D-amino acids in dairy products and their relevance to the gut microbiota: a mini-review

D-amino acids (D-AAs) are increasingly recognized as measurable components of dietary exposure, arising mainly from microbial racemization and food-processing-related conversion. Dairy products, particularly fermented dairy foods, provide a useful model for examining this exposure because proteolysis, microbial metabolism and processing conditions can reshape enantiomer-resolved amino-acid profiles and enrich selected D-AAs. In this review, we summarize current evidence regarding the occurrence of D-AAs in dairy products, their possible gastrointestinal fate after ingestion, and their potential relevance to the gut microbiota. Available studies indicate that dairy-derived D-AAs may become more accessible during digestion and may contribute transiently to intestinal luminal D-AA pools. Moreover, at least some orally delivered D-AAs can undergo host uptake, systemic distribution and clearance. However, dietary D-AAs enter an intestinal environment that already contains substantial microbiota-derived D-AA pools and are subject to active host regulation. Thus, current evidence does not support the conclusion that dairy-derived D-AAs reproducibly remodel the gut microbiota. Rather, they are better viewed as minor but potentially relevant, context-dependent contributors at the diet–microbiota interface. From a nutritional and safety perspective, dairy-derived D-AAs do not currently appear to pose major concerns under normal consumption conditions. Future studies should use standardized chiral analytical methods and well-controlled human dietary intervention studies to clarify exposure, gastrointestinal fate and functional relevance.