Dietary Fibre: more than a moving story
Ah, yes, roughage… What is it exactly and why should I care?
You might be surprised to learn that the exact definition of dietary fibre is hotly debated, but it’s generally agreed that it’s that part of a plant-derived food which resists digestion in the small intestine, though it may be partially fermented in the large intestine. It is also almost always of carbohydrate origin. (Ref)
One of the most obvious changes between diets a century ago and today is the loss of fibre, yet we tend not to give it much thought. Australian recommended intakes for men and women, respectively, are 30g and 25g (Ref) but the 2023 National Nutrition and Physical Activity Survey found fibre intakes were only about 75% of this. Don’t feel bad, these levels are quite normal in the developed world. (Ref, Ref)
South African and Indian men are getting their fibre but nobody else is.
Source: Burke Miller (2020). (Ref)
And this matters. Compared to the people eating the least fibre, the people eating the most tend to have less heart disease, cancer, diabetes, mental health conditions, constipation, they’re slimmer. And, if you still need more, they die less. (Ref, Ref) It also seems that, while 25 to 30g prevents constipation, higher amounts are better for disease reduction, which we’ll look at in in this article.
Any drawbacks?
Fibre can bind with food minerals, most commonly calcium, magnesium, zinc, and iron, and interfere with absorption. This can be reduced through soaking, sprouting and fermenting, or combining foods with other acids, such as other fermented foods, vinegars or vitamin C. Moreover, a healthy microbial population in the large intestine can partially compensate, with onsite fermentation of fibre, liberating minerals and making them available for absorption. (Ref, Ref, Ref)
There is also the issue of fibre intolerance. Most people will notice bloating and flatogenesis (apparently this is a term) with a change in their fibre habits but people with conditions such as inflammatory bowel disease (IBD), diverticular disease and irritable bowel syndrome (IBS) are particularly susceptible. IBD, including Crohn’s disease and ulcerative colitis, requires medical management, but, in general, fibre appears helpful; as it does in managing diverticular disease (though strong evidence is lacking). (Ref, Ref) IBD and IBS can co-occur in some people and have many of the same symptoms (abdominal pain, bloating, diarrhea or constipation), however IBS is a distinct condition. (Ref) In IBS, symptoms are often exacerbated by ingestion of fermentable carbohydrates, or FODMAPs (oligosaccharides, disaccharides, monosaccharides and fermentable polyols), where oligosaccharides are a common type of fibre. (Ref) IBS seems endemic amongst some cohorts of the population and will get its own article.
Fibre: Let’s break it down.
We need to get more complex here as it’s the properties of the fibre that determine its benefits. Fibre is commonly characterised by its solubility, fermentability and viscosity, where fibres that are fermentable and viscous also tend to be soluble, while insoluble fibres are non-viscous and only selectively fermentable. When you eat a plant-based food you’re usually getting more than one type of fibre, but different types of plant foods tend to excel in particular fibres, which underlines the importance of diversity in reaping all fibre’s advantages. This table summarises the situation. (Ref, Ref, Ref)
| Characteristics | Fibre Type | Good food sources |
|---|---|---|
| Insoluble | Cellulose | cereals, vegetables, legumes, pears and apples |
| Hemicellulose (partially soluble and partially fermentable) | Whole grains, nuts, legumes | |
| Resistant starch (generally insoluble, fermentable) | Types I-IV, think of cold potatoes, pasta and rice | |
| Lignins | Strawberries, peaches, flaxseed | |
| Soluble: Viscous and fermentable | Arabinoxylans | Psyllium, flax, bamboo shoots, pasta |
| β-Glucans | Oats, barley, rye, maize, wheat, rice, mushrooms | |
| Galactomannans | Locust bean, guar, taro, and fenugreek | |
| Pectins | Apple and citrus peel, cabbage, whole grains, beetroot, legumes. | |
| Soluble: low viscosity, fermentable | Fructans (Inulin and fructooligosaccharides (FOS)) | Jerusalem artichoke, chicory root, onions, garlic. Implicated in IBS. |
| Galacto-oligosaccharides | Legumes, almond, cashew, pistachios | |
| Dextrins | Cereals | |
| Laminarins | Brown seaweed | |
| Soluble: viscous, low fermentability | Alginate | Seaweed |
Solubility is a measure of how intact the fibre molecules remain when exposed to water at body temperature. Insoluble fibre’s actions include mechanically stimulating the large intestinal wall to, ahem, lubricate the stool; entrapping some minerals, both desirable, such as iron and calcium, and undesirable, such as heavy metals (Ref, Ref); and encapsulation of calories within cell walls, shielding them from digestion. Nuts are a good example of this, where, about a third of calories may remain undigested. (Ref)
Viscosity is how thick the fibre becomes. Viscous fibres absorb water in the digestive tract, which paradoxically slows down food passage in the small intestine and keeps things moving in the large intestine. In the small intestine, this confers a feeling of fullness and slows absorption. In the large intestine, non-fermentable, viscous fibre retains its moisture and volume, so improves stool consistency and bowel movement frequency. Along its passage, a viscous fibre bolus can also absorb some unwanted or harmful elements to reduce absorption, including fat, cholesterol, and heavy metals. (Ref)
Fermentability indicates whether a fibre is consumed by the gut microbiota. Bacterial fermentation in the colon generates short-chain fatty acid (SCFA) metabolites (principally, butyric, propionic and acetic acid), the basis of the microbiome’s health benefits. The different microbiome species each have their own preferred fibre substrates and generally only produce a single type of SFCA. Hence dietary fibre diversity is important to promote microbial diversity and optimise benefits.
