What does eating fibre do to your gut microbiome and your long-term health?
Fibre demonstrably feeds your gut bacteria, improves stool consistency and protects the gut wall. But how your microbiome and immune system respond varies considerably from person to person.
Fibre is literally food for your gut bacteria. Those bacteria ferment the fibre and in doing so produce short-chain fatty acids, such as butyrate. Butyrate is the primary fuel for the cells lining your gut wall and helps regulate your immune system. Soluble fibre is broken down more easily than insoluble fibre and therefore has a greater influence on which bacteria flourish. Each type of fibre has its own specific effect.
What happens with a structurally low fibre intake is telling. In mouse studies, fibre deprivation caused bacteria to start consuming the protective mucus layer of the gut wall as an alternative energy source. 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 the population level, a low-fibre diet is associated with higher rates of bowel disease, obesity and type 2 diabetes. That is an association, not a proven causal relationship.
A surprising finding from a study in healthy adults (18 participants per group, 17 weeks): eating more fibre did not increase the diversity of the gut microbiome. The existing bacteria became more active and produced more digestive enzymes, but new species did not automatically appear. The effect on the immune system was highly individual. There were three distinct individual response patterns, which were linked to how diverse a person's microbiome already was at the start. Fibre therefore does not do the same thing for everyone.
For people with constipation, a study in 242 adults (4 weeks) showed that psyllium, wheat bran and polydextrose significantly improved stool consistency: softer and easier to pass. Frequency did not improve measurably more than with placebo. Polydextrose did increase the amount of Bifidobacterium, a bacterium that is considered beneficial.
For metabolism, review studies indicate that fibre can improve blood sugar and fat metabolism via short-chain fatty acids, particularly in people with metabolic diseases. Think lower fasting blood glucose values and a more favourable cholesterol profile. A practical point: with 10 to 15 grams of extra fibre per day, bloating and flatulence are the most commonly reported side effects. Building up gradually clearly helps with that.
Based on 6 PMID sources, including randomised studies, mechanistic research and review studies. The mucus layer effects come from mouse studies. The diversity and immune data come from a small study in humans (18 participants per group). The constipation study included 242 participants. The metabolic data come largely from review studies.