Beyond protein and fat, red meat also carries a small molecule called L-carnitine (choline travels a similar road).It is not evil in itself — it is a regular in muscle, helping fat produce energy.
What decides its fate is who it meets next. As an aside, red meat's risk is not only this one path: heme iron catalyzing oxidation in the gut, N-nitroso compounds (NOC) in processed meat, and heterocyclic amines and PAHs from high-heat charring are each a separate hit. This scene follows only the TMAO line.
2 · Gut microbes turn it into TMA
In the gut, L-carnitine meets the gut microbiota living there; these bacteria metabolize it as food, and one product is a small molecule called TMA (trimethylamine).Here is the key: how much TMA forms depends on which microbes you have grown. Long-term vegetarians have shifted flora far less able to turn carnitine into TMA; omnivores produce TMA more efficiently. So this step is not meat turning toxic directly — the microbes do the conversion for it.
3 · Liver oxidizes TMA to TMAO
Gut-made TMA is absorbed and carried via the portal vein to the liver, where an enzyme in liver cells, FMO3, adds an oxygen and oxidizes it into TMAO (trimethylamine-N-oxide).TMA + O → TMAO
By now the once-harmless carnitine has been relayed by the gut-then-liver assembly line into a metabolite that enters the circulation. What actually links to cardiovascular risk is TMAO, not carnitine itself.
4 · TMAO speeds atherosclerosis in mice
In mice, TMAO in the blood accelerates atherosclerosis, while in humans what stands is only that omnivores make more TMAO than vegans.Once TMAO enters the blood, it accelerates atherosclerosis in mice (Koeth 2013): it disturbs cholesterol clearance and makes macrophages more likely to become foam cells. What stands in humans is the production difference: omnivores make more TMAO than vegans.
Connect the whole line: red meat's L-carnitine → gut microbes make TMA → the liver oxidizes it to TMAO. The lesson is not to quit meat but to see why moderation and a diverse microbiome matter.