L-Carnitine in omnivorous diets induces an atherogenic gut microbial pathway in humans

BACKGROUND.L-Carnitine, an abundant nutrient in red meat, accelerates atherosclerosis in mice via gut microbiota–dependent formation of trimethylamine (TMA) and trimethylamine N-oxide (TMAO) via a multistep pathway involving an atherogenic intermediate, &ggr;-butyrobetaine (&ggr;BB). The contribution of &ggr;BB in gut microbiota–dependent L-carnitine metabolism in humans is unknown. METHODS. Omnivores and vegans/vegetarians ingested deuterium-labeled L-carnitine (d3-L-carnitine) or &ggr;BB (d9-&ggr;BB), and both plasma metabolites and fecal polymicrobial transformations were examined at baseline, following oral antibiotics, or following chronic (≥2 months) L-carnitine supplementation. Human fecal commensals capable of performing each step of the L-carnitine→&ggr;BB→TMA transformation were identified. RESULTS. Studies with oral d3-L-carnitine or d9-&ggr;BB before versus after antibiotic exposure revealed gut microbiota contribution to the initial 2 steps in a metaorganismal L-carnitine→&ggr;BB→TMA→TMAO pathway in subjects. Moreover, a striking increase in d3-TMAO generation was observed in omnivores over vegans/vegetarians (>20-fold; P = 0.001) following oral d3-L-carnitine ingestion, whereas fasting endogenous plasma L-carnitine and &ggr;BB levels were similar in vegans/vegetarians (n = 32) versus omnivores (n = 40). Fecal metabolic transformation studies, and oral isotope tracer studies before versus after chronic L-carnitine supplementation, revealed that omnivores and vegans/vegetarians alike rapidly converted carnitine to &ggr;BB, whereas the second gut microbial transformation, &ggr;BB→TMA, was diet inducible (L-carnitine, omnivorous). Extensive anaerobic subculturing of human feces identified no single commensal capable of L-carnitine→TMA transformation, multiple community members that converted L-carnitine to &ggr;BB, and only 1 Clostridiales bacterium, Emergencia timonensis, that converted &ggr;BB to TMA. In coculture, E. timonensis promoted the complete L-carnitine→TMA transformation. CONCLUSION. In humans, dietary L-carnitine is converted into the atherosclerosis- and thrombosis-promoting metabolite TMAO via 2 sequential gut microbiota–dependent transformations: (a) initial rapid generation of the atherogenic intermediate &ggr;BB, followed by (b) transformation into TMA via low-abundance microbiota in omnivores, and to a markedly lower extent, in vegans/vegetarians. Gut microbiota &ggr;BB→TMA/TMAO transformation is induced by omnivorous dietary patterns and chronic L-carnitine exposure. TRIAL REGISTRATION. ClinicalTrials.gov NCT01731236. FUNDING. NIH and Office of Dietary Supplements grants HL103866, HL126827, and DK106000, and the Leducq Foundation.

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