Fibras Dietéticas

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The term ‘dietary fibre’ was first coined in 1953, and from 
the 1970s onwards attempts to provide formal defini-
tions 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 dis-
seminated 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).
Investigating the health effects of fibre is complicated 
by variations in the interventions. Studies can investi-
gate synthetic fibres consisting of only one type of mol-
ecule (for example, fructo- oligosaccharides), extracted 
fibres from naturally occurring plant sources consist-
ing 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 char-
acteristics 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 num-
ber 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 7 g per day there was a statisti-
cally 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, 
Dietary fibre in gastrointestinal health 
and disease
Samantha K. Gill, Megan Rossi, Balazs Bajka and Kevin Whelan ✉
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.
King’s College London, 
Department of Nutritional 
Sciences, London, UK.
✉e- mail: kevin.whelan@ 
kcl.ac.uk
https://doi.org/10.1038/ 
s41575-020-00375-4
REvIEWS
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mailto:kevin.whelan@
kcl.ac.uk
mailto:kevin.whelan@
kcl.ac.uk
https://doi.org/10.1038/s41575-020-00375-4
https://doi.org/10.1038/s41575-020-00375-4
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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 25 g per day and 29 g 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. Both observational analyses highlight 
the critical importance of the quantity of fibre required 
to elicit health benefits5,6. These well- established asso-
ciations 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 
20 g per day7.
As well as disease prevention, dietary fibre has the 
potential to be used as a therapeutic intervention, in par-
ticular for disorders of the gastrointestinal tract. National 
and international guidelines provide some recommen-
dations in relation to dietary fibre in the treatment of 
gastrointestinal disorders such as irritable bowel syn-
drome (IBS)8,9, inflammatory bowel disease (IBD)10 and 
diverticular disease11,12, and in the management of spe-
cific gastrointestinal symptoms such as constipation12,13. 
However, these recommendations are often limited, fail-
ing 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 dis-
orders. The potential of dietary fibre for gastrointesti-
nal health and as a therapeutic agent in gastrointestinal 
disorders is attributed to its effect on nutrient diges-
tion and absorption, improving glycaemic and lipae-
mic responses, regulating plasma cholesterol through 
limiting bile salt resorption, influencing gut transit, 
and microbiota growth and metabolism. Mechanistic 
research has highlighted the diverse physico chemical 
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 physicochem-
ical 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
Themajority of dietary fibres are the structural polysac-
charide components of plant cell walls (Fig. 1; TaBle 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 poly-
mer 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 com-
plexity. Indeed, both milling and cooking can also be 
important determinants of the physicochemical char-
acteristics of dietary fibres, improving starch digesti-
bility and degradation of plant- derived compounds16. 
However, some digestible polysaccharides can also 
be classified as dietary fibre due to their inaccessibil-
ity 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, viscos-
ity 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 amylo-
pectin) and cellulose. The former is composed of 
α- glucose monomers and the latter β- glucose. The cor-
responding 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 
Key points
•	Dietary	fibre	has	been	shown	to	have	a	number	of	important	associations	with	the	
development	and	management	of	various	diseases	and	with	mortality	in	
epidemiological	and	interventional	studies.
•	Dietary	fibre	has	physicochemical	characteristics	(for	example,	solubility,	viscosity,	
fermentability)	that	determine	its	functionality	in	the	gastrointestinal	tract,	including	
its	effects	on,	for	example,	micronutrient	availability,	gut	transit	time,	stool	formation	
and	microbial	specificity.
•	Current	dietary	fibre	recommendations	are	often	limited	and	conflicting,	and	fail		
to	provide	specific	types	and	doses	in	the	treatment	of	gastrointestinal	disorders	
including	irritable	bowel	syndrome,	inflammatory	bowel	disease,	diverticular	disease	
and	functional	constipation.
•	Future	research	that	considers	the	influence	of	differing	physicochemical	
characteristics	on	functionality	will	potentially	maximize	the	effect	of	clinically	
meaningful	symptom	improvement	in	gastrointestinal	disorders.
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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 solu-
bility, whereas branching in β- glucan decreases solubility. 
Additionally, some fibres, such as pectin or methyl cellu-
lose, contain side- chains along the polymer that provide 
resistance to digestion20 whilst also increasing solubility21.
The majority of current evidence has focused on solu-
bility 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 tran-
sit time (insoluble fibres such as cellulose, wheat bran 
and lignin)22. Thus, classifying fibres based upon solubil-
ity 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 num-
ber 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, solubil-
ity alone does not predict the physiological effects of 
fibre and, therefore, its functional properties. For exam-
ple, 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 diges-
tion 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, cel-
lulose 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 feed-
back mechanism that results in inhibition of gastro-
intestinal 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 char-
acteristics 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 poly-
saccharidesand ordered assembly polymers. Random 
coil polysaccharides increase viscosity through entan-
glement, thereby restricting the flow of the surround-
ing 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 sug-
gested to have multiple health benefits. Consumption of 
viscous dietary fibre has been shown to alter transit time 
Box 1 | Definition of dietary fibre and approaches to its chemical analysis
Dietary fibre definition
•	Dietary	fibre	as	defined	by	the	World	Health	Organization	and	Codex	Alimentarius	
(collection	of	internationally	recognized	standards,	guidelines	and	codes	of	practice)	
includes	all	carbohydrates	that	are	neither	digested	nor	absorbed	in	the	small	intestine	
and	have	a	degree	of	polymerization	(DP)	of	ten	or	more	monomeric	units205,206.
•	The	Codex	Alimentarius	dietary	fibre	definition	has	been	adopted	by	many	countries	
and	this	has	encouraged	international	consistency	in	nutrition	labelling,	food	
composition	tables	and	published	research.
•	There	is	flexibility	in	the	definition	enabling	regional	authorities	to	include	
carbohydrates	with	a	DP	of	three	to	nine	monomeric	units	in	the	fibre	definition.	
Subsequently,	the	European	Food	Safety	Authority	(EFSA)5	and	the	FDA6	adopted	the	
wider	definition	of	dietary	fibre	to	include	all	carbohydrates	that	are	neither	digested	
nor	absorbed	in	the	small	intestine	and	have	a	DP	of	three	or	more	monomeric	units.
•	Both	EFSA	and	the	FDA	specify	that	synthetic	and	extracted	fibres	that	are	not	
intrinsic	to	plant	cells	must	also	demonstrate	physiological	effects	to	human	health	
prior	to	being	declared	a	dietary	fibre.
•	Codex-	defined	fibre	includes	non-	starch	polysaccharides	such	as	cellulose,	
hemicelluloses	and	pectins,	resistant	starch	and	non-	digestible	oligosaccharides	such	
as	inulin	and	oligofructose,	as	well	as	lignins.	Thus,	foods	high	in	dietary	fibre	include	
wholegrains,	legumes,	vegetables,	fruits,	nuts	and	seeds.
