Is TMAO produced from dietary choline, phosphatidylcholine, and carnitine by gut microbes and the liver?

TMAO formation is a well-established host–microbiome metabolic pathway: dietary trimethylamine-containing nutrients are processed by intestinal microbes to trimethylamine (TMA), then predominantly oxidized by the liver to TMAO.

Clinical and human evidence

• Human stable-isotope feeding studies show that labeled phosphatidylcholine produces labeled plasma TMAO. Broad-spectrum antibiotics markedly suppress this response, with recovery after antibiotic withdrawal, demonstrating that intestinal microbes are required for this dietary pathway.
• Carnitine challenge studies similarly show near-elimination of labeled and native TMAO formation during antibiotic exposure and restoration after microbiota recovery.
• Dietary pattern matters: omnivores generated more TMAO from carnitine than vegans or vegetarians, consistent with substantial person-to-person variation in the responsible microbial metabolism.

Mechanism

• Choline is directly converted to TMA by microbial choline TMA-lyase, encoded by cutC with its activating gene cutD. Phosphatidylcholine can supply choline for this route.
• L-carnitine commonly follows a microbial sequence of carnitine → γ-butyrobetaine (γBB) → TMA; human isotope evidence supports this intermediate pathway, including enrichment of γBB-to-TMA conversion among omnivores.
• In the liver, NADPH-dependent flavin-containing monooxygenase 3 (FMO3) is the dominant TMA oxidase, accounting for at least 90% of hepatic TMA-oxidizing activity in available estimates. Human liver microsomes directly form TMAO from TMA.

Physiologic corroboration

• Following oral radiolabeled TMA, >95% of urinary radioactivity was TMAO, with about 95% recovered in urine within 24 hours.
• Reduced FMO3 activity in trimethylaminuria markedly impairs TMA oxidation and increases TMA excretion.

Bottom line

• The claim is strongly supported: gut microbes convert dietary choline/phosphatidylcholine and carnitine into TMA, and hepatic FMO3 converts TMA efficiently into TMAO.