Are plant-based diets better for your gut than mixed diets?
Plant-based eating is good for your gut bacteria, but a Mediterranean diet combined with fermented foods performs at least as well. In any case, cut back substantially on processed meat and refined sugars: that is the most concrete step you can take right now.
What you eat has a rapid and measurable effect on the bacteria in your gut. The composition of your gut microbiome can shift within just a few days of a dietary change. This works in both a positive and a negative direction.
A fully plant-based diet promotes fibre-fermenting bacteria such as Roseburia and Ruminococcus bromii. A fully animal-based diet does the opposite: it increases the presence of Bilophila wadsworthia, a bacterium linked to a higher risk of inflammatory bowel disease. This shift was already visible within days in a controlled study.
Yet 'plant-based' is not automatically better than 'mixed'. A Mediterranean diet, which does include fish, dairy and sometimes meat but leans heavily on vegetables, legumes, olive oil and whole grains, also scores excellently in multiple studies. In 612 older Europeans, one year of Mediterranean eating led to greater production of short-chain fatty acids (fats that nourish the gut lining and have anti-inflammatory effects), fewer harmful bile acids and reduced frailty. Another study in people with excess weight found that after eight weeks of Mediterranean eating there was an increase in the beneficial bacterium Faecalibacterium prausnitzii and higher butyrate production, independent of how many calories participants consumed.
Fermented foods (yoghurt, kefir, sauerkraut) achieved something in a 17-week randomised study that a high-fibre diet did not: they increased microbiome diversity and reduced inflammatory markers. A high-fibre diet did increase the microbiome's enzymatic capacity to break down carbohydrates, but the effect on diversity and inflammatory markers was variable and depended on how diverse someone's microbiome already was.
Red and processed meat, heavily processed products and large amounts of refined sugars are associated with a higher risk of inflammatory bowel diseases such as Crohn's disease and ulcerative colitis. Eating more fibre specifically lowers the risk of Crohn's disease, but does not clearly lower the risk of ulcerative colitis. That distinction matters: plant-based eating does not offer the same protection for every gut condition.
All claims are based on controlled human studies and an overarching review of systematic reviews and meta-analyses (PMID 38599319). The level of evidence is consistently 'moderate': there are multiple human studies, but no large independent RCTs that directly compare plant-based versus mixed diets on hard clinical endpoints.