Dietary fibre analysis
•	Analytical	methods	to	quantify	dietary	fibre	have	evolved	alongside	the	updated	
definitions.
•	The	Association	of	Official	Analytical	Chemists	(AOAC)	method	(2011.25)	is	
considered	the	most	reflective	of	the	current	Codex	definition,	capturing	and	
enabling	quantification	of	most	dietary	fibre	entities,	including	total	dietary	fibre	and	
its	insoluble	and	soluble	fractions	by	an	enzymatic–gravimetric	assay,	and	molecular	
weight	by	size	exclusion	chromatography	or	high-	performance	liquid	chromatography.
•	Some	food	databases	still	include	some	fibre	values	derived	from	outdated	AOAC	
methods	that	do	not	reflect	current	Codex	fibre	definitions.
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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 abil-
ity 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 dam-
age 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 pro-
tective 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 dif-
fusion across it. In the colon, increases in luminal vis-
cosity and water- holding capacity can in turn influence 
colonic bulk and transit time. The colonic contrac-
tions 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 occur-
ring in the colon, indeed with greater understanding of 
the physico chemical properties, it might be possible to 
affect microbiome composition49.
There are several mathematical equations and mod-
els, 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 
Digested
Cooked starches
Cell wallStarch granules
Soluble fibres
• Pectin
• β-Glucan
• Galactomannan
Insoluble fibres
• Cellulose
• Hemicellulose
• Lignin
Gelatinized (soluble)
Resistant starch
• RS-1: whole or partially milled
• RS-2: granules (high amylose)
• RS-3: retrograded (cooked and cooled)
• RS-4: chemically modified
• RS-5: starch–lipid complexes
Raw starches
Ungelatinized (insoluble)
Fig. 1 | physicochemical characteristics of dietary fibre and their location within the plant cell. Polysaccharides that 
contribute to the dietary fibre definition can be divided into two broad categories: non- starch polysaccharides that are a 
main component of plant cell walls, and resistant starch (RS), the plant’s energy store and a major carbohydrate source in 
the human diet. Their chemical structure, interactions with other cell wall components, food processing and digestion can 
all influence their solubility, viscosity and fermentability.
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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.
Fibre fermentability. Observational studies have con-
sistently shown differences in faecal microbiota com-
position between industrialized and rural populations. 
These differences have been attributed to differences 
in the typical westernized diet consisting of foods that 
are highly refined and low in dietary fibre, particularly 
fermentable fibre50–55. Unlike mammalian cells, some 
species of the gut microbiota possess enzymes able to 
hydrolyse the chemical bonds within some dietary fibres 
within plant- based foods56. Interestingly, a number of 
specialist primarydegraders known as ‘keystone species’ 
have been identified within the large intestine57. These 
species have a superior ability to degrade certain dietary 
fibres and release energy on which other bacterial com-
munities depend. For example, Ruminococcus bromii 
has been shown to specifically degrade certain types of 
resistant starch57.
Examples of fermentable fibres include inulin- type 
fructans, galacto- oligosaccharides and resistant starch 
(TaBle 1). All natural plant fibres have some degree of 
fermentability, even cellulose and lignin (low ferment-
ability), with only synthesized fibres such as methyl-
cellulose being completely non- fermentable58 (TaBle 1). 
Fibre fermentability has traditionally been assessed 
using in vitro fermentation models of the digestive tract. 
Although this technique provides valuable mechanistic 
insights into fermentation patterns and behaviour of dif-
ferent dietary fibres, the findings are not always reflected 
in vivo. For example in vitro models have indicated that 
psyllium is moderately fermented, whereas in vivo 
studies have demonstrated that it is poorly fermented59. 
This discrepancy is probably because the dilution and 
high- speed mechanical blending that occurs in vitro 
destroys the gel network, artificially exposing the fibres 
to enzymatic degradation. In vivo methods of measur ing 
fibre fermentability include quantification of short- chain 
fatty acids (SCFAs) in stools and hydrogen breath test-
ing, although these also have limited interpretability, 
and breath testing in particular is subject to limited 
reproducibility60. Advances in MRI have enabled meas-
urement of colonic gas volumes in response to differ-
ent fibres in humans61,62. Although still in its infancy 
in terms of quantifying fermentation, MRI overcomes 
many of the limitations of the other methods and can 
non- invasively and simultaneously assess the water con-
tent of the small intestine, colonic volumes and colonic 
gas volume63.
Greater intake of dietary fibre, particularly from 
foods high in the fermentable fibres, has been associ-
ated with higher stool SCFA concentrations in adults64. 
SCFAs have a number of key roles in the gastrointestinal 
tract. Animal studies have shown that SCFAs affect gas-
trointestinal motility by stimulating colonic contractile 
activity through increasing the number of excitatory 
cholinergic neurons65. SCFAs also have a mediatory role, 
bridging communication between the mucosal micro-
biota and the mucosal immune system, with preclini-
cal evidence suggesting notable anti- inflammatory and 
immunomodulatory effects with relevance to inflam-
matory disorders of the gut66. For example, SCFAs can 
influence intestinal adaptive immune responses through 
direct regulation of the size and function of the regu-
latory T cell pool, including proliferative capacity and 
gene expression in mice67. In animal studies, SCFAs have 
also been implicated in maintaining intestinal barrier 
integrity68 and regulating appetite via several mech-
anisms including the stimulation of gluconeogenesis 
in the liver69. Furthermore, SCFAs indirectly maintain 
gastrointestinal homeostasis via the reduction in lumi-
nal pH70, which could be important in preventing 
colonization and inhibiting growth of acid- sensitive 
enteropathogens (Fig. 2).
Table 1 | physicochemical characteristics of common dietary fibres
Fibre type Common sources physicochemical characteristicsa
solubility Viscosity Fermentability
Cellulose All green plant cell 
walls
Insoluble Non- viscous Low
Lignins All green plant cell 
walls
Insoluble Non- viscous Low
Arabinoxylans Wheat, psylliumb Low to 
medium
Medium Highb
β- Glucans Oat, barley, fungi Low to 
medium
Medium to 
high
High
Galactomannans Guar gum, fenugreek Medium 
to high
Medium to 
high
High
Pectins Fruits, vegetables, 
legumes
High Medium to 
high
High
Inulin Cereals, fruits, 
vegetables
Medium 
to high
Low to high High
Galacto- 
oligosaccharides
Pulses (e.g. beans, 
peas, lentils)
High Low High
Dextrins Cereals (e.g. wheat 
dextrins)
High Non- viscous 
to low
High
Alginate Seaweed High High Low
Methylcellulose Synthesized High High Non- 
fermentable
Resistant Starch
RS-1 (physically 
inaccessible)
Whole grains, 
legumes, raw fruits, 
vegetables
Insoluble Non- viscous High
RS-2 (starch 
conformation)
Cereals, raw 
legumes, raw fruits, 
vegetables
Low Non- viscous High
RS-3 
(retrograded)
Cooking and cooling 
of any starch source
Low Non- viscous 
to low
High
RS-4 (chemically 
modified)
Synthesized (e.g. 
acylated starches)
Low to 
high
Low to 
medium
High
RS-5 (starch–
lipid complex)
Synthesized 
(e.g. amylose 
and stearic acid)
Low Low Low
aPhysicochemical characteristics are not distinct entities but represent a continuum or 
gradient, and will vary depending on botanical origin, chemical structure and molecular 
weight, or whether the fibres are isolated or part of the cell wall matrix (Fig. 1). bDue to their 
structural features, wheat and psyllium sources of arabinoxylans are considered of only low 
fermentability.
