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Fibre for Constipation and Diarrhoea

Fiber has long been touted at the cure all for any bowel disorder. Whether it be constipation or diarrhoea, fiber has been seen to ‘fix your bowels’. In today’s podcast, we examine the evidence surrounding these claims, as well as look at the different fibers (soluble and insoluble) and, well basically, what is up with your sh*t!

Getting to know your sh*t.

We have ratings for everything. Unfortunately, we also rate poos. There is no other way to say it. Below is the infamous Bristol Stool Form Chart mentioned often in these notes so now is as good as time as any to explore this poo rating scale.

Where do you fit on the chart? Put simply, Type 2-4 is considered ‘normal’ so a Type 1 usually indicates constipation and any number above 5 indicates diarrhoea. The frequency of these movements is also a factor. More of this is discussed later in detail.

Fiber Facts

Misconception All Fibers Provide a Regularity Benefit

‘Regularity’ can be defined as the regular (eg, daily) elimination of bulky/soft/easy-to-pass stools. Constipation can be defined as infrequent (<3 bowel movements [BMs] per week) elimination of small/hard stools that are difficult to pass. Normal BM frequency is considered to be at least three BMs per week to 3/day. Although BM frequency is often used as a measure of regularity, it should not be the primary measure. For example, one person may strain to pass a single small, hard marble-like stool every day (eg, 7 BMs/wk), whereas another may have a bulky/soft/easy-to-pass stool every other day (eg, 3 to 4 BMs/wk). In this instance, the person with the higher BM frequency is constipated, whereas the other is not. The most important consideration for assessing a clinically meaningful regularity benefit with increased fiber consumption is evidence of a significant increase in both stool output (assessed as grams of stool per day for healthy subjects, can be assessed as grams per week in chronic constipation) and stool water content (%).

Stool water content is highly correlated with stool consistency and is the mechanism for both a stool softening effect and a stool bulking effect. There are two mechanisms by which fiber can provide a significant regularity benefit (laxative effect): large/coarse insoluble fiber particles (eg, wheat bran) have a mechanically irritating effect on large bowel mucosa, stimulating secretion of water and mucous, and soluble gel-forming fiber (eg, psyllium) has a high water-holding capacity that resists dehydration in the large bowel. For both mechanisms, fiber must resist fermentation to remain intact and present throughout the length of the large bowel (must be present in stool; prerequisite #1), and fiber must increase stool water content (prerequisite #2) leading to bulky/soft stools that are easy to pass. An appreciation for the strong correlation between stool water content and stool consistency can provide insights into why some functional fibers provide an effective regularity/laxative benefit, why some do not, and how some functional fibers can actually have a constipating effect.

It is all about the water content

Gut contents is normally a liquid (≥90% water) when it arrives in the cecum, and it is gradually dehydrated along the entire length of the large bowel, resulting in formed stool (≈75% water content) in the rectum. As discussed above, when considering the regularity/laxative effects of fiber in the large intestine, the isolated fiber must meet two prerequisites to provide a significant benefit. A fiber must resist fermentation to remain relatively intact and present throughout the length of the large bowel (be present in stool) because transit through the large bowel normally takes 1 or more days, and the large bowel is quite efficient at absorbing water along its entire length. A fiber that is readily fermented in the proximal large bowel cannot significantly affect the water content of stool in the distal bowel 1 or more days later. Exposure of the stool to the mucosa throughout the remainder of the large bowel, without the presence of intact fiber, would result in significant stool dehydration. A fiber must significantly increase the percent water content of stool, which is the primary driver for both softening stool and increasing stool bulk. The water content of stool is inversely proportional to stool viscosity.

