Does eating a varied diet help your gut bacteria?

Fermented products such as yoghurt, kefir and kimchi are the clear winner when it comes to bacterial diversity. A 17-week randomised study showed that people who ate large amounts of fermented foods gradually developed greater variety in their gut bacteria and had lower inflammatory markers in their blood. That is striking, because the group that ate a lot of plant-based fibre did not see that effect: their bacterial diversity did not rise measurably, although the bacteria did become more active in breaking down carbohydrates. Whether fibre helps you therefore depends on which bacteria you already have. People who already have a diverse gut microbiome appear to benefit more from additional fibre.

Eating broadly varied meals including wholegrains, legumes, broccoli, avocado and fermented dairy is associated, in a large American population study (nearly 3,900 people), with higher levels of substances that indicate a healthier gut flora. This is an observational association, not proof of cause and effect, but it aligns with what the randomised research shows.

What you do NOT eat matters at least as much. A lot of red meat, processed meat and white bread are linked to a less favourable gut flora. And there is a specific risk from heavily processed products: an emulsifier found in many ready-made meals (carboxymethylcellulose, or CMC) caused reduced bacterial diversity, fewer beneficial fatty acids in the stool and more gastrointestinal complaints in as little as 11 days. In two of the seven users, bacterial material penetrated the intestinal wall, a sign of inflammation. That is a concrete warning signal for anyone who eats a lot of processed products.

The underlying mechanism is well established: gut bacteria convert fibres, proteins, plant compounds and other food components into useful substances such as short-chain fatty acids and vitamins. What you eat therefore directly determines what your gut bacteria produce for you. Animal research also suggests that the ratio of proteins, carbohydrates and fats in your diet has even more influence on your gut flora than your genetic make-up, but this still needs to be confirmed in humans.