If a person’s body has a fishy smell, they may have trimethylaminuria. This is a rare disorder in which the body cannot 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 fishlike odor, although it can be described as smelling like other things.

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.

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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

Trimethylamine comes from foods that contain choline, carnitine, and trimethylamine N-oxide, known as TMAO. Examples of these foods include liver and eggs.

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 fishlike odor. When people are unable to break down trimethylamine as usual, the body releases the chemical through bodily fluids.

Due to this, people with trimethylamine give off a strong fishlike odor.

The main symptom of trimethylaminuria is a strong fishlike odor. The body releases excess trimethylaminuria through:

  • breath
  • sweat
  • urine
  • reproductive fluids

People may have a persistent strong odor 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 experience 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 fishlike 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.

It is an autosomal recessive disease. This means a person must have two mutated FMO3 genes, one from each parent, to develop the condition. If they have one faulty gene, they will be a carrier.

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.

These include:

  • L-carnitine, a supplement that is popular with athletes
  • choline, an ingredient used in treatments for Huntington’s and Alzheimer’s diseases
  • lecithin, an additive that features in a range of health supplements

Symptoms can occur when the FMO3 enzyme in the liver becomes unable to break down the excess quantities of triethylamine.

Various conditions can cause changes in a person’s body odor. Find out more here.

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.

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.

People can reduce the odor of trimethylamine by avoiding substances that increase production or levels of trimethylamine in the body, such as lecithin and choline.

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 a person’s urine. The National Human Genome Research Institute recommends:

  • 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 B2, may help increase any existing FMO3 enzyme activity in the body. People can take the recommended dosage 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 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 a person may be taking.

People who avoid certain foods and drinks to reduce their symptoms may benefit from working with a doctor or registered dietitian. These professionals can help them avoid nutrient deficiencies and maintain a healthy 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 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.