Change water content of the stool

As stool water content decreases, stool viscosity increases exponentially: liquid stool is ≈90% water content; soft stool is ≈77% water; formed stool is ≈75% water, and hard stool is ≤72% water. This 18% difference in stool water content (from 90% to 72%) represents a 240-fold increase in stool viscosity (from liquid to hard). By increasing stool water content, an effective fiber therapy will keep stools soft/formed, and significantly increase stool bulk, both of which make stools easy to pass without straining. An ineffective fiber would either have no significant effect on stool water content/stool bulk, or would add to the dry mass of stool, which would decrease the percentage of stool water content and result in harder stools. The following sections will discuss different fiber types (eg, insoluble fiber, soluble gel-forming fiber, and soluble nonviscous fiber) as they relate to a regularity benefit/laxative effect.[1]

Not all fibers are created equally

Although it is true that some fibers provide a regularity/laxative benefit, it is not true that all fibers have this effect. As with normalizing blood lipid levels and attenuating glucose response, the official guidelines cite a few studies that suggest a laxative effect for inulin, oligofructose, and fructooligosaccharides. In theory, fermentable fibers would increase the mass of bacteria, thereby increasing stool output.

In contrast to this theory, data from well-controlled RCTs show that fermentable fibers have no effect on stool output or stool softening. One study in 36 healthy subjects showed that a high dose of polydextrose (20 g/day for 10 days) resulted in a minimal (2 g stool per gram fiber; 2 g/g) but statistically significant effect on stool output. In contrast, a similar study in 21 healthy adults, with a higher dose (21 g/day) of polydextrose for 3 weeks, showed no effect on stool output. In addition, four other studies with more reasonable doses (4 to 12 g/day) for 3 to 4 weeks also failed to show a significant effect of polydextrose on stool output or other regularity/laxative outcome measures compared to the placebo. Of 21 studies that assessed a stool softening effect, 20 showed no effect of the fiber compared with the placebo.[2]

Misconception: Insoluble Fiber has High Water-Holding Capacity (Holds Water Like a Sponge) that Provides a Regularity/Laxative Benefit

For insoluble fiber, there continues to be a misconception that the observed increase in stool water content associated with its laxative effect is due to water-holding capacity. In reality, insoluble fiber has no significant interaction with water and no appreciable water-holding capacity in the large bowel, yet it can significantly increase both stool water content (soften stools) and stool bulk. The question is, How?

Insoluble fiber (wheat bran) irritates the gut

The answer: Insoluble particles have a mechanically irritating effect on the mucosa of the large bowel, stimulating secretion of water and mucous as a defence mechanism to protect from abrasion. Insoluble fiber (eg, wheat bran) is poorly fermented, so it remains relatively intact and present throughout the large bowel. The observation that coarse wheat bran had a greater laxative effect than fine wheat bran suggested that the insoluble particles themselves may have a direct effect in the large bowel. This observation led to several studies comparing insoluble wheat bran to swallowed inert plastic particles (plastic effect) at the same grams per day dose as the wheat bran. Note that plastic particles have no water-holding capacity and are not fermented by bacteria, so any observed laxative effect would be purely mechanical in nature.

Wheat fibre irritates me!

The studies clearly showed that swallowed plastic particles, cut to match the size and shape of wheat bran particles milled to different sizes, exhibited the same laxative effect: large/coarse particles had a profound laxative effect, whereas small/smooth particles had no effect. These studies confirmed that the laxative effect of insoluble fiber was due to mechanical irritation of the mucosa, causing secretion of water and mucous, leading to bulky/soft/easy-to-pass stools. One study assessed finely ground wheat bran and showed that it added to the dry mass of stool, effectively lowering the percent stool water content, which led to harder stools and reports from healthy subjects that they developed difficult/uncomfortable BMs during the wheat bran treatment period (constipating effect).

The bottom line.

In summary, for both coarse wheat bran and coarse plastic particles, the observed increase in stool output and the stool softening effect were due to mechanical irritation of the large bowel mucosa (plastic effect), stimulating secretion of water and mucous. Large/coarse particles can provide a significant laxative effect/regularity benefit, whereas fine/smooth particles can have a constipating effect, providing a rationale for why laxative-effect clinical data for insoluble fiber may appear inconsistent. When considering insoluble fiber for a clinical study or professional recommendation, attention must be paid to the particle size/coarseness of the final marketed product. Further, the lack of water-holding capacity and the mucosa irritating effect make insoluble fiber a poor choice for attenuating symptoms in irritable bowel syndrome.[3]

