Do gut microbes convert choline, carnitine, and betaine into TMAO linked with cardiovascular risk?

Dietary choline, L-carnitine, and betaine can enter a microbiota–liver metabolic pathway that produces TMAO. The biochemical pathway is well established; the clinical interpretation of a high blood TMAO concentration is substantially less specific.

Microbial and hepatic metabolism

• Choline: Under anaerobic conditions, microbial choline TMA-lyase (CutC, activated by CutD) cleaves choline to TMA and acetaldehyde. Isotope-labeled choline is converted to labeled TMA in human gut isolates and fecal communities.
• Carnitine: Human tracer and antibiotic-suppression studies support microbiota-dependent conversion, commonly through L-carnitine → γ-butyrobetaine (γBB) → TMA. Anaerobic bbu/gbu pathways mediate γBB conversion, while oxygen-dependent CntAB can directly cleave carnitine to TMA. Responses vary markedly among individuals and with dietary adaptation.
• Betaine: Anaerobic human fecal cultures convert isotope-labeled betaine to TMA, plausibly through selenium-dependent glycine-betaine reductase involving grdH. Its quantitative contribution in living humans appears less well defined than that of choline or carnitine.
• Liver: Hepatic FMO3 is the dominant TMA N-oxygenase, accounting for at least 90% of hepatic TMA oxidation. Human liver microsomes produce 2.9–9.1 nmol TMAO/min/mg protein; impaired FMO3 function reduces TMAO formation.

TMAO and cardiovascular interpretation

• Higher circulating TMAO is associated with cardiovascular events in prospective cohorts (11-study pooled adjusted HR 1.23, 95% CI 1.07–1.42; I²=31.4%), a modest association rather than proof of causality.
• TMAO is predominantly excreted in urine. Advanced CKD is associated with approximately 67.9 μmol/L higher TMAO, and TMAO correlates inversely with GFR (r=−0.45). Renal function is therefore especially important in a 71-year-old man when interpreting any result.

Bottom line

• The pathway is real, but elevated TMAO is a context-dependent marker—not a specific test of microbial metabolism or an independently validated cardiovascular-risk measure. Diet, kidney function, and hepatic FMO3 activity materially shape circulating TMAO.