People who eat identical meals display different blood glucose levels afterwards, thanks in part to differences in their gut microbes.

Large spikes in blood glucose after eating increase the risk of type 2 diabetes, so dietary guidelines rank foods based on their glycaemic index — an indicator of their effects on blood glucose. Eran Elinav and Eran Segal of the Weizmann Institute of Science in Rehovot, Israel, and their colleagues continuously monitored the diets and lifestyles of 800 people over a week, and found that meals with the same glycaemic index caused widely different glucose levels in participants. By analysing data on the participants' gut microbiomes, physical activity and other clinical factors, the team created personalized diets for 26 people and found that these resulted in lower glucose levels after meals than did non-personalized diets.

The study could partly explain the limited efficacy of universal dietary guidelines, the authors say.

Cell 163, 1079–1094 (2015)