Misconception: If Fiber Provides a Significant Laxative Benefit, Too Much of that Fiber Can Cause Diarrhea

In theory, this may be true for the mechanically irritating effects of insoluble fiber, particularly in patients with irritable bowel syndrome. In contrast, if a gel-forming soluble fiber can resist fermentation and retain its high water-holding capacity throughout the large bowel, it can provide a dichotomous, stool normalizing effect to soften hard stool (increase BM frequency) in constipation, and firm loose/liquid stool (decrease BM frequency) in diarrhea. Psyllium has been shown to soften hard stool/reduce symptoms in patients with chronic constipation and improve stool form/reduce symptoms in chronic diarrhea, lactulose-induced diarrhea, Crohn’s disease, and phenolphthalein-induced diarrhea. Clinical studies have also shown psyllium to be effective for normalizing stool form and reducing symptoms in irritable bowel syndrome.

Misconception: Fiber Exerts a Laxative/Regularity Benefit by Stimulating Large Bowel Motility

To understand how fiber exerts a laxative effect, it is important to understand the motor activity of large bowel, where ≈95% of motor events are segmental (mixing) pressure waves that facilitate the absorption of water and electrolytes, and the remaining ≈5% are propagating pressure waves (peristalsis) that propel contents toward the anus. Propagating pressure waves occur over a wide range of amplitudes and propagating rates, from high amplitude (>100 mm Hg), slowly propagating (≤1 cm/second), infrequent (≤6/day) pressure waves that are lumen-occluding events (propel all contents), to low amplitude (10 mm Hg), rapidly propagating (≥10 cm/second), frequent (≥30/day) pressure waves that only propel gas. Between these extremes are a range of medium amplitude/propagating rate pressure waves that propel lower viscosity substrates, like soft stool and liquids.

How rapidly a substrate transits the large bowel is a function of viscosity. Gas, the lowest viscosity present in the large bowel, is easily propelled by all propagating pressure waves, but primarily by the small/frequent/fast waves that act like a squeegee to propel intestinal gas rapidly past other luminal contents (gurgling sound). Gas can traverse the entire length of the large bowel in <30 minutes (≈14 flatulence episodes per day). Liquid stool is propelled by all but the small/frequent/fast gas waves, resulting in rapid transit through the large bowel (≈1 to 2 hours) and the potential for frequent BMs (eg, diarrhea). Formed stool is only propelled by high amplitude, infrequent, slow moving pressure waves, which is why solid contents may require days to transit the large bowel (≈1 BM per day). If stool becomes very small and hard, it may no longer be effectively propelled by normal pressure waves, and may require intervention for evacuation (eg, enema).[4]

An effective fiber for laxation does not alter large bowel motility, but instead exerts a regularity benefit by altering the viscosity of stool. With constipation, hard stools would only be propelled by a few of the highest amplitude contractions (or none at all, requiring intervention). A stool softening effect would decrease stool viscosity, making more of the existing motor events propulsive, increasing both colonic transit rate and the frequency of bulky/soft/easy-to-pass stools, thereby relieving symptoms of constipation. With diarrhea, a stool normalizing/firming effect would increase the viscosity of stools, making fewer of the existing motor events propulsive, slowing transit and decreasing BM frequency. An effective fiber can alter the viscosity of stool, thereby altering transit rate, but has no significant effect on motility in the large bowel.[5]

Soluble fibre from green bananas beats constipation in kids.

A recent study was undertaken to evaluate the effect of combinations of green banana biomass and laxatives in children and adolescents with chronic constipation.

This was a randomized study of 80 children and adolescents with functional constipation according to the Rome IV Criteria for constipation, who were divided into five groups: (1) green banana biomass alone; (2) green banana biomass plus PEG 3350 with electrolytes; (3) green banana biomass plus sodium picosulfate; (4) PEG 3350 with electrolytes alone; and (5) sodium picosulfate alone.