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Many factors can affect the rate, site and extent of 
fibre fermentation, including the composition of the 
microbiota (degradation capacity) with different micro-
organisms shown to preferentially metabolize different 
fibres71 and the availability of other substrates (that is, 
proteins that have escaped digestion)72. Nonetheless, the 
key factor influencing fibre fermentability is thought to 
be its physicochemical characteristics (for example, sol-
ubility, viscosity, accessibility). This has been shown in 
an in vitro fermentation study using healthy human stool 
samples from three donors to investigate the effect of 
15 dietary fibres on SCFA production. Despite marked 
inter- individual differences in gut microbiota composi-
tion between donors, SCFA production (concentrations 
and proportions) from the different fibres was reproduc-
ible between samples. More specifically, rhamnose pro-
duced the highest proportion of propionate followed by 
galactomannans, whereas fructans and other α- glucans 
and β- glucans produced the highest proportion of 
butyrate73.
The effect of fermentable fibres on SCFA produc-
tion has been consistently shown in vitro74–77. By con-
trast, interpretation of human interventional studies is 
somewhat limited given that approximately 95% of 
SCFAs are absorbed by colonocytes, and therefore fae-
cal SCFA concentrations only represent ~5% of the total 
SCFAs produced. Thus, faecal SCFAs better reflect the 
dynamic processes of both SCFA production and SCFA 
absorption, the former being affected by fibre source, 
colonic transit time and the gut microbiota and the latter 
being greatly affected by colonic transit time78.
Reducing fermentable fibre intake might reduce bac-
terial diversity. Several animal studies have shown that 
gut microbiota deprived of fermentable fibres shift to 
degrading and extracting alternative energy from the 
glycoprotein- rich mucus layer that acts as a protec-
tive and mechanical barrier to pathogens79,80. Indeed, 
reduced availability of fermentable fibres leads to a 
thinner mucus layer81–83, which in turn might compro-
mise intestinal epithelial integrity and increase pathogen 
susceptibility. Reduced availability of fermentable fibres 
also causes a shift from a stable microbial intestinal 
environment to one that is temporarily or permanently 
altered, often characterized by reduced bacterial diver-
sity and richness, a state that is commonly referred to as 
dysbiosis84. A dysbiotic intestinal environment is a com-
mon feature of a number of gastrointestinal disorders; 
for example, decreases in abundance of bifidobacteria85,86 
and Firmicutes87–89 have been commonly observed in 
cohorts of people with IBS and IBD, respectively.
The food matrix. The nature of the food matrix in which 
a dietary fibre is delivered will markedlyinfluence the 
extent of its physiological function. More specifically, 
the particle size and integrity of the plant cell walls 
affects the dissolution of soluble fibre90. This process 
has the potential to substantially affect luminal viscosity 
and reduce the rate of fermentation. Furthermore, the 
Fibre
↑ Solubility
↓ pH
↑ Viscosity → ↓ Glucose
and lipid absorption rate
Bile acids
↓ Reabsorption
↑ Stool bulk
Microbial growth
Dendritic
cell
Immunomodulation
Inhibit
pathogens
↓ pH
Supports
tight
junction
integrityLarge fibre
particles
T cell
SCFA
• Acetate
• Propionate
• Butyrate
Liver
Ca2+
↑ Colonic muscular contraction
Large intestineSmall intestine
Intestinal
epithelial
cell
Iron
Phytate
↓ Gluconeogenesis
↑ Water binding
Fig. 2 | Mechanisms by which different dietary fibres affect the gastrointestinal tract. There are several mechanisms 
by which the physicochemical characteristics (solubility, viscosity and fermentability) of dietary fibre might affect its 
functional properties in the gastrointestinal tract including influencing glucose and lipid absorption, contributing to stool 
output (frequency, consistency and weight) and stimulating changes in microbial composition and metabolite production 
including the production of short- chain fatty acids (SCFAs).
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cell wall integrity can also encapsulate the intracellular 
starch91, therefore reducing digestion by endogenous 
enzymes and increasing the substrate available for 
microbial fermentation (RS-1; Fig. 1).
Particle size can influence fibre fermentation, 
although to date, the majority of research has been con-
ducted in vitro, with limited translational research inves-
tigating the effects in humans. Reduced particle size of 
fibres has been associated with increased SCFA concen-
trations in an in vitro fermentation system with human 
faecal inoculum92, suggesting greater bacterial fermen-
tation. This outcome is likely to have been due to the 
fact that smaller particles have a greater external surface 
area exposed to bacterial enzymes. Similarly, the physical 
act of chewing and grinding high- fibre foods can have a 
notable role in particle size kinetics by increasing surface 
area and total pore volume as well as structural modifi-
cation. For example, reducing the particle size of coconut 
residue from 1,127 μm to 550 μm led to an increase in 
hydration properties, including water- holding, retention 
and swelling capacity93. Equally, in human studies, the 
physical and mechanical effects of large and/or coarse 
insoluble fibre particles on the colonic mucosa has been 
shown to stimulate the secretion of water and mucus 
into the lumen, contributing to stool output (that is, 
consistency and weight)94,95.
Porosity determines the degree to which enzymes or 
bacteria can diffuse into particles, which can substan-
tially influence the fermentability of a fibre. Low porosity 
of a food matrix can result from maintenance of cell wall 
structures in the small intestine and, therefore, inability 
of digestive enzymes to access intracellular starch, lead-
ing to increased RS-1, as is the case, for example, with 
whole chickpeas17. Low porosity of a food matrix in the 
large intestine can also impede fermentative degradation 
in humans96. Dietary fibre preparations high in insoluble 
fibres such as cellulose are likely to have low porosity, 
whereas those high in soluble fibres such as pectin have 
high porosity, and these differences in the porosity of 
fibres contribute to differences in their fermentability 
(TaBle 1).
Functional characteristics of fibre
Micronutrient bioavailability. Dietary fibre can influ-
ence nutrient bioavailability beyond merely limit-
ing micronutrient accessibility within a food matrix. 
