Six months of Mediterranean eating did not change the gut microbiome in this UK trial

The MedEx-UK randomised controlled trial tested a 24-week Mediterranean diet programme, alone or together with a physical activity programme, against people's usual lifestyle. A secondary analysis published in the British Journal of Nutrition on 29 September 2026 studied stool samples from 93 participants, 47 of whom gave samples both before and after.

Mediterranean diet scores rose clearly in both diet groups compared with the control group, while activity levels did not differ significantly between groups. Yet the researchers found no significant effects on gut bacterial diversity, the overall make-up of the bacterial community, the amounts of individual bacteria, short-chain fatty acids, bile acids or other faecal metabolites. How closely people followed the diet was not linked with the microbiome results either.

The authors offer one explanation: in a personalised programme, each person changed different parts of their diet, so microbiome responses varied widely and averaged out. A null result in a small group does not show that diet never matters for gut bacteria. It shows that a real-world diet change did not move the average microbiome in six months, and it says nothing against the diet's other benefits, which this analysis did not test.