Background: Human gut bacteria can synthesize proteinogenic amino acids and produce a range of metabolites via protein fermentation, some known to exert beneficial or harmful physiological effects on the host. However, the effects of the type and amount of dietary protein consumed on these metabolic processes, as well as the effects of the microbiota-derived amino acids and related metabolites on the host health are still predominantly unknown.\ud Scope and approach:This review provides an up-to-date description of the dominant pathways/genes involved in amino acid metabolism in gut bacteria, and provides an inventory of metabolic intermediates derived from bacterial protein fermentation that may affect human health. Advances in understanding bacterial protein fermentation pathways and metabolites generated at a global level via the implementation of ‘omics’ technologies are reviewed. Finally, the impact of dietary protein intake and high-protein diets on human health is discussed.\ud Key findings and conclusions:The intestinal microbiota is able to synthesize amino acids, but the net result of amino acid production and utilization, according to dietary patterns still needs to be determined. The amount of ingested dietary protein appears to modify both the diversity and composition of the intestinal microbiota as well as the luminal environment of the intestinal epithelium and peripheral tissues. The understanding of the consequences of such changes on the host physiology and pathophysiology is still in an early stage but major progress is expected in the near future with the investigation of host-microbe omics profiles from well-controlled human intervention studies.This works is supported by the European Union's Seventh Framework Program under the grant agreement no 613979 (MyNewGut).Peer reviewe

Background: Although high-protein diets (HPDs) are frequently consumed for body-weight control, little is known about the consequences for gut microbiota composition and metabolic activity and for large intestine mucosal homeostasis. Moreover, the effects of HPDs according to the source of protein need to be considered in this context.