l-Carnitine in omnivorous diets induces an atherogenic gut microbial pathway in humans
Robert A. Koeth, … , Jose Carlos Garcia-Garcia, Stanley L. Hazen
Robert A. Koeth, … , Jose Carlos Garcia-Garcia, Stanley L. Hazen
Published December 10, 2018
Citation Information: J Clin Invest. 2019;129(1):373-387. https://doi.org/10.1172/JCI94601.
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Clinical Research and Public Health Cardiology Vascular biology

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 microbiota–dependent 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 (d3-l-carnitine) or γBB (d9-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 d3-l-carnitine or d9-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 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 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. FUNDING. NIH and Office of Dietary Supplements grants HL103866, HL126827, and DK106000, and the Leducq Foundation.

Authors

Robert A. Koeth, Betzabe Rachel Lam-Galvez, Jennifer Kirsop, Zeneng Wang, Bruce S. Levison, Xiaodong Gu, Matthew F. Copeland, David Bartlett, David B. Cody, Hong J. Dai, Miranda K. Culley, Xinmin S. Li, Xiaoming Fu, Yuping Wu, Lin Li, Joseph A. DiDonato, W.H. Wilson Tang, Jose Carlos Garcia-Garcia, Stanley L. Hazen

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Figure 3

γBB is a major gut microbiota metabolite of l-carnitine, and TMA formation from γBB is influenced by dietary habits.

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γBB is a major gut microbiota metabolite of l-carnitine, and TMA formati...
(A) Plasma d3-TMAO and d3-γBB concentrations in vegans/vegetarians (d3-TMAO, n = 9; d3-γBB, n = 9) versus omnivores (d3-TMAO, n = 15; d3-γBB, n = 12) participating in an oral d3-l-carnitine (250 mg) challenge. The left panel illustrates the marked differences in d3-TMAO generation previously reported in omnivores versus vegans/vegetarians. The right panel shows a small difference in plasma d3-γBB concentration between omnivores and vegans/vegetarians. Data represent mean ± SEM. A Mann-Whitney test was used to compare the AUCs between dietary groups. (B) Box-and-whisker plots of fasting plasma concentrations of γBB from subjects (n = 9) before versus after 1 week of oral broad-spectrum antibiotics to suppress gut microbiota. Boxes represent the 25th, 50th, and 75th percentiles, and whiskers represent the 10th and 90th percentiles. Differences were assessed using a Wilcoxon matched-pairs test. (C) Fasting plasma concentrations of γBB in vegans/vegetarians (n = 32) versus omnivores (n = 40). Boxes represent the 25th, 50th, and 75th percentiles, and whiskers represent the 10th and 90th percentiles. A Mann-Whitney test was used to assess differences between groups. (D) Baseline human fecal metabolite studies in vegans/vegetarians and omnivores (n = 10 each group). Fecal samples were incubated anaerobically with d3-l-carnitine, and d3-TMA and d3-γBB were quantified by LC-MS/MS. Data are expressed as mean ± SEM. A Mann-Whitney test was used to assess differences between groups. (E) Baseline human fecal metabolite studies in vegans/vegetarians (n = 10) versus omnivores (n = 10). Fecal samples were incubated with d3-l-carnitine or d9-γBB as indicated. Metabolites were quantified by LC-MS/MS. Data are expressed as mean ± SEM. A Mann-Whitney test was used to assess differences between groups.