미칩니다.
• 현재 식이섬유 권장량은 제한적이고 상충되는 경우가 많으며, 과민성 대장 증후군, 염증성 장 질환, 게실 질환, 기능성 변비 등 위장 장애 치료에 필요한 구체적인 유형과 용량을 제공하지 못하고 있습니다.
• 다양한 물리화학적 특성이 기능성에 미치는 영향을 고려한 향후 연구는 위장 장애의 임상적으로 의미 있는 증상 개선 효과를 극대화할 수 있을 것입니다.

식이섬유와 소화기 건강 요약

이 문서는 **식이섬유(Dietary Fiber)**가 소화기 건강과 질병 관리에 미치는 영향을 설명합니다. 주요 내용을 아래와 같이 요약합니다.

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1. 식이섬유의 종류

✅ 수용성 섬유 (Soluble Fiber):

• 물에 녹아 젤을 형성하고 소화를 늦춤.
• 예: 펙틴, 베타글루칸, 갈락토만난

✅ 불용성 섬유 (Insoluble Fiber):

• 물에 녹지 않으며 변의 부피를 늘리고 장 통과 속도를 높임.
• 예: 셀룰로스, 헤미셀룰로스, 리그닌

✅ 저항성 전분 (Resistant Starch):

• 소장에서 소화되지 않고 장내 미생물에 의해 발효됨.
• 예: RS-1, RS-2, RS-3

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2. 주요 건강 효과

✅ 심혈관 건강:

• 식이섬유는 심혈관 질환, 뇌졸중, 관상동맥 질환 위험을 감소시킵니다.

✅ 소화기 건강:

• 장 운동 개선, 변비 완화, 변의 형태와 빈도 개선에 도움을 줍니다.

✅ 대사 건강:

• 혈당과 콜레스테롤 수치를 안정화하고 체중 관리를 돕습니다.

✅ 암 예방:

• 식이섬유 섭취는 대장암 위험을 낮추는 것과 관련이 있습니다.

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3. 작용 기전

✅ 장내 미생물(Gut Microbiota):

• 발효 가능한 섬유는 유익균(Bifidobacterium, Lactobacillus) 성장을 촉진합니다.

✅ 단쇄 지방산(SCFA) 생성:

• 발효 과정에서 생성되는 아세테이트, 프로피온산, 부티르산은 장 건강 유지와 염증 조절에 기여합니다.

✅ 장 통과 촉진:

• 불용성 섬유는 변의 부피를 늘려 장 운동을 빠르게 하고,
  수용성 섬유는 소화 속도를 조절합니다.

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4. 임상적 적용

✅ 과민성 대장 증후군(IBS):

• **수용성 섬유(예: 차전자피, Psyllium)**는 증상을 개선,
  **불용성 섬유(예: 밀기울)**는 증상을 악화시킬 수 있습니다.

✅ 염증성 장 질환(IBD):

• 발효성 섬유는 장 건강을 지원하지만 개인별 내성에 따라 조절이 필요합니다.

✅ 기능성 변비:

• 식이섬유는 변의 빈도와 형태를 개선하는 데 효과적입니다.

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5. 권장 섭취량

✅ 성인: 하루 25~35g의 식이섬유 섭취 권장
✅ 식품: 통곡물, 콩류, 과일, 채소, 견과류, 씨앗류

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6. 도전 과제 및 향후 연구

✅ 개인화 필요:

• 장내 미생물 구성과 건강 상태에 따라 식이섬유의 효과가 다르게 나타남.

✅ 연구 방향:

• 질병별 최적의 섬유 종류, 용량, 조합에 대한 연구가 필요합니다.

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💡 요약:

• 식이섬유는 소화기 건강, 심혈관 질환 예방, 대사 건강 개선에 필수적입니다.
• 개인 맞춤형 접근과 균형 잡힌 식단이 효과적인 활용의 핵심입니다.

The term ‘dietary fibre’ was first coined in 1953, and from the 1970s onwards attempts to provide formal definitions have continued in light of the growing evidence of its associated health benefits1,2 . In 2009, following nearly 20 years of discussions, the World Health Organization and Codex Alimentarius provided a globally disseminated and updated definition (Box 1). Analytical methods to quantify dietary fibre have evolved alongside the updated definitions, although data derived from these updated methods are not comprehensively available in all food composition databases (Box 1).

