Fish odor syndrome – A rare metabolic condition that makes sweat smell like fish

Summary

Disease name: Trimethylaminuria (TMAU), also known as "fish odor syndrome"Affected populations: TMAU is a rare metabolic condition that causes a person to smell like rotten fish. The condition is more common in women than in men, and there's evidence that female sex hormones, such as progesterone, can exacerbate patients' symptoms.The exact prevalence of TMAU is unknown, and estimates of global cases vary greatly, ranging from 1 in a million to 1 in 200,000 people.Causes: Patients with TMAU smell like fish due to a buildup of a chemical called trimethylamine in their body. Trimethylamine is produced by bacteria in the gut as a byproduct of the digestion of certain foods, including eggs, liver, legumes and specific kinds of seafood, such as fish, squid and crabs.Related: Why can't we smell ourselves as well as we smell others?Normally, an enzyme in the body breaks down trimethylamine into an odorless chemical, known as trimethylamine N-oxide, which is then excreted via urine. This enzyme is encoded by a gene called FMO3.In patients with TMAU, though, this enzymatic process doesn't occur, so trimethylamine accumulates in the body and ends up being released in excess quantities in patients' sweat, urine and breath. This makes them smell like rotten fish.Get the world’s most fascinating discoveries delivered straight to your inbox.Most cases of TMAU are caused by mutations in the FMO3 gene that prevent the enzyme it encodes from working properly. In these instances, patients inherit the disease in an autosomal recessive manner, meaning they must inherit two copies of the mutated FMO3 gene — one from each parent — to develop the condition.More rarely, TMAU can be caused by consuming a large quantity of foods that lead to trimethylamine production. It can also result from liver failure and certain medical treatments, such as testosterone replacement therapy, which impact the processing and production of trimethylamine, respectively. Hormonal changes brought about by the m...