Cereals containing dietary fibre (such as wheat) are a 
vital source of non- haem iron; however, other cereal 
components (for example, bran fractions) contain con-
comitant factors such as phytate that reduce absorption 
of iron, zinc and calcium97. Additionally, in wheat, the 
aleurone contains the majority of the iron, but is encap-
sulated by the cell walls (predominantly dietary fibre)98. 
Studies have shown that micro- milling to disrupt these 
structures results in increased mineral bioavailability 
in vitro99. In contrast to iron, several studies in humans 
have shown that consumption of specific fibres, such 
as fructans100 and galacto- oligosaccharides101, could 
increase absorption of dietary calcium. Based on exper-
imental findings, the proposed mechanism involves 
decreased colonic pH resulting from SCFA produc-
tion, which in turn can increase calcium solubility, 
thereby increasing passive transport of calcium across 
the intestine.
The influence of dietary fibre consumption on vita-
min absorption remains unclear. Several studies have 
demonstrated that some fibres improve absorption 
of some water- soluble and fat- soluble vitamins102,103, 
whereas others have shown no effect104. For example, 
a study in African Americans has shown that fibre can 
enhance the net colonic bacterial biosynthesis of B vita-
mins such as folate105, whereas other studies in women 
have shown increased faecal excretion of vitamins 
following dietary fibre consumption106. The variety of 
fibre sources with varying physico- chemical properties 
demonstrates that further work is required.
Gut transit time. Abnormal whole- gut transit times are 
found in some gastrointestinal disorders, which some 
types of fibre could normalize. For example, evidence 
from animal models demonstrates that, as found in rats, 
fermentable fibres can indirectly modulate contractile 
activity via production of SCFAs107, whereas, as found 
in dogs, non- fermentable fibres might contribute to 
stool weight that increases colonic volume and, there-
fore, stimulates contractility108. Indeed, a systematic 
review and meta- analysis in healthy populations found 
that transit time decreased in a dose- dependent man-
ner by 0.78 h per additional 1 g per day of wheat fibre109. 
Additionally, wheat fibre particle size has been shown 
to influence stool output, whereby coarse wheat fibre 
resulted in higher stool weight (mean 219.4 g per day) 
than fine wheat fibre (199 g per day) in one study of 
21 healthy humans110.
In the 1970s and 1980s, studies measuring gut transit 
time using radio- opaque markers and scintigraphy 
showed that dietary fibre interventions reduce gut transit 
time in humans111–114. One study in healthy humans 
showed a decrease in transit time with wheat bran sup-
plementation from 70 h to approximately 46 h (reF.115). 
Insoluble, poorly fermented fibres (for example, wheat 
bran) have a greater effect on reducing gut transit time 
in healthy individuals and to a lesser extent in those 
with constipation116 than fermentable fibres that do not 
remain physically intact throughout the colon.
More recently, studies investigating the use of a wire-
less motility capsule that measures, among other things, 
transit time have been reported. Transit times measured 
with this device correlate with those obtained using 
radio- opaque markers and scintigraphy117,118 and the 
device has been shown to provide a more practical and 
less invasive method for determining gastric emptying 
time and whole gut transit time. For example, one con-
trolled crossover trial in healthy individuals with back-
ground fibre intakes of 14–15 g per day who consumed 
an additional 9 g per day of wheat bran or a low- fibre 
control diet for 3 days found that whole- gut transit time 
and colonic transit time were lower in those receiving 
wheat bran supplementation than in those receiving the 
control diet (−8.9 h and −10.8 h, respectively)119.
Stool forming. For a dietary fibre to exert effects on stool 
output (that is, frequency, consistency and weight), it 
must possess certain physical characteristics. Wheat bran 
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supplementationhas been shown to increase stool fre-
quency from 1 per day to 1.5 per day115, although the 
type of bowel movement (complete or spontaneous) was 
not reported, whereas further studies in healthy humans 
have shown that supplementation with a cellulose– 
pectin combination120 or psyllium121 and high- fibre 
breakfast cereals122 increase stool frequency.
Both soluble, viscous fibres and insoluble, non- 
viscous fibres can contribute to improvements in stool 
consistency and stool weight (bulk). Soluble, viscous 
fibres (for example, psyllium) with a high water- holding 
capacity that are resistant to fermentation and form a 
viscoelastic substance in the gastrointestinal tract, con-
tribute to softening hard stool and increasing stool bulk, 
making them easier to pass123,124. These properties also 
assist with diarrhoea by firming loose stools and slow-
ing transit time125–127. By contrast, insoluble, non- viscous 
fibres (for example, coarse wheat bran) can contribute to 
improvements in stool consistency and stool weight via 
the mechanical stimulation of the intestinal mucosa95,128. 
Previous studies in adults with self- reported constipa-
tion have shown improvements in stool consistency 
with increased consumption of rye bread129 and fibre 
bread130. Theoretically, fermentable fibres might con-
tribute, in part, to stool weight via increasing microbial 
mass, but the effect size is limited compared with that 
of non- fermentable fibres. For example, results from 
six trials showed that RS-2 supplementation increased 
stool weight (+38 g per day, 95% CI 23–53 g per day; 
P < 0.001) at doses ranging from +21.5 to +37 g per day3. 
A review of interventional trials investigating outcomes 
in healthy humans concluded that fermentability deter-
mines the role of fibre in total stool weight, with less 
fermentable fibres from cereals contributing most to 
stool weight131.
Microbial specificity (prebiotics). Some fermentable 
fibres are also classed as prebiotics, a term whose defi-
nition has been updated to “a substrate that is selectively 
utilized by host microorganisms conferring a health 
benefit”132. Examples of prebiotic fibres include the 
inulin- type fibres and galacto- oligosaccharides.
Prebiotic fibres are known for their rapid fermenta-
tive capacity and subsequent release of SCFAs, in par-
ticular acetate, but selectively stimulate the growth of 
only a specific range of genera and/or species (that is, 
Bifidobacterium and Lactobacillus)133–135. This selectiv-
ity is due to specific gene clusters within the bacterial 
genome that dictate the saccharolytic enzymes they 
produce and their phenotypic ability to selectively 
metabolize the prebiotic substrate136,137.
The first landmark study demonstrating the prebio-
tic effect in eight healthy volunteers found that supple-
mentation with 15 g per day of oligofructose or inulin 
increased luminal bifidobacterial levels by almost one 
log10 (reF.138). Subsequent research has demonstrated a 
dose- dependent effect on luminal Bifidobacterium levels 
of oligofructose121 and galacto- oligosaccharides139 sup-
plementation. Published in 2018, a systematic review 
and meta- analysis of 64 randomized controlled trials 
(RCTs) in humans demonstrated that fibre, and in par-
ticular fructans and galacto- oligosaccharides and other 
candidate prebiotic fibres, increases the abundance of 
Bifidobacterium and Lactobacillus species133.