'식이섬유'라는 용어는 1953년에 처음 만들어졌으며, 1970년대부터 식이섬유와 관련된 건강상의 이점에 대한 증거가 증가함에 따라 공식적인 정의를 제공하려는 시도가 계속되었습니다1,2. 

2009년, 거의 20년 동안의 논의 끝에 세계보건기구(WHO)와 국제식품규격위원회(Codex Alimentarius)는 전 세계적으로 보급되고 업데이트된 정의를 제공했습니다(박스 1). 

식이섬유를 정량화하는 분석 방법은 정의가 업데이트되면서 함께 발전해 왔지만, 이러한 업데이트된 방법에서 도출된 데이터가 모든 식품 구성 데이터베이스에서 포괄적으로 제공되는 것은 아닙니다(박스 1).

식이섬유 정의 • 

세계보건기구(WHO)와 국제식품규격위원회(Codex Alimentarius)에서 정의한 식이섬유

(소화되지 않고 소장에서 흡수되지 않는 모든 탄수화물을 포함하며, 

중합도가 높고 단량체 단위가 많거나 많을 때를 의미함)205,206. 

• 국제식품규격위원회(Codex Alimentarius)의 식이섬유 정의는 많은 국가에서 채택되었으며, 

이로 인해 영양 성분 표시, 식품 구성표, 발표된 연구의 국제적 일관성이 촉진되었습니다.

 • 탄수화물을 식이섬유 정의에 포함시킬 수 있도록 탄수화물의 분자량이 3~9인 단량체를 포함하도록 하는 유연한 정의가 있습니다. 그 후, 유럽식품안전청(EFSA)5과 미국식품의약국(FDA)6은 소장에서 소화되거나 흡수되지 않고, 분자량이 3개 이상의 단량체 단위인 모든 탄수화물을 식이섬유에 포함시키기 위해 식이섬유의 광범위한 정의를 채택했습니다.

 • EFSA와 FDA는 식물 세포에 내재되어 있지 않은 합성 섬유와 추출 섬유도 식이섬유로 선언되기 전에 인체 건강에 대한 생리적 효과를 입증해야 한다고 명시하고 있습니다.

 • 코덱스에 정의된 식이섬유에는 셀룰로오스, 헤미셀룰로오스, 펙틴과 같은 비전분 다당류, 저항성 전분, 이눌린과 올리고프룩토오스와 같은 소화되지 않는 올리고당, 리그닌 등이 포함됩니다. 따라서 식이섬유가 풍부한 식품에는 통곡물, 콩류, 채소, 과일, 견과류, 씨앗류가 포함됩니다.

Dietary fibre analysis • Analytical methodsto quantify dietary fibre have evolved alongside the updated definitions. • The Association of Official AnalyticalChemists(AOAC) method (2011.25) is considered the mostreflective ofthe currentCodex definition, capturing and enabling quantification of most dietary fibre entities, including total dietary fibre and itsinsoluble and soluble fractions by an enzymatic–gravimetric assay, and molecular weight by size exclusion chromatography or high-performance liquid chromatography. • Some food databasesstill include some fibre values derived from outdated AOAC methodsthat do notreflect currentCodex fibre definitions.

식이섬유 분석 • 식이섬유를 정량화하는 분석 방법은 정의가 업데이트되면서 함께 발전해 왔습니다. • AOAC(Association of Official Analytical Chemists) 방법(2011.25)은 현재 Codex 정의에 가장 잘 반영된 것으로 간주되며, 총식이섬유와 그 불용성 및 용해성 분획을 포함한 대부분의 식이섬유 실체를 효소-중량 분석법으로 정량화하고, 분자량을 크기 배제 크로마토그래피 또는 고성능 액체 크로마토그래피로 측정할 수 있도록 합니다. • 일부 식품 데이터베이스에는 현재 Codex 섬유 정의에 반영되지 않은 구식 AOAC 방법에서 파생된 섬유질 값이 포함되어 있습니다.

