Trimethylaminuria (TMAU; primary trimethylaminuria), also known as fish odor syndrome or fish malodor syndrome, is a rare metabolic disorder that causes a defect in the normal production of an enzyme named flavin-containing monooxygenase 3 (FMO3). When FMO3 is not working correctly or if not enough enzyme is produced, the body loses the ability to properly convert trimethylamine (TMA) from precursor compounds in food digestion into trimethylamine oxide (TMAO), through a process called N-oxidation. Trimethylamine then builds up and is released in the person’s sweat, urine, and breath, giving off a strong fishy odor or strong body odor. A variant of TMAU (secondary trimethylaminuria or TMAU2) exists where there is no genetic cause, yet excessive TMA is secreted, possibly due to intestinal dysbiosis, altered metabolism, or hormonal causes.
