Is TMAO produced by gut microbes and then oxidized by the liver?

TMAO is a host–microbe co-metabolite: its production commonly requires both microbial processing of dietary substrates and subsequent host hepatic oxidation. The proposed sequence is strongly supported by human mechanistic studies.

Clinical and human mechanistic evidence

• In deuterium-labeled phosphatidylcholine challenge studies, labeled TMAO appeared in plasma and urine, demonstrating conversion of a dietary choline-containing precursor into circulating and excreted TMAO.
• Broad-spectrum antibiotics nearly abolished total and labeled TMAO production; TMAO formation returned after antibiotic withdrawal and microbiota recovery. This reversible intervention provides strong evidence that gut microbial activity is required for this dietary route.
• Labeled L-carnitine studies independently support the same overall pathway: after steak plus labeled carnitine, TMAO rose in omnivores, was suppressed during antibiotic exposure, and reappeared with microbiota recovery.

Mechanistic explanation

• Gut microbes convert dietary choline-containing phosphatidylcholine and L-carnitine to trimethylamine (TMA). For L-carnitine, human follow-up work identifies a two-step microbial route through γ-butyrobetaine (γBB): L-carnitine → γBB → TMA.
• Host flavin-containing monooxygenases in the liver, including FMO3, oxidize TMA to trimethylamine N-oxide (TMAO). The tracer studies establish the net precursor-to-TMAO sequence, consistent with this established enzymatic mechanism.

Practical interpretation

• TMAO concentration is not solely a readout of microbial TMA production: saltwater fish and seafood provide preformed TMAO, and kidney clearance is important, with concentrations rising as GFR declines.
• Dietary pattern and individual microbiota composition can substantially affect production capacity.

Bottom line

• The claim is well supported: gut microbial conversion of dietary precursors to TMA, followed by hepatic FMO-mediated oxidation to TMAO, is an established human metabolic pathway.