Is TMAO formed from gut microbial conversion of choline and carnitine followed by liver oxidation?

Trimethylamine N-oxide (TMAO) formation is a sequential host–microbiome pathway rather than a direct effect of consuming choline or carnitine. The claim is well supported by human intervention and mechanistic enzyme evidence.

Clinical and metabolic evidence

• Human isotope-challenge studies show that ingested phosphatidylcholine and L-carnitine give rise to labeled circulating and urinary TMAO. During treatment with poorly absorbed broad-spectrum antibiotics, labeled TMAO production was nearly eliminated or abolished and returned after antibiotics were stopped, demonstrating that gut microbes are required for the TMA-generating step.
• Choline and betaine could still increase despite antibiotic suppression of labeled TMAO, consistent with microbial dependence being specific to conversion toward TMA rather than absorption of all dietary choline-derived compounds.
• The carnitine pathway includes a documented intermediate: L-carnitine → γ-butyrobetaine → TMA → TMAO. Post-carnitine TMAO production was markedly lower in vegans/vegetarians than in omnivores, indicating that habitual diet can shape microbial capacity for this metabolism.

Mechanism

• After intestinal production and absorption of TMA, the liver predominantly converts it to TMAO through flavin-containing monooxygenase 3 (FMO3). This is an NADPH- and molecular-oxygen-dependent N-oxidation reaction.
• Recombinant human FMO3 directly catalyzes TMA-to-TMAO conversion. Reduced FMO3 activity or pathogenic variants impair TMA N-oxygenation and cause trimethylaminuria, while experimental FMO3 modulation changes circulating TMAO in the expected direction.
• Other flavin-containing monooxygenases may contribute, but FMO3 is the principal hepatic enzyme.

Bottom line

• The stated pathway is scientifically well established: gut microbial metabolism converts dietary choline and carnitine—often via γ-butyrobetaine for carnitine—to TMA, and hepatic FMO3 predominantly oxidizes TMA to TMAO.