Metabolic disorders such as obesity and type 2 diabetes (T2D) represent a growing unmet clinical need. Accumulating evidence shows that the collection of microbes residing within the human intestinal tract influence host metabolism. Diet is one of the most important factors shaping the gut microbiome. So far, mainly microbial metabolism of dietary fibers has been studied, with less emphasis on dietary proteins. Here we investigated how existing metagenome data available from different ethnicities is associated with different dietary patterns and test how they respond to diets high and low in proteins. To this end, we first performed cross-sectional analysis of two European cohorts (MetaCardis, n = 1759; HELIUS, n = 1528). There was no significant association between protein intake (total, animal, or plant) with either gut microbiota alpha diversity or beta diversity, regardless of ethnicity, but daily intake of (animal) protein was associated with glucose and lipid metabolism and T2D, which was independent of ethnic background. Next, we performed a dietary intervention with high or low protein for 12 weeks to invesitage the effect on the microbiota in individuals of Caribbean or Caucasian descent as well as with or without T2D, but did not find any effect on weight, glucose and lipid metabolism, nor microbiota composition. However, we did observe a significant change in plasma metabolites eg phenylacetylglutamine was increased following high protein intake, whereas indole 3 propionic acid was increased inlow protein diet. Third, using bioreactors we observed that the gut microbiota is associated with altered production of amino acid derived metabolites, especially branched chain and aromatic amino acids. However, we did not observe strong effects of media containing low or high protein content, which may be explained by that the low-protein media may contain too high protein levels to detect a difference, or alternatively the chemical environment is different in the bioreactors. One main focus in the grant was to further explore how the gut microbiota produces imidazole propionate and we first validated our findings in different ethnicities. Finally, we adminsiteredoral histidine before and after oral antibiotics to individuals with or without T2D an surprisingly found that ImP levels increased after antibiotics, suggestig that the low diverse envrionment promote ImP production. In summary, we have found that dietary protein intake has no major impact on gut microbiota composition and that production of bioactive metabolites is determined by both microbial composition, chemical environment, and substrate availability. Exploring this interaction more closely may provide increased understanding for how gut microbiota affects protein metabolism and production of bioactive amino acid derived metabolites.

The consortium has identified that diet, including animal protein, is associated with a small but significant effect on gut microbiota compositionFurthermore, we have demonstrated that the gut microbiota can produce bioactive metabolites from amino acids, but that these may not primarily originate from the diet, but rather by altered metabolic capacity in the microbiota.

Indeed, patients with type 2 diabetes have an altered microbiota that produce increased levels of imidazole propionate, which is confirmed in patients of different ethnicities. Interestingly, antibiotics change the dynamics in the gut leading to increased levels of imidazole propionate and we are currently exploring which bacteria that are responsible. Collectively, we found that protein composition (notably animal proteins) in human diet have a relatively small effect on gut microbiota composition, but that the gut microbiota in type 2 diabetes processes histidine alternatively, which may contribute to diabetes development both in subject with a Caucasian or Caribbean background.