Dietary fibre – the best-supported component of food

What dietary fibre is and how it works

Dietary fibre is the part of plant food that the small intestine does not break down. It reaches the large intestine undigested, where bacteria live on it. The idea that a lack of it promotes Western diseases goes back to the surgeon Denis Burkitt, who between 1966 and 1972 brought together observations by several doctors into this thesis and was afterwards simply called “Fibre Man”.

What happens in the gut can be measured. In a randomised Stanford study over 17 weeks with 18 participants per group, the diversity of the gut flora stayed the same on a high-fibre diet, but the bacteria expanded their enzymes for breaking down fibre. The immune response followed three different patterns, depending on how diverse the gut flora was at the start. So the body responds, and not in the same way for everyone.

What the studies show

The Lancet 2019, cohort studies

Systematic review with meta-analyses, commissioned by the World Health Organization: 185 prospective studies with just under 135 million person-years, excluding people with chronic disease. Highest versus lowest intake meant 15 to 30 percent lower all-cause and cardiovascular mortality and 16 to 24 percent less coronary heart disease, stroke, type 2 diabetes and colorectal cancer. For every 8 grams more per day, deaths and new cases fell by 5 to 27 percent, depending on the endpoint. The greatest risk reduction was at 25 to 29 grams a day; the dose-response curves suggest that more could bring further benefit.

The Lancet 2019, randomised trials

58 randomised trials with 4,635 adults compared higher with lower fibre intake over a few weeks. With higher intake, body weight was 0.37 kilograms lower, systolic blood pressure 1.27 millimetres of mercury lower and total cholesterol 0.15 millimoles per litre lower. Trials with dietary supplements were excluded.

The glycaemic index in the same comparison

The same paper also examined diets with a low glycaemic index or low glycaemic load. There, risk reductions were smaller or absent, with low to very low certainty of evidence. Dietary fibre is rated moderate in the same table.

Stanford study, Cell 2021

Randomised study over 17 weeks with 18 healthy adults per group, high-fibre versus fermented-food diet. On the high-fibre diet, the diversity of the gut flora remained stable, and the bacteria expanded their fibre-degrading enzymes. The prespecified primary endpoint, a measure of the immune response, did not change; instead, three trajectories emerged, depending on the baseline flora.

Three bean feeding studies, Nutrition Journal 2011

Participants ate half a cup of beans daily for 8 or 12 weeks. In the first week, fewer than half reported more flatulence, and with black-eyed peas only 19 percent. At least seven in ten of those affected said it was gone by the second or third week. Even on the control diet without beans, 3 to 11 percent reported more flatulence.

Where the data stop

All figures on mortality and disease come from observation. People who eat a lot of fibre smoke less, move more and eat less meat; the studies adjust for this as well as they can, but it never fully succeeds. The randomised trials did not follow anyone through to a heart attack: the 15 to 30 percent apply to people who have eaten this way for decades, the 0.37 kilograms to a few weeks with a few grams more. Moderate does not mean shaky here, but that later work may still shift the result. For amounts beyond 30 grams a day, the data are missing.

The second limit concerns the shelf. What was measured was what people ate, and an apple contains not only fibre but also potassium, polyphenols, water, volume and chewing time. Nobody knows which part had the effect. Individual fibres such as psyllium husk or prebiotics have their own studies on cholesterol and blood sugar, with other endpoints; they do not inherit the big numbers.

How to do it

The goal is 25 to 29 grams a day; that is where the Lancet analysis found the greatest risk reduction. In its 2023 guideline, the World Health Organization recommends at least 25 grams, explicitly from fibre naturally occurring in food; the German Nutrition Society gives a guide value of at least 30 grams. This was measured with food, not powder: whole grains, pulses, vegetables, fruit, nuts. If you are well below that, you have the biggest lever; the curves suggest that more still helps, but that is not certain.

Increase the amount gradually and drink enough with it. In the bean studies, flatulence subsided after two to three weeks in at least seven in ten of those affected; the gut flora needs this time to adjust.

Safety

Fibre from food is not a new active ingredient but part of every plant-based diet. If you increase your intake abruptly, you will often get a feeling of fullness and flatulence, which decrease as the gut flora adapts; in the bean studies, this was over by the second or third week in at least seven in ten of those affected. Drinking enough is part of it. If you have a bowel condition, discuss a major change with a doctor first.

Sources

• Reynolds et al., The Lancet 2019 — systematic reviews and meta-analyses on carbohydrate quality, 185 prospective studies and 58 randomised trials
• WHO, guideline on carbohydrate intake for adults and children, 2023 — at least 25 grams of naturally occurring dietary fibre per day
• German Nutrition Society (DGE), reference value for dietary fibre — guide value of at least 30 grams per day for adults
• Stanford diet study — Wastyk et al., Cell 2021, high-fibre versus fermented-food diet, 17 weeks
• Three bean feeding studies — Winham and Hutchins, Nutrition Journal 2011, flatulence with half a cup of beans daily