Trimethylamine N-oxide (TMAO) is an oxidation product of trimethylamine and a common metabolite in animals and humans. In particular, trimethylamine-N-oxide is biosynthesized endogenously from trimethylamine, which is derived from choline, which can be derived from dietary lecithin (phosphatidylcholines) or dietary carnitine. TMAO decomposes to trimethylamine (TMA), which is the main odorant that is characteristic of degrading seafood. TMAO is an osmolyte that the body will use to counteract the effects of increased concentrations of urea (due to kidney failure) and high levels can be used as a biomarker for kidney problems. It has been identified as a uremic toxin according to the European Uremic Toxin Working Group (PMID:22626821). Fish odor syndrome or trimethylaminuria is a defect in the production of the enzyme flavin containing monooxygenase 3 (FMO3) causing incomplete breakdown of trimethylamine from choline-containing food into trimethylamine oxide. Trimethylamine then builds up and is released in the person's sweat, urine, and breath, giving off a strong fishy odor. The concentration of TMAO in the blood increases after consuming foods containing carnitine or lecithin (phosphatidylcholines), if the bacteria that convert those substances to TMAO are present in the gut (PMID:23614584). High concentrations of carnitine are found in red meat, some energy drinks, and certain dietary supplements; lecithin is found in eggs and is commonly used as an ingredient in processed food. High levels of TMAO are found in many seafoods. Some types of normal gut bacteria (e.g. species of Acinetobacter) in the human gut convert dietary carnitine and dietary lecithin to TMAO (PMID:21475195). TMAO alters cholesterol metabolism in the intestines, in the liver and in arterial wall. When TMAO is present, cholesterol metabolism is altered and there is an increased deposition of cholesterol within, and decreased removal of cholesterol from, peripheral cells such as those in the artery wall (PMID:23563705). Urinary TMAO is a biomarker for the consumption of fish, especially cold-water fish. Trimethylamine N-oxide is found to be associated with maple syrup urine disease and propionic acidemia, which are inborn errors of metabolism. TMAO can also be found in Bacteroidetes, Ruminococcus (PMID:26687352).