However, large inter- individual variability in 
gut microbiota responses to dietary fibre have been 
reported140–144. A crossover RCT in 34 healthy partici-
pants found that those with a habitually high dietary fibre 
intake had a greater gut microbial response to pre biotics 
(that is, increases in abundance of Bifidobacterium and 
Faecalibacterium, and decreases in Coprococcus, Dorea 
and Ruminococcus) compared with those with habitu-
ally low dietary fibre intake, suggesting that individuals 
following a habitual high- fibre diet are more likely to 
benefit from an inulin- type fructan prebiotic145.
Dietary fibre in gut disorders
Given the substantial inter- condition and inter- 
individual variability in response to dietary fibre, there 
remains the complex challenge of unravelling which 
fibres are most appropriate for which gastrointestinal 
disorders. Consideration of the diverse physicochem-
ical characteristics of fibre and how these translate to 
functional characteristics is fundamental to optimizing 
any clinical benefit (Fig. 3). Consideration should also 
be given to maintaining the balance between optimiz-
ing symptom benefit (that is, management and main-
tenance) and limiting symptom exacerbation (that 
is, tolerance). The manipulation of dietary fibre is a 
common approach in clinical practice for many gastro-
intestinal disorders and is commonly recommended as 
first- line therapy in the management of several gastro-
intestinal symptoms (TaBle 2), despite the limited num-
ber of RCTs across gastrointestinal disorders, all of 
which used heterogeneous methodologies (for example, 
fibre type, amount, duration).
Irritable bowel syndrome. IBS is a functional gastroin-
testinal disorder characterized by recurrent abdominal 
pain and change in stool habit (that is, constipation, diar-
rhoea or both), often alongside abdominal bloating and 
distension146. The mechanisms underpinning IBS symp-
toms include visceral hypersensitivity, and alterations in 
gut–brain interactions, immune activation, gut motil-
ity and the gut microbiota. Since the revised Rome IV 
criteria were introduced in 2016, estimated worldwide 
prevalence of IBS has decreased from 11.7% (Rome III) 
to 5.7% (Rome IV)147.
Guidelines for fibre consumption in individuals 
with IBS vary. The National Institute for Health and 
Care Excellence8 in the UK recommends that resistant 
starch intake should be reduced, whereas the World 
Gastroenterology Organization Global Guidelines148 
suggest that fibre- rich foods or fibre supplements (for 
example, psyllium) should be encouraged and that insolu 
ble fibres that might exacerbate symptoms should be 
limited.
To date, a number of systematic reviews and meta- 
analyses have concluded that some fibres are beneficial in 
reducing IBS symptoms and improving stool frequency 
and consistency, although results are inconsistent with 
wide variation in responses149–151. Benefits seem to be 
limited to soluble fibre (RR of having continued symp-
toms after supplementation 0.83, 95% CI 0.73–0.94)152, 
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compared with other fibres such as bran (RR 0.90, 95% 
CI 0.79–1.03). The ongoing changes in diagnostic cri-
teria (for example, Rome Criteria for IBS), and lack of 
standardization in outcome measures between studies 
presents major challenges when attempting to com-
pare findings of previous studies. Further rigorous and 
long- term RCTs are required, and there is an urgent 
need to assess the different functionalities of dietary 
fibres in subgroups of individuals with IBS (for exam-
ple, specifically those with constipation- predominant 
IBS or diarrhoea- predominant IBS) to enable a better 
understanding of its therapeutic potential. Interestingly, 
a three- period, crossover mechanistic study revealed 
that despite similar physiological responses to prebio-
tics between patients with IBS and healthy individuals 
as controls, only those with IBS experienced symptoms 
when challenged with 40 g of fructose or inulin whereas 
the healthy controls did not, suggesting that visceral 
hypersensitivity to colonic gas is involved in the induc-
tion of symptoms, rather than excessive gas production 
per se153.
A limited number of clinical trials investigating 
prebiotic supplementation (for example, oligofructose, 
fructo- oligosaccharide and β- galacto- oligosaccharides) 
ranging from 3.5 g per day to 20 g per day over 4–12 weeks 
have been conducted in IBS populations, with mixed 
results154–157. A systematic review and meta- analysis pub-
lished in 2019 of 11 RCTsincluding 729 patients showed 
that although the abundance of bifidobacteria increased 
following prebiotic supplementation, there were no 
differences in response rates, or severity of abdominal 
pain, bloating, flatulence or quality of life158. Notably, 
subgroup analysis showed improvement in flatulence 
with lower prebiotic doses (≤6 g per day; standardized 
mean difference (SMD) −0.35, 95% CI −0.71 to 0.00; 
P = 0.05) and non- inulin- type fructans (for example, 
galacto- oligosaccharide, guar gum: SMD −0.34, 95% CI 
−0.66 to −0.01; P = 0.04). Although little current evidence 
exists for the use of prebiotics in IBS management158,159, 
emerging evidence suggests that second- generation can-
didate prebiotics such as pectin and partially hydrolysed 
guar gum have bifidogenic properties160 with potential 
therapeutic use in IBS161, possibly due to their viscosity 
characteristics and, therefore, slower fermentation rate.
Inflammatory bowel disease. IBD encompasses Crohn’s 
disease and ulcerative colitis, both chronic, relapsing 
gastrointestinal disorders of which the pathogenesis 
remains incompletely understood, although a dysreg-
ulated mucosal inflammatory response in genetically 
susceptible individuals is responsible for the initiation 
and maintenance of IBD162. This process involves altera-
tions in immunological factors (for example, T and B cell 
regulation) and microbial factors (for example, diversity 
and functionality of bacteria such as Faecalibacterium 
prausnitzii)163. The prevalence of IBD exceeds 0.3% 
across North America, Oceania and Europe164. The goal 
of IBD treatment is to halt disease progression, main-
tain remission and prevent recurrence of inflammatory 
episodes.
There continues to be debate on the clinical benefits 
of dietary fibre in IBD165 despite plausible mechanisms 
for its therapeutic potential, including the production of 
SCFAs (particularly butyrate) that could attenuate intes-
tinal inflammation through upregulation or downregu-
lation of cytokine expression (for example, IL-10, IFNγ 
↑ 
V
is
co
si
ty
↑ Solubility
↑ F
er
me
nt
ab
ilit
y
Physicochemical 
• Soluble
• Non-viscous
• Fermentable 
• e.g. Inulin, GOS
Functional
• SCFA production
• Prebiotic
Physicochemical
• Soluble
• Viscous
• Fermentable
• e.g. Pectin,
 galactomannan
Functional
• Nutrient bioavailability
• SCFA production
Physicochemical
• Medium solubility
• Medium viscosity
• Low fermentability
• e.g. Psyllium
Functional
• Nutrient bioavailability
• Faster transit
• Water holding
Physicochemical
• Insoluble
• Non-viscous
• Non-fermentable 
• e.g. Cellulose in wheat
 bran
Functional
• Faster transit
• Stool bulking
Fig. 3 | spectrum of physicochemical characteristics of dietary fibre. The physicochemical characteristics of fibre 
(solubility, viscosity and fermentability) form a continuum and work in concert to determine its functional properties in the 
gastrointestinal tract. The combination of these three physicochemical characteristics determines the functional effects 
of fibres in the gut. For example, fibres in the left- hand, bottom, near corner (insoluble, non- viscous, non- fermentable) have 
functions relating to gut transit time; fibres in the right- hand, bottom, far corner (soluble, non- viscous, fermentable) 
have functions relating to microbiome and fermentation; and fibres in the right- hand, top, far corner (soluble, viscous, 
fermentable) have functions relating to microbiome, fermentation and nutrient bioavailability. Fibres in intermediate 
positions would be predicted to have intermediate functional properties. GOS, galacto- oligosaccharides; SCFA, 
short- chain fatty acid.
