TMAO

Treat TMAO as a research marker, not a number to chase. The reasons to keep red meat moderate and to eat it with vegetables, legumes and whole grains stand without it — see Red and processed meat.

How your body makes it

Red meat is the main dietary source of L-carnitine, and it also supplies choline, though eggs and liver supply far more. Gut bacteria turn both into trimethylamine (TMA), by two separate routes: choline through an enzyme encoded by a bacterial gene called cutC,^[1] carnitine through an unrelated system, cntA/cntB.^[2] The liver then converts TMA to TMAO, which circulates in the blood.

The case against it

Blood TMAO has been found to rise in step with the amount of plaque in the arteries, in a study summarised by a 2025 review.^[3] In laboratory work, drawn mainly from rodents, TMAO drives cholesterol build-up in the immune cells of the artery wall (turning them into the "foam cells" of plaque) and makes platelets more reactive, which could raise the risk of clots. That makes it a consistent marker with a plausible mechanism.

Why the case isn't closed

• Genetic studies haven't found a causal link. These use inherited differences in TMAO levels to get around confounding, and the ones that would show cause have not found one.^[4]
• Choline, not TMAO, predicted events in one large analysis. Across five Dutch cohorts of 7,834 people, higher blood choline went with about 17% more cardiovascular events, while blood TMAO itself did not predict them, and a genetic test of the whole pathway found nothing.^[5]
• Fish is linked to it more than meat. The same analysis linked fish, not meat, to blood TMAO — awkward for the theory, since fish is one of the foods recommended in place of red meat.^[6]

What diet trials show

Swapping red meat for plant-based alternatives lowers it fast, with trade-offs. In a crossover trial, 41 adults — a third of them with established cardiovascular disease — added a daily serving of red meat or of plant-based meat alternatives to their diet for six days each. The plant diet lowered TMAO, the trial's main measure, and slightly lowered LDL ("bad") cholesterol. Two results ran the other way: body weight rose slightly, by 0.6 kg, and so did NT-proBNP, a blood marker of strain on the heart.^[7]

Fibre taken with meat did not lower it. A randomised crossover pilot gave 13 volunteers a fibre supplement or a placebo alongside beef. The fibre reduced how much of the bacterial cutC gene was present, the first step of the choline route, but TMAO production did not differ between fibre and placebo. A lower TMAO response showed up only in a subgroup picked out after the fact, people who ate meat fewer than three times a week.^[8] The mechanism moved and the endpoint did not, in thirteen people.

What to do

1. Keep red meat moderate for the reasons that don't depend on TMAO — the cancer, diabetes and heart signals in the main article.
2. Eat meat alongside vegetables, legumes and whole grains. It is sound advice for other reasons; lowering TMAO is not yet one of them.
3. Don't cut fish over TMAO. Fish was linked to higher blood TMAO in the Dutch data and remains a good replacement for red meat.

Further reading

• Craciun S et al. Microbial conversion of choline to trimethylamine requires a glycyl radical enzyme. PNAS 2012.^[9]
• Zhu Y et al. Carnitine metabolism to trimethylamine by an unusual Rieske-type oxygenase from human microbiota. PNAS 2014.^[10]
• Andreu-Sánchez S et al. Unraveling interindividual variation of trimethylamine N-oxide and its precursors at the population level. iMeta 2024.^[11]
• Ferreira JP et al. Processed plant-based meat-supplemented diet versus red meat-based supplemented diet randomized cross-over trial Finding Optimal Oral Diet-1 (FOOD-1) trial. Sci Rep 2026.^[12]
• Haas M et al. Effect of dietary fiber on trimethylamine-N-oxide production after beef consumption and on gut microbiota: MEATMARK - a randomized cross-over study. Eur J Clin Nutr 2025.^[13]