What do fibres do to your gut bacteria?
Fibres change your gut bacteria selectively: they stimulate beneficial bacteria such as Bifidobacterium, but do not automatically make the bacterial community more diverse. Fructans and galacto-oligosaccharides have the strongest backing for this effect.
Fibres clearly and consistently increase the amount of Bifidobacterium in your gut. That finding came from 64 studies combined, covering more than 2,000 people. Lactobacillus increases too, but that effect is noticeably smaller.
Not all fibres are equally good at this. Fructans (such as inulin, found in chicory and onion) and galacto-oligosaccharides are the most effective types. Inulin also raises the levels of Anaerostipes and Faecalibacterium, two bacteria that are beneficial for the gut lining, while at the same time lowering the level of Bacteroides. The change is therefore selective, not broad.
Something that surprises many people: fibres do not automatically make your gut bacteria more diverse. The total number and variety of bacterial species do not change in any measurable way. Fibres favour certain species, but that sometimes comes at the expense of others.
Fibres also prompt bacteria to produce short-chain fatty acids, substances that are good for the gut lining. Butyrate appears to increase slightly, but the results are not consistent. Whether you actually produce more of it depends on which fibres you eat, in what quantity, and what your gut bacteria already look like.
A separate point on safety: fibre supplements in high doses can cause digestive complaints, such as increased gas or a bloated feeling. A small study found that a combination of fibres and polyphenols protected the gut lining under extreme conditions, but also caused more digestive discomfort. Fibres from ordinary food are well tolerated by most healthy people; with supplements it is wise to build up slowly.
This answer draws mainly on a large summary of 64 studies (covering more than 2,000 participants in total), supplemented by several other overviews on inulin and specific fibre types. The evidence for the effect on Bifidobacterium is solid; the evidence on diversity and short-chain fatty acids is less consistent. The researchers behind the chicory-fibre study report a commercial interest on the part of one of the authors.
What does a high-fat diet do to your gut microbiome?
A high-fat diet noticeably disrupts the gut microbiome: less bacterial diversity, more harmful species and a leaky gut wall that fuels low-grade inflammation. That relationship is consistent in animal research and plausible in humans, so if you want to protect your gut health, it pays to keep an eye on the proportion of fat in your diet.
What does alcohol do to your gut microbiome?
Chronic heavy drinking demonstrably disrupts the gut microbiome, with consequences for the heart and liver. Moderate drinking was not examined in these studies, but the mechanisms identified provide a clear picture of how harm arises.
What does smoking do to your gut and gut microbiome?
Smoking disrupts your gut microbiome and increases the risk of intestinal inflammation, gastrointestinal complaints and certain forms of cancer. Quitting smoking is the most direct way to reduce these risks.
Can your gut microbiome influence inflammation in your body?
Your gut microbiome can influence inflammatory processes throughout your body via the gut wall. Eating more fibre and exercising regularly are the most well-supported ways to influence this favourably.
Can your gut microbiome control your appetite and hunger?
Yes, your gut microbiome can influence your hunger and eating behaviour via a direct connection to your brain. Eating more fibre is the best thing you can do right now to support that route.
What does fasting one day a week do to your gut microbiome?
Intermittent fasting demonstrably changes your gut microbiome, but the effects are likely temporary if you stop; for healthy people, the benefit is not yet well substantiated.