Investigating the health effects of fibre is complicated by variations in the interventions. Studies can investigate synthetic fibres consisting of only one type of molecule (for example, fructo-oligosaccharides), extracted fibres from naturally occurring plant sources consisting of one, or a limited number of fibres (for example, alginate or psyllium), single foods containing a limited number of naturally occurring fibres that are intrinsic and intact in plant cells (such as prunes or wholegrain cereals), and high-fibre diets consisting of a wide range of different naturally occurring fibres from a wide range of different foods. The variations in fibre interventions have created numerous challenges in interpreting and applying the findings. Firstly, in vitro and animal studies have frequently used synthetic or extracted fibres in supplemental form, but whose physicochemical characteristics such as molecular weight and bioaccessibility might be different when consumed as whole foods and as part of diets that can affect their functional properties, and secondly because high-fibre foods and diets contain other nutrients and food components (such as vitamins and polyphenols) that could be beneficial to health and, therefore, identifying the effect of fibre alone can be challenging. Dietary fibre has been shown in an extensive number of epidemiological and interventional studies to have important associations with the development and management of various diseases and with mortality.

For example, in 2015, the Scientific Advisory Committee on Nutrition (SACN)3 in the UK performed meta-analyses of epidemiological studies of fibre in the prevention of disease, and showed that for each increase in dietary fibre intake from food of 7g per day there was a statistically significantly reduced risk of cardiovascular disease (relative risk (RR) 0.91, 95% CI 0.88–0.94; P< 0.001), haemorrhagic plus ischaemic stroke (RR 0.93, 95% CI 0.88–0.98; P=0.002), colorectal cancer (CRC; RR 0.92, 95% CI 0.87–0.97; P=0.002), rectal cancer (RR 0.91, 95% CI 0.86–0.97; P = 0.007) and diabetes (RR 0.94, 95% CI 0.90–0.97; P=0.001). In 2019, a meta-analysis of 185 epidemiological cohort studies including just under 135 million person-years echoed these findings, showing that risk reduction is greatest when dietary fibre intake from food is between 25g per day and 29g per day. This higher fibre intake was associated with reduced risk of all-cause mortality (RR 0.85, 95% CI 0.79–0.91) and mortality from coronary heart disease (RR 0.69, 95% CI 0.60–0.81) and cancer (RR 0.87, 95% CI 0.79–0.95), and with lower incidence of coronary heart disease (RR 0.76, 95% CI 0.69–0.83), stroke (RR 0.78, 95% CI 0.69–0.88), type 2 diabetes mellitus (RR 0.84, 05% CI 0.78–0.90) and CRC (RR 0.84, 95% CI 0.78–0.89)4 compared with lower fibre intake.

섬유의 건강에 미치는 영향을 조사하는 것은 개입의 다양성으로 인해 복잡합니다. 

연구에서는 

한 가지 유형의 분자(예를 들어, 프락토올리고당)로만 구성된 합성 섬유, 

하나 또는 제한된 수의 섬유로 구성된 자연 발생 식물 원료에서 추출한 섬유(예를 들어, 알긴산 또는 차전자피), 

식물 세포에 내재되어 있는 제한된 수의 자연 발생 섬유를 함유한 단일 식품(예를 들어, 자두 또는 통곡물 시리얼)을 

조사할 수 있습니다. 

그리고 

다양한 식품에서 추출한 다양한 천연 섬유로 구성된 고섬유질 식단. 

섬유질 개입의 다양성은 

연구 결과를 해석하고 적용하는 데 많은 어려움을 야기했습니다. 

첫째, 

체외 및 동물 연구에서 보충제로 합성 섬유나 추출 섬유를 자주 사용하지만, 

이러한 섬유는 전체 식품으로 섭취하거나 식단의 일부로 섭취할 때 

분자량, 생체 접근성 등의 물리화학적 특성이 달라 기능적 특성에 영향을 미칠 수 있습니다. 

둘째, 

고섬유질 식품과 식단에는 

건강에 유익한 다른 영양소와 식품 성분(비타민, 폴리페놀 등)이 포함되어 있기 때문에 

섬유질 단독의 효과를 파악하는 것이 어려울 수 있습니다. 

식이섬유는 

수많은 역학 및 중재 연구에서 

다양한 질병의 발생과 관리, 그리고 

사망률과 중요한 관련이 있는 것으로 나타났습니다.