| SFCA metabolite | Produced by | Action | Substrate |
|---|---|---|---|
| Acetate | Bifidobacteria | ↑ satiety | Pectin, inulin, Galacto-oligosaccharides |
| Propionate | Firmicutes and Bacteroidetes and Negativicutes classes Lachnospiraceae | ↑ satiety, ↓cholesterol and lipid synthesis, ↑ insulin sensitivity | β-glucan, some dextrins and laminarins |
| Butyrate | Firmicutes, Actinobacteria, Fusobacteria and Proteobacteria | ↓ inflammation, ↓ cancer | Resistant starch, FOS, acetate, propionate |
SFCAs apparently do it all! Some of these mechanisms are still not entirely established however.
Data source: Xiong et al (2022). (Ref) COX-2: cyclooxygenase-2; GPR41: G-protein-coupled receptor 41; IL: interleukin; NF-κB: nuclear factor-kappa B; SITR: small intestinal transit rate (this should be large intestinal transit rate!); WCF: water content of faeces.
Fibre health benefits – almost a panacea
Before we get started, bear in mind that eating more fibre entails eating less processed food and more phytonutrients and may also correlate with other healthy lifestyle habits. When using population studies, researchers invariably adjust their results for the most obvious health factors, like, age, exercise, weight and smoking but undoubtedly there are corollary benefits that come with high fibre foods that flatter the data.
Without doubt, advantages are partly derived from fibre’s relationship with lower body weight and lower chronic inflammation, which are both implicated in chronic disease development (and indeed are themselves interrelated (Ref)).
Weight management
As discussed in this article, higher fibre diets tend to lower energy density and slow eating rate, so appetite signals can be recognised before too much is eaten. Fibre-rich food is also digested more slowly and, if still unabsorbed nutrients reach the furthest part of the small intestine (the ileum), this signals to the body that it has enough digesting work on its hands and precipitates the release of appetite-reducing hormones, known as the “ileal brake”. (Ref) In addition, SFCAs may have a systemic effect on fat accumulation. (Ref, Ref)
The additive effect, however, is very small and supportive evidence is largely observational. One large European study found that per 10 g/day higher fibre intake, annual body weight and waist circumference gains were 39g and 0.08 cm less, respectively (you’d need very large numbers, which they had, to get statistical significance, which they did, from such a small difference!). Cereals (wheat, rye, barley etc) showed the greatest effect ( -77 g/y, -0.10 cm/y). (Ref)
Inflammation
As for inflammation, butyrate is particularly active in the colon in maintaining the ideal anaerobic environment, which suppresses colonic inflammation. (Ref) Systemically, SFCAs may also reduce inflammation. In one observational study, the tertile with the highest fibre intakes had 36% and 43% lower levels of two blood inflammatory markers plasminogen activator inhibitor-1 and resistin. While a British study found an inverse association between fibre intake and a common blood inflammatory marker, C-reactive protein (CRP). (Ref)
The three tables below summarises large studies examining the impact of an incremental dose of fibre on population disease risk.
| Condition | Fibre increment | Risk reduction |
|---|---|---|
| g/d | ||
| Mortality | 10 | 11% |
| Cardiovascular disease | 7 | 9% |
| Stroke | 7 | 7% |
| Gastric cancer | 10 | 44% |
| Colorectal cancer | 10 | 11% |
| Colon cancer | 7 | 7% |
| Rectal cancer | 7 | 9% |
| Colorectal adenoma | 10 | 9% |
| Breast cancer | 10 | 5% |
| Ovarian cancer* | 5 | 6% |
| Kidney cancer* | 10 | 6% |
| Endometrial cancer | 5g/1000kJ | 18% |
| Condition | Fibre increment | Risk reduction |
|---|---|---|
| g/d | ||
| Pancreatic | 10 | 12% |
Source: Mao et al (2017). (Ref)
| Condition | Fibre increment | Risk reduction |
|---|---|---|
| g/d | ||
| Haemorrhagic stroke | 10 | 14% |
| Ischaemic stroke | 10 | 17% |
Source: Chen et al (2013). (Ref)
Cardiovascular Disease (CVD)
In the latest data (2023), CVD remained the single biggest cause of death in Australia.