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and IL-1β) by colonic epithelial cells according to in vitro 
data67,166,167. A number of studies have found alterations 
in the gut microbiota (for example, reduced abundance 
of Bifidobacterium and Faecalibacterium prausnitzii) in 
patients with Crohn’s disease that might be amenable to 
prebiotic fibre supplementation168,169. Previous studies 
have generally found lower stool SCFA concentrations 
in patients with IBD than in healthy individuals170–172, 
suggesting a reduced fermentative capacity and an 
impairment in SCFA production in this patient group. 
Collectively, it is conceivable that fibre (dietary or sup-
plement form) might prevent IBD, maintain or restore 
intestinal epithelial integrity in IBD, although studies 
in humans of fibre in the prevention, maintenance and 
treatment of IBD are extremely limited165.
In terms of the risk of developing IBD, a prospective 
cohort study in 170,776 women in the Nurses’ Health 
Cohort Study followed up over 26 years identified 269 
and 338 cases of Crohn’s disease and ulcerative colitis, 
respectively173. Compared with women with the lowest 
energy- adjusted dietary fibre intake, intake in the high-
est quintile (median of 24.3 g per day) was associated 
with a 40% reduction in risk of Crohn’s disease (hazard 
ratio for Crohn’s disease, 0.59, 95% CI 0.39–0.90), but 
not ulcerative colitis173. On the contrary, a meta- analysis 
of 14 case–control studies found an association between 
higher intakes of vegetables and lower risk of ulcerative 
colitis (OR 0.71, 95% CI 0.58–0.88), but not Crohn’s 
disease (OR 0.66, 95% CI 0.40–1.09), whereas a higher 
consumption of fruit was associated with lower risk 
of both ulcerative colitis (OR 0.69, 95% CI 0.49–0.96) 
and Crohn’s disease (OR 0.57, 95% CI 0.44–0.74)174. 
However, a large prospective study including 401,326 
participants recruited from across eight European coun-
tries found no associations between intakes of total fibre 
or fibre from specific sources and the development of 
IBD175; however, this study was in a population of peo-
ple recruited in their middle age, beyond the age at 
which IBD commonly develops (mean age at recruit-
ment 49.6–51.6 years, range 20–80 years). Furthermore, 
given the nature of these observational studies, recall 
bias is likely.
Only a limited number of clinical trials investigating 
the use of fibre in the maintenance and treatment of IBD 
have been undertaken, and they have been summarized 
in a systematic review165. For maintenance of remission, 
of the four studies included in patients with ulcerative 
colitis (n = 213), two found positive effects on disease 
activity: one larger RCT found continued remission at 
12 months across all groups (psyllium versus mesalamine 
Table 2 | Guidelines and recommendations for the use of fibre in gastrointestinal disorders
Gastrointestinal 
disorder
Dietary fibre recommendations level of evidence
IBS- C Soluble fibre in supplement form (e.g. psyllium, methylcellulose, 
partially hydrolysed guar gum)8,12,148–150
Meta- analyses of RCTs
Adjust fibre intake according to symptoms including adding naturally 
occurring sources (e.g. oats or linseeds)8
Professional consensus
Ground linseeds (6–24 g per day); increase gradually over a 3- month 
period9
Small number of RCTs
IBS- D Reduce intake of insoluble fibre, such as wholemeal or high- fibre flour 
and breads, cereals high in bran, and whole grains such as brown rice8
Systematic review 
of RCTs
Soluble fibre in supplement form (e.g. psyllium)148,150,204 Meta- analyses of RCTs
Adjust fibre intake according to symptoms8 Professional consensus
Inflammatory 
bowel disease
Encourage a varied diet to meet energy and nutrient requirements, 
including dietary fibre, including a wide variety of fruit and vegetables, 
cereals, grains, nuts and seeds10
Professional consensus
Consider limiting fibre and fibrous foods in patients with strictures12 Professional consensus
Dietary fibre should not be restricted, unless intestinal obstruction12 Professional consensus
Ulcerative colitis might be more amenable to fibre supplement 
interventions than Crohn’s disease165
Systematic review 
of RCTs
Diverticular 
disease
Low- fibre diet during active diverticulitis to ‘minimize irritation’12Professional consensus
High- fibre diet from mixed sources to prevent diverticulitis Observational studies 
and professional 
consensus
Functional 
constipation
Encourage gradual increases (weeks rather than days) in fibre (or by 
adding fibre supplements) to minimize gastrointestinal discomfort 
including bloating and flatulence aiming for 20–30 g per day, being 
aware that beneficial effects might be seen after several weeks12,13
Meta- analyses of RCTs
Encourage whole- fibre foods with additional components 
with laxating effects (e.g. sorbitol) such as prunes or apricots13
Small number of RCTs and 
professional consensus
High- dose psyllium (>15 g/day)124 Meta- analysis of RCTs
Guidelines are based upon research and professional consensus, but frequently the evidence is not from high- quality clinical trials. 
IBS- C, irritable bowel syndrome with constipation; IBS- D, irritable bowel syndrome with diarrhoea; RCTs, randomized controlled 
trials.
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versus psyllium plus mesalamine), whereas a smaller 
unblinded RCT found a lower rate of treatment failure 
at 12 months for psyllium plus mesalamine (28%) ver-
sus mesalamine alone (35%). Four studies in patients 
with Crohn’s disease (n = 465) found equivalence in 
the number of patients with deteriorating disease at 
24 months between the high- fibre group (mean intake 
27 g per day) and the low- fibre group (mean intake 15 g 
per day), one study found negative outcomes in patients 
consuming a high- fibre diet (33.4 ± 1.8 g per day, of 
which 2.9 ± 0.3 g per day was from raw fruit and vegeta-
bles) with markedly higher treatment failure and shorter 
time to relapse (1.4 versus 2.8 months) than patients 
consuming a low- fibre exclusion diet165. By contrast, an 
observational cohort study in 1,619 patients with IBD 
found that a higher fibre intake was associated with 
reduced risk of flare in those with Crohn’s disease 
(adjusted OR 0.58, 95% CI 0.37–0.90), but not in those 
with ulcerative colitis (adjusted OR 1.82, 95% CI 0.92–
3.60), although intakes were measured using a 26- item 
self- reported, retrospective dietary survey176. For treat-
ment of active disease, the systematic review included 
five trials in patients with ulcerative colitis (n = 114) and 
showed positive effects of fibre (for example, germinated 
barley, combined oligofructose–inulin) on disease activ-
ity. Although five trials in patients with Crohn’s disease 
(n = 193) showed no positive effect of fibre, three studies 
showed equivalent effects of a high- fibre diet compared 
with another dietary intervention in cohorts with active, 
inactive or mixed disease stages165.
