Trimethylaminuria is a rare disorder in which the body is unable to break down the chemical trimethylamine.

Trimethylamine has a very strong smell, similar to that of rotting fish. In people with trimethylaminuria, trimethylamine builds up in the body, causing it to give off a strong fish-like odor.

Trimethylaminuria does not have any associated health problems, but the strong odor may affect people socially and psychologically.

Here, we look at the causes, symptoms, and treatment of trimethylaminuria.

a man wiping sweat off his brow as it smells of fish due to TrimethylaminuriaShare on Pinterest
A person with trimethylaminuria may have sweat that has a strong fish-like odor.

Trimethylaminuria is a rare disorder that causes a person to have an excess of the chemical trimethylamine in the body.

People may also refer to trimethylaminuria as:

  • fish odor syndrome
  • fish malodor syndrome
  • stale fish syndrome
  • TMAuria
  • TMAU

The intestines produce a chemical called trimethylamine when people eat certain foods, including:

  • eggs
  • liver
  • legumes
  • fish
  • some types of vegetables

Usually, an enzyme breaks down trimethylamine as part of the digestion process.

Some people have a mutation in the gene that controls this enzyme, which prevents it from breaking down certain chemicals properly. As a result, trimethylamine builds up in the body.

Trimethylamine has a strong fish-like odor. When people are unable to break trimethylamine down as usual, the body releases the chemical through bodily fluids.

Due to this, people with trimethylamine give off a strong fish-like odor.

The main symptom of trimethylaminuria is a strong fish-like odor. The body releases excess trimethylaminuria through:

  • breath
  • sweat
  • urine
  • reproductive fluids

People may have a strong odor all of the time or a milder odor that can change in intensity.

Certain factors, such as sweating, may increase the smell. People may find that the odor worsens with exercise or stress.

Trimethylaminuria seems to be more common in females than in males. Although there is not yet a clear reason for this, researchers suggest that female sex hormones, such as estrogen and progesterone, could play a role.

Females may have more severe symptoms:

  • just before menstruation
  • during menstruation
  • after taking oral contraceptives
  • around menopause

Stress levels and diet may also play a part in triggering symptoms.

People with trimethylaminuria do not usually have any symptoms other than a fish-like odor, and the disorder does not cause any other physical health issues.

However, some people may find that the strong odor affects their mental, emotional, or social health. These individuals may socially isolate themselves or experience depression due to the condition.

Trimethylaminuria is usually an inherited condition that occurs due to an affected flavin-containing monooxygenase 3 (FMO3) gene.

A mutation in the FMO3 gene affects the FMO3 enzyme. This enzyme converts trimethylamine to trimethylamine N-oxide, which has no smell.

If the FMO3 enzyme does not work properly, the body is unable to break down trimethylamine, and the chemical builds up in the body. The body releases the strong-smelling chemical through bodily fluids, such as sweat and urine.

The symptoms of trimethylaminuria vary greatly among individuals. Researchers believe that different types of mutations in the FMO3 gene can affect the intensity of the symptoms.

In some cases, people may develop secondary trimethylaminuria from large doses of trimethylamine or products that trigger trimethylamine production.

This type of the condition can occur when the FMO3 enzyme in the liver becomes unable to break down the excess quantities of trimethylamine.

A doctor may be able to diagnose trimethylaminuria by asking a person about their symptoms and carrying out a few tests.

A urine test can show whether a person has high levels of trimethylamine in their urine.

A doctor may give the person an oral dose of choline first, as this causes the production of trimethylamine.

People may also undergo genetic testing, which can show whether there is a mutation in the FMO3 gene that causes trimethylaminuria.

There is currently no cure for trimethylamine, so treatment focuses on managing and reducing symptoms.

One of the main ways in which people can reduce the odor of trimethylamine is by avoiding certain foods that contain trimethylamine or choline, which triggers trimethylamine production.

The milk from wheat-fed cows contains trimethylamine, while foods that contain choline include:

  • eggs
  • liver
  • kidney
  • beans
  • peanuts
  • peas
  • soy products
  • brassica vegetables, such as cabbage, cauliflower, broccoli, and Brussels sprouts
  • lecithin, including fish oil supplements that contain lecithin

Trimethylamine N-oxide is present in seafood, including fish, cephalopods (such as squid and octopus), and crustaceans (such as crabs and lobsters). It is also in freshwater fish at lower levels.

Other ways to reduce symptoms include:

  • taking a small dose of antibiotics, which can reduce bacteria in the gut to help prevent the production of trimethylamine
  • taking a laxative to lessen the time that food takes to pass through the digestive tract, which can help reduce the amount of trimethylamine that the gut produces
  • if possible, avoiding situations or activities that cause excessive sweating, such as heavy exercise or emotional upset and stress

Certain supplements may help reduce the amount of trimethylamine in the urine. The National Human Genome Research Institute recommend:

  • 750 milligrams (mg) of activated charcoal twice a day, for 10 days
  • 60 mg of copper chlorophyllin after meals three times a day, for 3 weeks

Riboflavin, or vitamin B-2, may help increase any existing FMO3 enzyme activity in the body. People can take the recommended intake of 30–40 mg between three and five times a day with meals.

People can also avoid using alkaline soaps and body lotions with a high pH level. Using slightly acidic soaps or body lotions with a pH of 5.5–6.5 instead can help wash off trimethylamine more easily from the skin.

If trimethylaminuria has a psychological or social impact on a person, they should speak to a doctor or counselor. Family or relationship counseling may also be helpful.

If a person suspects that they have trimethylaminuria, they can see their doctor for a diagnosis.

A doctor can help put together a treatment plan that includes dietary and lifestyle changes.

They can also advise on any supplements and check that these will not interact with other medications that the person may be taking.

People who are avoiding certain foods and drinks to reduce their symptoms may benefit from working alongside a doctor or registered dietitian. These professionals can help them avoid any nutrient deficiencies and maintain a healthful lifestyle.

Trimethylaminuria is a rare condition that causes a buildup of the chemical trimethylamine in the body.

The body releases the excess trimethylamine through sweat, urine, breath, and reproductive fluids, giving off a strong fishy odor.

Many people with trimethylaminuria, particularly those with mild-to-moderate symptoms, will be able to reduce the fish smell with dietary and lifestyle changes.

Trimethylaminuria does not cause any other physical health problems, and people with the condition are usually in good health otherwise.

It is important that people seek help from a healthcare professional if trimethylaminuria causes social isolation, depression, anxiety, or any other psychological issues.