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

Abstract

BACKGROUND. l-Carnitine, an abundant nutrient in red meat, accelerates atherosclerosis in mice via gut microbiotadependent formation of trimethylamine (TMA) and trimethylamine N-oxide (TMAO) via a multistep pathway involving an atherogenic intermediate, γ-butyrobetaine (γBB). The contribution of γBB in gut microbiota-dependent l-carnitine metabolism in humans is unknown. METHODS. Omnivores and vegans/vegetarians ingested deuterium-labeled l-carnitine (d 3-l-carnitine) or γBB (d 9-γBB), 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→γBB→TMA transformation were identified. RESULTS. Studies with oral d 3-l-carnitine or d 9-γBB before versus after antibiotic exposure revealed gut microbiota contribution to the initial 2 steps in a metaorganismal l-carnitine →γBB→TMA→TMAO pathway in subjects. Moreover, a striking increase in d 3-TMAO generation was observed in omnivores over vegans/vegetarians (>20-fold; P = 0.001) following oral d 3-l-carnitine ingestion, whereas fasting endogenous plasma l-carnitine and γBB 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 γBB, whereas the second gut microbial transformation, γBB→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 γBB, and only 1 Clostridiales bacterium, Emergencia timonensis, that converted γBB 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 γBB, followed by (b) transformation into TMA via low-abundance microbiota in omnivores, and to a markedly lower extent, in vegans/vegetarians. Gut microbiota γBB→TMA/TMAO transformation is induced by omnivorous dietary patterns and chronic l-carnitine exposure. TRIAL REGISTRATION. ClinicalTrials.gov NCT01731236.

Role of gut microbial metabolites in nonalcoholic fatty liver disease

“…Metabolomic studies have found that the profile of microbial metabolites is altered in NAFLD, which is commonly considered a secondary outcome of the altered gut microbial profile . Nevertheless, a number of metabolites are produced by gut bacteria utilizing dietary nutrients as precursors, indicating that diet has an important impact on gut microbial metabolites, in addition to its capacity to remodel directly the microbial profile . Therefore, gut microbial metabolites may serve as mediators in the gut–liver axis, highlighting the involvement of host–diet–microbiota–metabolite interactions in NAFLD.…”

Gut microbiota: a promising target against cardiometabolic diseases

“…TMAO and a TMAO precursor, y-butyrobetaine, have been associated with atherosclerosis [97,109]. In addition to this, TMAO has been associated with major adverse cardiovascular events.…”

Understanding immune–microbiota interactions in the intestine

“…For example, primary intestinal organoids represent a particularly useful tool to study human epithelium–microbe interactions for several reasons, including: they can be generated directly from humans (with or without disease‐associated genetic polymorphisms); they can be genetically modified using clustered regularly interspaced short palindromic repeats (CRISPR)/Cas technology; they recapitulate important epithelial cell lineages and physical features of the intestinal tract; and they can be seeded with microbes of interest . Another promising approach is to identify factors like metabolites that can impact a response of interest and work to identify the microbes that are responsible for their production . As knowledge of the key mechanisms of host–microbiota interactions are revealed, our accuracy in predicting immunomodulatory activity based on the genomic content of a strain or community should markedly improve.…”

“…93 Another promising approach is to identify factors like metabolites that can impact a response of interest and work to identify the microbes that are responsible for their production. 94,95 As knowledge of the key mechanisms of host-microbiota interactions are revealed, our accuracy in predicting immunomodulatory activity based on the genomic content of a strain or community should markedly improve.…”