Trimethylaminuria (TMAU), also known as fish odor syndrome or fish malodor syndrome, is a rare metabolic disorder that causes a defect in the normal production of the enzyme 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 break down trimethylamine (TMA) from precursor compounds in food digestion into trimethylamine oxide (TMAO) through a process called N-oxygenation. 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. The odor is not necessarily fishy, and the type of odor can vary from time to time. In that sense, the alternate names referring solely to a fishy odor are misnomers.

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