예를 들어, 

2015년 영국의 SACN(영국의 영양에 관한 과학 자문 위원회)3은 

질병 예방에 대한 식이섬유 섭취와 관련된 역학 연구의 메타 분석을 수행했고, 

식이섬유 섭취량이 하루 7g 증가할 때마다 

심혈관 질환의 위험이 통계적으로 유의미하게 감소한다는 것을 보여주었습니다(상대 위험도(RR) 0.91, 95% CI 0.88–0. 94; P< 0.001), 

출혈성 및 허혈성 뇌졸중(RR 0.93, 95% CI 0.88–0.98; P=0.002), 

대장암(CRC; RR 0.92, 95% CI 0.87–0. 97; P=0.002), 

직장암(RR 0.91, 95% CI 0.86–0.97; P = 0.007) 및 

당뇨병(RR 0.94, 95% CI 0.90–0.97; P=0.001). 

2019년에 

1억 3,500만 명에 가까운 인원-년(person-year)을 대상으로 한 

185건의 역학 코호트 연구에 대한 메타 분석에서도 이러한 결과가 확인되었으며, 

식이섬유 섭취량이 

하루 25g에서 29g 사이일 때 위험 감소 효과가 가장 큰 것으로 나타났습니다. 

이 높은 섬유질 섭취는 

모든 원인으로 인한 사망 위험 감소(RR 0.85, 95% CI 0.79–0.91)와 

관상동맥 질환으로 인한 사망 위험 감소(RR 0.69, 95% CI 0.60–0.81) 및 

암으로 인한 사망 위험 감소(RR 0.87, 95% CI 0.79–0.95)와 관련이 있었습니다. 

그리고 

관상동맥 질환의 발생률이 낮습니다(RR 0.76, 95% CI 0.69–0.83). 

섬유소 섭취량이 낮은 사람에 비해 대장암(RR 0.78, 95% CI 0.69–0.88), 

제2형 당뇨병(RR 0.84, 95% CI 0.78–0.90), 

대장암(RR 0.84, 95% CI 0.78–0.89)4에 걸릴 위험이 낮습니다.

Both observational analyses highlight the critical importance of the quantity of fibre required to elicit health benefits5,6 . These well-established associations between dietary fibre intake and health have resulted in the majority of countries recommending a daily intake for adults of 25–35 g per day. Despite this recommendation, the average intake of dietary fibre by adults worldwide remains low, typically under 20g per day7 . As well as disease prevention, dietary fibre has the potential to be used as a therapeutic intervention, in particular for disorders of the gastrointestinal tract. National and international guidelines provide some recommendations in relation to dietary fibre in the treatment of gastrointestinal disorders such as irritable bowel syndrome (IBS)8,9 , inflammatory bowel disease (IBD)10 and diverticular disease11,12, and in the management of specific gastrointestinal symptoms such as constipation12,13. However, these recommendations are often limited, failing to provide specifics in terms of the type and dose of fibre, and are sometimes even conflicting.

The limited number and quality of studies as well as the variations in the fibre interventions (including fibre type, source, dose and duration of treatment) represent key challenges to providing recommendations for the therapeutic use of dietary fibre in the treatment of gastrointestinal disorders. The potential of dietary fibre for gastrointestinal health and as a therapeutic agent in gastrointestinal disorders is attributed to its effect on nutrient digestion and absorption, improving glycaemic and lipaemic responses, regulating plasma cholesterol through limiting bile salt resorption, influencing gut transit, and microbiota growth and metabolism. Mechanistic research has highlighted the diverse physicochemical characteristics of different dietary fibres, such as solubility, viscosity and fermentability, all of which determine their function in the upper and lower gastrointestinal tracts. The aim of this Review is to discuss the physicochemical and functional characteristics of dietary fibres and the effect of these factors on the clinical application of fibre in the management of gastrointestinal disorders, with a focus on studies in humans wherever possible.