Collectively, based on these results, although there 
is limited evidence for the value of dietary fibre in the 
maintenance or treatment of IBD, dietary fibre should 
not be unnecessarily restricted in patients with IBD, 
unless intestinal strictures are present and there is risk 
of obstruction. Overall, the results suggest that ulcerative 
colitis might be more amenable to dietary fibre inter-
ventions than Crohn’s disease, potentially due to the 
formation of SCFAs at the site of disease165. Historically, 
it has been common in clinical practice to recommend 
reducing high- fibre foods during relapse, although this 
practice is not evidence- based, and patients should be 
monitored in regard to their tolerance to fibre during 
both remission and relapse.
Diverticular disease. Diverticular disease refers to 
herniation of the mucosa and submucosa through the 
muscular layer of the colonic wall177. The pathogenesis 
of diverticular disease relates to colonic smooth muscle 
overactivity, thickening of the colonic wall and/or genet-
ics, in addition to lifestyle factors such as fibre intake 
and physical activity, although associations remain 
inconsistent178. The incidence of diverticular disease is 
highest in economically developed countries with cases 
continuing to increase, and is strongly associated with 
age, with a prevalence of 5% in those under 40 years 
and up to 65% in those over 65 years179. Although the 
majority of patients with diverticular disease remain 
asymptomatic (~80%), inflammation of a diverticulum 
presents as diverticulitis that can vary in duration and 
severity, but can be complicated by fistulae, abscesses, 
obstruction and perforation180.
A prospective cohort study in 46,295 men investigat-
ing the risk of developing diverticular disease, found a 
positive association between a Western dietary pattern 
and an increased risk of diverticulitis, in particular, 
a higher consumption of red meat and lower consump-
tion of dietary fibre181. Similar results were found in the 
Million Women Study including 690,075 women in 
the UK whereby the relative risk of diverticular disease 
for a 5 g per day greater fibre intake was 0.86 (95% CI 
0.84–0.88). Notably, the source of fibre seemed to influ-
ence disease risk whereby an additional 5 g per day of 
fibre from fruit (RR 0.81, 95% CI 0.77–0.86; P < 0.01) or 
cereals (RR 0.84, 95% CI 0.81–0.88; P < 0.01) was associ-
ated with significant reductions in risk, an additional 5 g 
per day of fibre from vegetables was not associated with 
risk (RR 1.03, 95% CI 0.93–1.14; P = 0.634) whereas an 
additional 1 g per day of fibre from potatoes was asso-
ciated with a greater risk (RR 1.04, 95% CI 1.02–1.07; 
P = 0.002)182. By contrast, one observational case–control 
study in 2,104 participants undergoing colonoscopy 
found that a high- fibre diet was not protective against 
diverticular disease183; in fact, those with the highest 
quartile of fibre intake showed an increased prevalence 
of diverticular disease (prevalence ratio 1.30, 95% CI 
1.13–1.50). However, it is important to note that dietary 
intake was measured up to 12 weeks following colon-
oscopy and it is not possible to exclude alterations in diet 
as a consequence, rather than a cause, of the diverticular 
disease diagnosis. Overall, a meta- analysis of five pro-
spective cohort studies (19,282 cases, 865,829 partici-
pants) found a reduced risk of diverticular disease, with 
a relative risk of 0.74 (95% CI 0.71–0.78) for every addi-
tional 10 g per day of total fibre intake184. Again, the level 
of protection varied with the fibre source, with an addi-
tional 10 g per day of fibre from fruit providing the great-
est protection (RR 0.56, 95% CI 0.37–0.84), followed by 
cereals (RR 0.74, 95% CI 0.67–0.81) and vegetables (RR 
0.80, 95% CI 0.45–1.44)184. The proposed mechanisms 
by which fibre reduces the risk of diverticular disease 
include increased stool bulk, decreased colonic pressure 
and therefore reduced herniation.
In terms of fibre being used in the management of 
acute diverticulitis, there is no consensus on fibre intake. 
Some guidelines suggest a low- fibre diet to ‘minimize 
irritation’12 and a gradual increase to 20–30 g per day 
through diet or as fibre supplements once inflammation 
has resolved185. These recommendations are often used 
in clinical practice, although they are based on physio-
logical rationale or uncontrolled studies, and not robust 
clinical trials. A systematic review published in 2018 rec-
ommended that patients with uncomplicated diverticu-
litis should be placed on a liberalized diet (for example, 
solid food, no bowel rest or nil by mouth), as opposed 
to dietary restrictions, and a high- fibre diet that meets 
individualized nutrient requirements, with or without 
fibre supplementation. However, it was recognized that 
recommendations were based on a limited number of 
low- quality studies186.
In terms of fibre being used in the management 
of uncomplicated diverticular disease, a systematic 
review from 2012 (reF.187) found only three RCTs of 
sufficient quality, although they yielded inconsistent 
Nature reviews | GastroenteroloGy & HepatoloGy
R e v i e w s
findings188–190. In 2019, a systematic review of nine con-
trolled or uncontrolled trials in patients with asymp-
tomatic orsymptomatic uncomplicated diverticular 
disease identified only one study investigating the effect 
of a high- fibre diet with bran on the risk of developing 
diverticulitis, but this study did not have a control group 
against which to compare the risk191. In the remaining 
studies, fibre supplementation improved stool weight 
(mean difference, MD, +42 g per day; P < 0.00001) but 
did not affect gastrointestinal symptoms (SMD −0.13; 
P = 0.16) or gut transit time (MD −3.70; P = 0.32)191. 
Overall, in those with diverticular disease there is lim-
ited evidence for an effect of fibre in preventing acute 
diverticulitis.
Functional constipation. Functional constipation is 
one of the most common functional bowel disorders 
and is characterized by symptoms of difficult or infre-
quent stool passage, or incomplete defaecation, with-
out structural cause146. Unlike IBS in which abdominal 
pain must be present, functional constipation does 
not have abdominal pain as a predominant symptom, 
and although not considered to be a serious condition, 
it can lead to complications such as faecal impaction, 
bowel perforation and haemorrhoids146, and the symp-
toms experienced are varied and place a burden on the 
patient192. At present, there are limited data available on 
the pathophysiology of functional constipation, although 
lifestyle factors, including low fibre intake and low levels 
of physical activity are associated with the presence of 
constipation and may play a role in its aetiology192. The 
majority of studies have focused on chronic constipa-
tion, the estimated global prevalence of which is 14%123, 
although variations in symptoms used in self- reporting 
constipation result in varying numbers presenting to 
their doctor193.