Primary outcome measure was the reduction of the proportion of patients with Bristol Stool Form Scale ratings 1 or 2. Secondary outcome measures were: increase of the proportion of >3 bowel movements/week and reduction of the proportion of fecal incontinence, straining on defecation, painful defecation, blood in stool, abdominal pain, and decreased laxative doses.

Based on consumption of green banana biomass alone, a statistically significant reduction was observed in the proportion of children with Bristol Stool Form Scale rating 1 or 2, straining on defecation, painful defecation, and abdominal pain. Conversely, no reduction was observed in fecal incontinence episodes/week, blood in stool, and no increase was observed in the proportion of children with >3 bowel movements/week. The percentage of children who required decreased laxative dose was high when green banana biomass was associated with sodium picosulfate (87%), and PEG 3350 with electrolytes (63%). Green banana biomass alone and associated with laxatives was well tolerated, and no adverse effects were reported.

What this means is that green banana biomass is advantageous as an adjunct therapy on functional constipation, mainly for reducing doses of laxatives.[6]  It also has the advantage of having an extremely low toxicity. Another advantage was that while the green bananas helped with constipation for the kids who needed it, it didn’t affect the kids that didn’t need a laxative effect. On the other hand, a laxative would simply increase bowel motions even in kids that don’t need it, potentially causing nutritional insufficiencies.

Is oat bran any good?

Colorectal cancer risk

Two studies were carried out in participants with a history of colorectal adenomas (Table 2). A 2-week oat-bran intervention had no significant effect on putative risk measures for colorectal cancer (colonic mucosal labelling index, mucosal labelling pattern and micronuclei per crypt) in post-polypectomy and non-polyp Canadian patients.

Similarly, 64 g/d of oat-bran for 6 weeks had no significant effect on measures of rectal epithelial cell proliferation (number of crypts, number of labelled cells/crypt column, number of cells/crypt column, total labelling index and percentage of labelled cells within compartments) in twenty Australian patients with recent adenomas.[7]

What about eating whole grain foods?

There is evidence that consumption of foods containing dietary fiber decreases the risk of colorectal cancer (CRC). Whole grains contain dietary fiber, as well as a range of micronutrients and bioactive compounds, but the association between the consumption of whole grains and the risk of CRC remains less studied.

The aim of a recent study was to investigate the association between whole-grain bread consumption and CRC incidence among Norwegian women. Dietary intake was estimated from the food-frequency questionnaires of 78,254 women in the cohort (median age: 55 years), and these women were then followed up for CRC incidence. During the 9 years of median follow-up, 795 women were diagnosed with CRC (316 proximal, 193 distal, 218 rectal).

Associations between whole-grain bread consumption and the risk of CRC (including colorectal subsites) were investigated and overall, no association (positive or negative) between whole-grain bread consumption and CRC was found.[8]

Government Fibre Recommendations

0-6 MonthsNo intake has been set
7-12 MonthsNo Intake has been set
Children and Adolecents
1-3 Years14g/Day
4-8 Years18g/Day
9-13 Years24g/Day
14-18 Years28g/Day
9-13 Years20g/Day
14-18 Years22g/Day
19-30 yr30 g/day
51-70 yr30 g/day
>70 yr30 g/day
19-30 yr25 g/day
31-50 yr25 g/day
51-70 yr25 g/day
>70 yr25 g/day
14-18 yr25 g/day
19-30 yr28 g/day
31-50 yr28 g/day
14-18 yr27 g/day
19-30 yr30 g/day
31-50 yr30 g/day

Daily Fibre Intake

Recommendations by life stage and gender

Daily Recommended Food Intake

Australian Dietary Guidelines: Recommended daily intakes

AgeVegetables and legumesFruitGrains (cereal)Lean meat, fish, poultry, eggs, nuts, seeds, legumes, beansMilk, yoghurt, cheese & alternativesAllowance for additional serves from any food group*
Girls and boys
1–2 years
Girls and boys
2-3 years
Girls 4-8 years4.51.541.51.50-1
Girls 9-11 years5242.530-3
Boys 4-8 years4.51.541.520-2.5
Boys 9-11 years5252.52.50-3
Girls 12-13 years5252.53.50-2.5
Girls 14-18 years5272.53.50-2.5
Boys 12-13 years5.5262.53.50-3
Boys 14-18 years5.5272.53.50-5
19-50 years
51-70 years
19-50 years
51-70 years
70+ years
Men 70+ years524.52.53.50-2.5
Pregnant Under 18years5283.53.50-3
Pregnant Over 18 Years528.53.52.50-2.5
Breast Feeding Under 18 Years5.5292.540-3
Breast Feeding over 18 Years7.5192.52.50-2.5