Ranked causes of death in Australia in 2023
Source: ABS (2024). (Ref)
Improved insulin sensitivity, lower body weight and a less inflammatory milieu, create a favourable cardiovascular environment but fibre appears to have benefits beyond the metabolic, specific to the cardiovascular system.
Much of the advantage is likely due to viscous fibre’s ability to reduce blood cholesterol, with β-glucan, psyllium and guar gum particularly viscous and therefore particularly effective. The mechanism is as follows: viscous fibre entraps cholesterol-containing bile (necessary for fat digestion) in the intestine, from whence it is eliminated in the stool. Normally, bile is reabsorbed further down in the digestive tract and recycled via the liver, but lacking resorbed bile, the liver must draw cholesterol out of the blood to make more bile. Consequently, blood cholesterol, particularly the more harmful low-density lipoprotein cholesterol (LDL-C), is reduced. (Ref) Butyrate may also offer a direct benefit via its anti-hypertensive effect and reduction of atherosclerotic plaque inflammation. (Ref)
The following chart illustrates the relationship between fibre intake and coronary heart disease (a large subset of CVD) and suggests fibre intakes above currently recommended amounts may be more protective.
Coronary heart disease is still going down at 27g/d, so maybe we need more?
Source: Pereira et al (2004). (Ref)
Cancer
Organ specific cancers with strong or moderte evidence for the benefits of dietary fibre
Source: Hu et al (2023). (Ref)
Its may be helpful to also know which of these cancers are the most dangerous. You note that colorectal cancer is the seventh leading cause of death overall in Australia, though breast and prostate cancer carry much greater sex-specific risk.
Organ specific cancer mortalities as a proportion of all cancer deaths
Source: ABS (2024). (Ref)
In the colorectum
Short chain fatty acids, particularly butyrate, foster the protective mucous layer and reduce free radicals which damage colon cell DNA. (Ref) Meanwhile, fibre can decrease stool transit time, while both soluble and insoluble fibre can bind toxins. In concert, these three mechanisms, reduce colon cell contact with carcinogenic compounds. (Ref) The European Prospective Investigation on Cancer study found colorectal cancer incidence was 42% lower between people with the lowest fibre consumption (15g/day) and the highest (35g/day). (Ref)
The relationship between daily fibre intake and colorectal cancer risk. Likewise higher fibre intakes than recommended are suggested here.
Source: Aune et al (2011). (Ref)
In the rest of the digestive tract
Along, the digestive tract, many of the cancer suppressive properties of fibre may lie in its carcinogen binding capacity, for instance in the acidic gastric environment, fibre can bind and neutralise nitrite, a carcinogen present in preserved meats and a byproduct of meat breakdown. Meanwhile, in the oesophagus, protective effects may stem from fibre’s protective effects against oesophageal reflux. (Ref)
And elsewhere
Fibre’s protective effects against breast cancer pertain to both pre and post menopusal women with soluble dietary fiber slightly more protective than insoluble. (Ref) Lower breast cancer, along with lower rates of endometrial cancer, likely stems from reduced circulating oestrogen, which, like cholesterol, is excreted in bile, then bound in soluble fibre and eliminated. Indirectly, lower cholesterol, a substrate for oestrogen production, may also be a factor.
The risk of female reproductive (breast, endometrial and ovarian) and prostate cancers, along with pancreatic cancer is also linked to glycaemic load and insulin sensitivity, which are both inversely linked to fibre intake. (Ref, Ref)
Type 2 Diabetes (T2D)
Look back at the earlier chart on causes of Australian deaths and note than diabetes ranks in sixth place. Body fat and inflammation are key contributors to T2D and are themselves interrelated. Visceral adiposity (internal abdominal fat) is particularly undesirable as it is pro-inflammatory, which is generally understood to explain its close relationship to chronic disease, especially, T2D. While age, gender, genetics and race can influence a propensity for visceral adiposity, overall body fat is an overriding contributor. (Ref, Ref)
Increasing visceral adiposity increases diabetes risk.
