'Fish odor syndrome': A rare metabolic condition that makes sweat smell like rotten fish

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.

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 menstrual cycle can also cause a transient form of TMAU.

Symptoms: Symptoms of TMAU may be present from birth or arise later in life, normally near the start of puberty (roughly around age 8 to 13 in females and 9 to 14 in males), when many hormonal changes happen.

Some patients with TMAU have a strong fishy odor all the time, while the smell may come and go for others with the condition. A patient's stress levels and diet can worsen their symptoms by increasing their sweat production and levels of trimethylamine, respectively.

TMAU is not deadly, but the condition can have devastating effects on patients' quality of life, by impeding their relationships with others and their career, for instance. These impacts can considerably impede their mental health and may lead to symptoms of depression, anxiety and suicidal thoughts in some.

Treatments: There is no cure for TMAU. However, health care providers may recommend that patients avoid foods that contain trimethylamine or substances that can be broken down into the chemical. These include milk from wheat-fed cows, as well as eggs, liver, kidney, seafood and peas.

Patients may also be advised to wash their skin with a slightly acidic soap or shampoo, to avoid strenuous exercise that causes sweating, to wash their clothes frequently and to use antiperspirant. They may also be advised to take measures to reduce their stress levels.

Additionally, doctors can prescribe low doses of antibiotics to reduce the amount of bacteria in the gut that metabolize trimethylamine. They may also prescribe activated charcoal, which binds to and reduces the amount of trimethylamine that can be absorbed from the gut. (Activated charcoal can interact with many medications, though, so it should be used with caution.)