The Australian Dietary Guidelines recommend the number of ‘standard serves’ we should consume from the five core food groups each day, for a nutritious and balanced diet.

The recommended intakes are an average to aim for each day. While it is normal to eat more of some foods on some days, and less on other days, what matters most is that you eat as close to these amounts as possible. This will ensure you’re getting an adequate amount of energy and nutrients in your overall diet.

Separate Infant Feeding Guidelines exist (for healthcare workers) for children under 2 years of age, or click you can to download a brochure ‘Giving your baby the best start’.

Visit for further information on the Australian Dietary Guidelines and recommended intakes.
Recommended daily intakes

Recommended average number of standard serves per day

* Additional serves are allowed for taller or more active people. Nutrition Australia encourages people to choose additional extra serves from the five core food groups, and to limit serves of ‘discretionary foods’ to a maximum of one serve per day (approx. 600kJ).[10]

Some Problems with these guidelines

Pregnant Women – Gestational Diabetes


1 serve of grains = 125cal of carbohydrates

8 serves of grains = 1,000cal of carbohydrates

What does 30g of fibre look like?


[1] J Acad Nutr Diet. 2017 Feb;117(2):251-264. Understanding the Physics of Functional Fibers in the Gastrointestinal Tract: An Evidence-Based Approach to Resolving Enduring Misconceptions about Insoluble and Soluble Fiber. McRorie JW Jr, McKeown NM.

[2] J Acad Nutr Diet. 2017 Feb;117(2):251-264. Understanding the Physics of Functional Fibers in the Gastrointestinal Tract: An Evidence-Based Approach to Resolving Enduring Misconceptions about Insoluble and Soluble Fiber. McRorie JW Jr, McKeown NM.

[3] J Acad Nutr Diet. 2017 Feb;117(2):251-264. Understanding the Physics of Functional Fibers in the Gastrointestinal Tract: An Evidence-Based Approach to Resolving Enduring Misconceptions about Insoluble and Soluble Fiber. McRorie JW Jr, McKeown NM.

[4] J Acad Nutr Diet. 2017 Feb;117(2):251-264. Understanding the Physics of Functional Fibers in the Gastrointestinal Tract: An Evidence-Based Approach to Resolving Enduring Misconceptions about Insoluble and Soluble Fiber. McRorie JW Jr, McKeown NM.

[5] J Acad Nutr Diet. 2017 Feb;117(2):251-264. Understanding the Physics of Functional Fibers in the Gastrointestinal Tract: An Evidence-Based Approach to Resolving Enduring Misconceptions about Insoluble and Soluble Fiber. McRorie JW Jr, McKeown NM.

[6] J Pediatr (Rio J). 2018 Jan 5. pii: S0021-7557(17)30638-1. Combinations of laxatives and green banana biomass on the treatment of functional constipation in children and adolescents: a randomized study.

Cassettari VMG1, Machado NC2, Lourenção PLTA3, Carvalho MA2, Ortolan EVP3.

[7] Br J Nutr. 2014 Oct;112 Suppl 2:S31-43. Oats and bowel disease: a systematic literature review. Thies F1, Masson LF2, Boffetta P3, Kris-Etherton P4.

[8] Nutrients. 2016 Jan; 8(1): 40. Consumption of Whole-Grain Bread and Risk of Colorectal Cancer among Norwegian Women (the NOWAC Study) Toril Bakken,1,* Tonje Braaten,1 Anja Olsen,2 Cecilie Kyrø,2 Eiliv Lund,1 and Guri Skeie1