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 Medicine 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 4

l-Carnitine supplementation enhances the synthetic capacity of gut microbiota to form TMAO.

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l-Carnitine supplementation enhances the synthetic capacity of gut micr...
(A) Plasma d3-TMAO concentrations in sequential venous blood draws after oral d3-l-carnitine challenge in vegans (n = 7) and omnivores (n = 8) at baseline, visit 1 (V1, 1 month), and visit 2 (V2, 2–3 months). Data represent mean ± SEM. A zero-inflated linear mixed-effects model reveals that plasma d3-TMAO is significantly higher after l-carnitine supplementation. (B) Plasma TMAO concentrations in vegans (n = 7) and omnivores (n =10) at baseline and following daily l-carnitine supplementation at visit 2 (V1, 1 month), and visit 3 (V2, 2 months). Boxes represent the 25th, 50th, and 75th percentiles, and whiskers represent the 10th and 90th percentiles. A repeated-measures 1-way ANOVA test was used to assess differences between baseline visits. (C) Plasma d3-γBB concentrations in sequential venous blood draws after oral d3-l-carnitine challenge in vegans/vegetarians (n = 7) and omnivores (n = 5) at baseline, visit 1 (V1, 1 month), and visit 2 (V2, 2 months). Data represent mean ± SEM. A zero-inflated linear mixed-effects model reveals that plasma d3-γBB production is not significantly higher after l-carnitine supplementation in vegans/vegetarians or omnivores.