Physicochemical characteristics

The majority of dietary fibres are the structural polysaccharide components of plant cell walls (Fig. 1; Tale 1). Cell walls contain multiple polysaccharides and the complexity in elucidating their functions results from the variety of sources and their functions within the cell. This aspect is most evident in the variation in their molecular structure, which includes the composition of the polymer subunits, but also extends to the polymer linkages and side-chains (esterification)14. These differences in molecular structure of dietary fibres can substantially alter their physicochemical properties and their behaviour in the gastrointestinal tract. For example, their resistance to intestinal digestion can result from the spatial orientation of polymer subunits, branching, or the presence of side-chains15. Food processing provides an additional level of complexity. Indeed, both milling and cooking can also be important determinants of the physicochemical characteristics of dietary fibres, improving starch digestibility and degradation of plant-derived compounds16.

However, some digestible polysaccharides can also be classified as dietary fibre due to their inaccessibility to digestive enzymes within the food matrix, such as type 1 resistant starch (RS-1; as in whole grains) or type 3 resistant starch (RS-3; retrograded), in which resistance can be conferred following cooking and cooling17. The consequence of these small variations in structure is that dietary fibres can have very different physicochemical characteristics (for example, viscosity and fermentability) that influence their functional effects (such as gut transit time or the microbiota) in the gastrointestinal tract.

Fibre solubility.

Solubility refers to the extent to which dietary fibres can dissolve in water. Unlike insoluble fibres that remain as discrete particles, soluble fibres have a high affinity for water18. In cases in which it is necessary to divide the dietary fibre content into soluble and insoluble fibre fractions, the enzymatic–gravimetric assay is often used for routine analysis (Association of Official Analytical Chemists method 2011.25). Examples of carbohydrate polymers whose structure affects their solubility are starch (amylose and amylopectin) and cellulose. The former is composed of α-glucose monomers and the latter β-glucose. The corresponding secondary structures result in starch being soluble (most of which is digested in the small intestine) and cellulose being insoluble (and therefore classified as dietary fibre)18. However, although β-glucose monomer linkages can result in β(1,4)-cellulose being insoluble, they can result in β(1,3)(1,4)-glucan (mixed linkages in β-glucan) being soluble19. Similarly, branching of the polymer structure, such as in amylopectin, β-glucan or inulin, can also affect solubility. Interestingly, the branching in amylopectin can result in increased solubility, whereas branching in β-glucan decreases solubility. Additionally, some fibres, such as pectin or methyl cellulose, contain side-chains along the polymer that provide resistance to digestion20 whilst also increasing solubility21. The majority of current evidence has focused on solubility as a characteristic of fibre in relation to its effect on the upper gastrointestinal tract through the regulation of gastric emptying and nutrient absorption. Indeed, early in vitro studies of isolated fibres allowed the distinction between those that primarily affect small intestinal lipid and glucose absorption and those that primarily affect colonic function such as stool bulking and reduced transit time (insoluble fibres such as cellulose, wheat bran and lignin)22. Thus, classifying fibres based upon solubility was for many decades used to allude to differentiation of their functional properties. However, in 2003 the Food and Agriculture Organization of the United Nations proposed that these conventionally classified terms relating to solubility should be phased out for a number of reasons23. Firstly, measuring and classifying fibre solubility in vitro is method-dependent. Secondly, the varying pH conditions within the gastrointestinal tract (such as stomach versus colon) and between individuals might affect fibre solubility in vivo24,25. Thirdly, solubility alone does not predict the physiological effects of fibre and, therefore, its functional properties. For example, both psyllium (soluble) and cellulose (insoluble) have been shown to improve glycaemic control, transit time and stool output, albeit via different mechanisms. Glycaemic control in humans is improved by psyllium26 through a mechanism involving increased viscosity of intestinal contents, whereas in rats, cellulose has been shown to affect glycaemia via inhibition of starch digestion by binding α-amylase27, thereby reducing glucose absorption28. A further challenge to the use of fibre solubility as an indicator of functionality is that, in reality, whole fibrous foods are often a complex mix of soluble and insoluble fibres (for example, resistant starch, hemicelluloses, cellulose and lignin) and, therefore, simultaneously exert different physiological effects in the gastrointestinal tract. For example, apples contain soluble (pectins) and insoluble (cellulose) fibre fractions. It has been suggested that the effects of both soluble (that is, swelling via water absorption) and insoluble (that is, bulking) fibres in the ileum might activate the ileal brake (negative feedback mechanism that results in inhibition of gastrointestinal motility and secretion) via mediators such as glucagon-like peptide 1 (GLP1) and GLP2 according to animal research29. Nonetheless, although solubility per se is a poor indicator of physiological function in isolation, it has a profound effect on other factors that have since gained recognition for their specific physiological and microbial actions in the gastrointestinal tract such as viscosity and fermentability.

