What does eating fibre do to your gut microbiome and your long-term health?

Eating fibre is perhaps the most direct way to feed your gut microbiome. Gut bacteria ferment fibre and in doing so produce short-chain fatty acids, including butyrate, propionate and acetate. Butyrate is the primary fuel source for the cells lining the gut wall and plays a central role in regulating the immune system, both within the gut itself and beyond it. This mechanism is well supported by multiple studies. Soluble fibres are broken down more readily than insoluble fibres and therefore have a greater influence on which bacteria grow and how they function. Not all fibres do the same thing: each type has its own specific effect on the bacterial population.

What happens when you structurally eat too little fibre is telling. Mouse experiments using an artificial human gut microbiome show that, in the absence of fibre, gut bacteria begin breaking down the protective mucus layer of the gut wall as an alternative energy source. In those mouse models, this led to severe intestinal inflammation following exposure to a pathogen. This has not yet been demonstrated directly in humans, but the mechanism is biologically plausible. At a population level, a low-fibre diet -- as is common in Western countries -- is associated with a higher prevalence of bowel diseases, obesity, type 2 diabetes and metabolic syndrome. This is an association, not causal evidence.

A surprising finding from a randomised study in healthy adults (18 participants per group, 17 weeks) is that eating more fibre did not increase the diversity of the gut microbiome. More enzymes were produced to break down fibre: the existing bacteria became more active, but new species did not automatically join them. Furthermore, the effect on the immune system proved to be highly individual. Average inflammatory markers did not change, but three clearly distinguishable individual response patterns were identified, which were linked to how diverse a person's gut microbiome already was at the start of the study. Those who already had a diverse gut microbiome responded differently from those who started with less diversity. This is a small study, but the finding suggests that fibre does not do the same thing for everyone.

For people with chronic constipation, a double-blind study in 242 adults (4 weeks) shows that supplementation with psyllium, wheat bran and polydextrose significantly improved stool consistency: stools became softer and easier to pass. Frequency -- that is, how often a person uses the toilet -- did not improve measurably beyond placebo. Polydextrose, a soluble fibre, did increase the amount of Bifidobacterium in the gut, a bacterium that is generally considered beneficial. Psyllium and wheat bran showed a rise in the bacterium Anaerostipes, which may be linked to a higher bowel movement frequency, but that relationship remains associative and has not been proven causal.

For metabolism, review studies indicate that a higher fibre intake can improve glucose and fat metabolism via short-chain fatty acid production, particularly in people with metabolic diseases. Think lower fasting blood glucose levels and a more favourable cholesterol profile. This evidence comes largely from review studies, not from one large randomised trial, and should therefore be interpreted with some caution.

A practical point to bear in mind: with supplementation of 10 to 15 grams of fibre per day, bloating and flatulence are the most commonly reported side effects. These are genuine complaints, particularly for people who are not accustomed to a high-fibre intake. Building up gradually is a logical approach to limiting this, even though that recommendation does not appear literally in the studies reviewed.