A number of large cohort studies have shown 
positive associations between high intakes of dietary 
fibre and stool frequency194,195. A systematic review 
and meta- analysis of seven RCTs concluded that fibre 
is effective in treating chronic constipation in adults 
compared with placebo. For example, fibre (dietary; 
for example, wheat bran) and in supplement form (for 
example, psyllium) including prebiotics (for example, 
inulin) was associated with increased stool frequency 
(SMD 0.39, 95% CI 0.03–0.76; P = 0.03) and more 
normalized stool consistency (SMD 0.35, 95% CI 
0.04–0.65; P = 0.02). In particular, subgroup analysis 
suggested that a high dose (>15 g per day) of psyllium 
was the most effective in increasing stool frequency 
and improving stool consistency124. However, fibre 
can induce other gastrointestinal symptoms such as 
flatulence (compared with placebo: SMD 0.56, 95% 
CI 0.12–1.00; P = 0.01). This meta- analysis further 
highlights the importance of choosing fibres with the 
most appropriate physicochemical characteristics to 
provide the preferred functional benefit. For exam-
ple, non- viscous, highly fermentable fibres such as 
prebiotic fibres (inulin and galacto- oligosaccharides) 
did not consistently have any benefit over placebo in 
increasing stool frequency and stool consistency. This 
is because these fibres are almost completely fermented 
in the colon so that and their water‐holding capacity is 
lost. By contrast, viscous and poorly fermented fibres 
such as psyllium retain their physicochemical charac-
teristics (such as high- water holding and gel- forming 
capacity) throughout the gastrointestinal tract59. 
Indeed, although insoluble fibres containing large 
and/or coarse particles can provide a regulatory 
benefit94,196, there are fibres (wheat dextrin and finely 
ground wheat bran) that have been shown to contribute 
only to the dry mass of stool, resulting in decreased 
stool water content and a constipating effect, potentially 
exacerbating symptoms in those with constipation197. 
This finding might, in part, explain the disparities in 
the findings of previous studies showing laxative effects 
of insoluble fibre198.
The effect of different quantities of different fibres 
on health have been previously highlighted3. One review 
that summarized the research of Burkitt, who advocated 
the intake of >50 g per day of dietary fibre for preven-
tion of chronic disease (for example, colon cancer)199, 
noted that fibre intakes of >35 g per day seem to be 
more effective in reducing chronic disease than lower 
intakes. Moreover, it has been suggested that the mini-
mal effects of fibre supplementation on health outcomes 
shown in many of the intervention trials in humans to 
date could simply reflect the insufficient quantity of fibre 
supplement provided in these studies199.
Future research
Fibre co- administration. Currently, extracted and 
isolated fibres (such as inulin or psyllium) from vari-
ous sources are commonly added to food products to 
enrich the fibre content, in order to assist people in 
achieving the dietary recommendations for dietary fibre.
It is plausible that co- administration of different 
fibres (for example, combined isolated fibres) might 
provide a ‘dual treatment’ by driving different func-
tionalities that target separate gastrointestinal features 
(that its, the correction of dysbiosis, and normalization 
of stool form and transit time), potentially maximizing 
the effect of clinically meaningful symptom improve-
ment. Yet, to date, there is limited research in the area 
of co- administration. A 3- week randomized crossover 
block design study in 19 healthy volunteers found that 
wheat bran plus resistant starch (12 g per day and 22 g 
per day, respectively) produced greater benefits (such 
as increased stool output, reduced transit time, reduced 
faecal pH (a proxy for luminal pH, for which lower 
values are associated with the suppression of potentially 
pathogenic microorganisms), and increased concentra-
tions of acetate and butyrate) than wheat bran (12 g per 
day) alone200. Furthermore, a study in animals demon-
strated that wheat bran combined with resistant starch 
versus wheat bran alone can shift fermentation distally, 
thus potentially improving the luminal environment and 
providing protective effects further along the colon201. 
In addition, an in vitro study using faecal microbiota 
from healthy donors to compare degradation profiles of 
single fibres (arabinoxylan, chondroitin sulfate, galac-
tomannan, polygalacturonic acid, xyloglucan) versus 
a combination of these showed slower utilization of 
some soluble dietary fibres when present in a mixture, 
www.nature.com/nrgastro
R e v i e w s
suggesting that this strategy could be used as a means of 
delivering fibres to the more distal regions of the colon202. 
Further research is required to determine whether fibre 
combinations might be efficacious in the management 
of gastrointestinal and other disorders. As our under-
standing of the physicochemical characteristics of 
dietary fibre advances, co- administration of known 
fibres with different functional characteristics is likely 
to offer greater therapeutic utility.
Natural fibres. Natural sources of dietary fibre are 
increasingly used as they often contain many differ-
ent fibres. For this reason, they might hold some of the 
benefits of co- administration, have synergistic effects 
and offer diverse functional characteristics that could 
offer therapeutic potential in the management of a 
range of gastrointestinal disorders. Examples of novel 
plant- based fibres include prickly pear, galactoman-
nan, plantain peel, ivy gourd, Gnetum africanum, yacon 
root, Moringa oleifera, which have unique combinations 
of different fibres and are rich sources of other bioac-
tive compounds, such as polyphenols, that have poten-
tial anti- inflammatory and antibacterial properties203. 
However, there are a limited number of studies of natural 
fibres in the management of gastrointestinal disorders.
Conclusions
Manipulating and/or increasing fibre intake is a promising 
therapeutic strategy in the prevention and management 
of many gastrointestinal disorders. Physicochemical 
characteristicssuch as solubility, viscosity and fermen-
tability drive different functionalities in the gastro-
intestinal tract, and therefore underpin their therapeutic 
potential. Current guidelines and recommendations 
reflect earlier studies that have used a wide range of die-
tary fibres with different physicochemical and functional 
characteristics. The lack of consistency and reporting of 
these characteristics in studies to date has limited the 
clinical utility of dietary fibre for managing gastrointes-
tinal disorders. There is an urgent need for well- designed 
RCTs to determine which physicochemical character-
istics, and therefore which fibre sources, and in what 
doses and durations are optimal for clinically mean-
ingful gastrointestinal health benefits. The utility of 
co- administration of different fibres with differing 
physiological effects, or novel, naturally occurring die-
tary fibres with dual physiological properties has yet to 
be explored and holds promise as a therapeutic strategy 
across several gastrointestinal disorders.
Published online xx xx xxxx
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