Does circulating TMAO link dietary choline/carnitine and gut microbes to higher cardiovascular risk?

Circulating trimethylamine N-oxide (TMAO) is a meta-organismal metabolite that serves as a prominent biomarker for cardiovascular health, linking dietary intake and gut microbial activity to host vascular pathology.

Biological mechanisms

• Microbial fermentation: Intestinal microbiota process dietary choline and L-carnitine under anaerobic conditions. Choline is cleaved by the glycyl radical enzyme complex choline-TMA lyase (CutC/D), while L-carnitine is processed by carnitine monooxygenase (CntA/B) to yield trimethylamine (TMA).
• Hepatic oxidation: Following absorption into the portal circulation, the host hepatic enzyme flavin-containing monooxygenase 3 (FMO3) catalyzes the conversion of TMA into circulating TMAO.

Clinical evidence and cardiovascular risk

• Cardiovascular events: Elevated circulating TMAO is consistently linked to increased risk of incident atherosclerotic cardiovascular disease (ASCVD) and major adverse cardiovascular events (MACE). In primary prevention, individuals in the highest TMAO categories exhibit a 20% to 30% higher risk of ASCVD, whereas in secondary prevention cohorts, elevated levels are linked to a 2- to 5-fold higher risk of recurrent events and mortality.
• Causality and subclinical disease: Longitudinal data from the CARDIA cohort show no independent association between TMAO and 10-year coronary artery calcium (CAC) progression in young-to-middle-aged adults. Furthermore, Mendelian randomization studies indicate that TMAO is likely a downstream biomarker rather than an upstream causal driver of disease, and major clinical guidelines do not currently recommend routine testing.

Bottom line

• Circulating TMAO is produced via a dual-genome pathway requiring gut microbial generation of TMA (via CutC/D and CntA/B) and host hepatic oxidation (via FMO3). While it is a robust prognostic biomarker for ASCVD and MACE, current evidence indicates it functions as a predictive marker rather than an established causal target.