Source: Zhou et al (2024). (Ref) VAI: visceral adiposity index
Beyond body weight and inflammation, the capacity of viscous fibre to slow nutrient absorption may work in the two-hour period post meals thus: ↓ post meal glucose peak —> ↓ insulin release —> ↓ reactive hypoglycaemia —> ↓ hunger. (Ref) So the pancreas is conserved and fewer calories may be ingested. Slower nutrient absorption in the intestine means that, some hours later, when the food reaches the last section of the small intestine (the ileum), nutrients are more likely to be incompletely absorbed. This stimulates the release of GLP-1 (of weight-loss drug fame), which has insulin modulating effects. (Ref)
Moving further down the digestive tract into the colon, fibre stimulates SFCAs, which have many posited actions (this is a fast-moving area of research). Some proposed mechanisms include the systemic, anti-inflammatory actions of butyrate, acetate’s stimulation on GLP-1 receptors in the brain (recall GLP-1 was first exploited as a diabetic target before the weight loss industry got very excited indeed) and propionate’s stimulatory effect on insulin output. (Ref)
The dashed lines are confidence intervals (researchers are 95% confident the actual range lies within the dashed range). As with CVD and colorectal cancer, this suggests fibre intakes above currently recommended amounts are more protective.
The relationship between fibre intake and diabetes mortality. Again, higher than doses than dietary recommendations appear indicated.
Source: Reynolds et al (2020). (Ref)
Mental Health
The microbiota of people with mental health disorders is frequently characterised by fewer anti-inflammatory bacteria and more pro-inflammatory bacteria than that of the wider population. (Ref) This may be effect as much as cause, but never the less, an inflammatory environment is as unhelpful to the neural system as elsewhere. Along with modulation of neuroinflammation, bacteria such as Bifidobacterium and Lactobacillus, appear to themselves produce neurotransmitters, including serotonin, which directly influence the brain. (Ref)
However, with regard to mental health, the evidence is, as yet, less abundant. Nevertheless, observational studies show a significant inverse relationship between fibre intake and both anxiety and depression. (Ref) Randomised controlled trials using fibre supplements failed to find an benefit however, which may be due to trial duration but also suggests than other companions to fibre (low processing, phytonutrients etc) factor into mental health. (Ref)
Constipation
Constipation is a function of both regularity (>3 times per week) and stool consistency. The Bristol Stool Chart is a popular affixture to many a GP’s wall. It’s funny and unlovely but a valuable tool to assess normalcy and removes the need for too much discussion on a subject many may find embarrassing. You’re aiming for the middle (3-5).
The Bristol Stool Chart.
Source: Research Gate (Ref)
It’s not a small issue either. In an Australian cross-sectional survey, chronic constipation was self-reported by almost a quarter of Australian adults, with women almost 50% more likely than men to be affected. (Ref) And, yes, fibre is a key pillar in promoting laxation, as it promotes a more hydrated stool, to confer a softer consistency. Softer consistency also increases propulsion rate through the bowel and thus bowel movement regularity. Fibre acts on two fronts: insoluble non-fermentable fibre, particularly large, coarse particles, stimulate water and mucous secretion in the large bowel, to soften stools. Interestingly though fine insoluble fibre can actually have a constipating effect. Meanwhile, (non-fermentable) viscous fibre retains water in the colon to deliver stool water content. Clearly water is a key player too thus adequate hydration helps.
Fibre fermentation disarranges the fibre molecular structure and thus negates any potential laxative effects of these fibres. Good pro-laxative fibres include psyllium (the active ingredient in Metamucil) and coarse wheat bran. (Ref)
Conclusion
Are you convinced fibre’s the best thing since since in sliced bread? This article reflects the current state of play but the science, especially that regarding SFCAs, is rapidly evolving. For practical application, fortunately, fibre is mandatorily included on the food label nutrition panel. But high fibre doesn’t mean it’s not highly processed – manufacturers have latched onto this one. I’ll tell you what you know already – include a generous diversity of minimally processed plant based foods in your diet, being mindful of your comfort, and you’ll be heading in the right direction. If you have one of the nutrition tracking apps I mentioned here, then you could consider tracking your fibre intake for a few days. And this is an excellent resource from Food Standards Australia New Zealand: AUSNUT 2023 – Food nutrient profiles (Excel, 1.4MB)*. To feel good about your efforts, your bathroom habits are obviously a key performance indicator, but you can also look to blood cholesterol, H2A1c, CRP and waist circumference for some positive affirmation over three months or so (improvements will be subtle, so manage your expectations). Please feel free to pass on this article to your friends and if you would like individual advice, I’m happy to help.
*The prior version (AUSNUT 2011-2013) had more granular fibre detail but I suspect has been removed for accuracy reasons. Shame though, as it appears to have been expunged from the internet entirely, so I can’t share.