Fibre viscosity.

Viscosity is the degree of resistance to flow. It is generally associated with soluble dietary fibres (such as gums, pectins, β-glucans and psyllium) and relates to the ability of a fibre, when hydrated, to thicken in a concentration-dependent manner30. Some forms of fibre, such as pectins, have the capacity to form gel networks. In the gastrointestinal tract, this process can begin in the mouth and continues throughout the digestive tract31. There are several physicochemical characteristics that contribute to the viscosity potential of fibre, including the length and structure of the polymer as well as its charge. These factors affect the ‘type’ of gel formed and the critical concentration required for the formation of a viscoelastic gel. Broadly, viscous fibres can be categorized into two groups: random coil polysaccharides and ordered assembly polymers. Random coil polysaccharides increase viscosity through entanglement, thereby restricting the flow of the surrounding solvent32. Examples include the neutral polymers β-glucans, psyllium and guar galactomannan, in which generally the longer the polymer (that is, the higher the molecular weight), the greater the entanglement that occurs and, therefore, the lower the concentration required to increase viscosity33. By contrast, ordered assembly polymers, such as some pectins and alginate, form a gel network in the presence of divalent ions (that is, Ca2+)33. Increasing gut luminal viscosity has been suggested to have multiple health benefits. 

Consumption of viscous dietary fibre has been shown to alter transit time in the upper gut, including decreasing gastric emptying rate and modulating small intestinal transit34. Increased luminal viscosity has been suggested to play a part in major regulatory effects of dietary fibre consumption, including delaying digestion, decreasing postprandial glycaemia35 and lipaemia36–38 and increasing satiety in humans39. The effect of the viscoelastic properties in the small intestine are less well defined, particularly as the effect of digestive secretions that dilute luminal contents are difficult to replicate and test in vivo. Indeed, a study investigating the effects of simulated gastric and small intestine digestion in vitro on the thickening ability of six soluble fibres from different sources found substantial differences in their viscosity profiles. For example, xanthan gum retained viscosity more than all the other fibres40. Viscosity remains the accepted model for the cholesterol-lowering capacity of β-glucan. Increased luminal viscosity decreases diffusion of bile salts, preventing their resorption in the distal ileum41. Malabsorbed primary bile salts entering the colon can be de-conjugated by bacterial hydrolases to produce secondary bile acids, which have been shown to increase the risk of CRC through induction of epithelial cell hyperproliferation and increased oxidative DNA damage in vitro42. Although the mechanism remains unclear, as does the interaction between bile acids and dietary fibre43, this elevated CRC risk is not observed in vivo. Indeed there is substantial epidemiological evidence suggesting the opposite: that diets high in fibre are protective against CRC in humans44. Additionally, in vitro studies have suggested that dietary fibres (for example, rice bran fibre, cellulose) might interact with digestive enzymes, inhibiting the rate of nutrient digestion27,45,46. An additional mechanism has been proposed, whereby an interaction between dietary fibres and the mucus layer results in localized increases in viscosity adjacent to the brush border in pigs47,48 regulating nutrient diffusion across it. In the colon, increases in luminal viscosity and water-holding capacity can in turn influence colonic bulk and transit time. The colonic contractions moving luminal content between compartments might also reduce localized viscosity by shear thinning and alter colonic transit, particularly with fibres that are able to form disordered networks when hydrated (such as pectins). The consequences of these changes are likely to influence the extent of fermentation occurring in the colon, indeed with greater understanding of the physicochemical properties, it might be possible to affect microbiome composition49. There are several mathematical equations and models, as well as rheological measurements to determine the viscosity of a solution. While the two most common analytical techniques of rheometry (measures the flow of a fluid) and viscometry (measures the viscosity of a fluid) are effective at determining viscosity of isolated fibre solutions, their validity in vivo remains equivocal as intestinal luminal contents are extremely heterogeneous and the effect of muscular contractions, peristalsis and mixing in the gastrointestinal tract cannot be replicated in vitro.