Fish Odor Syndrome (Trimethylaminuria)

*Fish odor syndrome (trimethylaminuria) facts Medically Edited by: Charles P. Davis, MD, PhD

• Fish odor syndrome (trimethylaminuria) is a genetic disease; symptoms are often present from birth.
• Fish odor syndrome is characterized by an offensive body odor and the smell of rotting fish due to the excessive excretion of trimethylaminuria (TMA) in the urine, sweat, and breath of affected individuals.
• Fish odor syndrome is caused by mutations in the FMO3 gene.
• Other causes of odor body can occur as a result from an excess of proteins in the diet, or from an increase in bacteria that normally produce trimethylamine in the GI tract (also may make fish odor syndrome worse); liver disease, kidney disease, poor hygiene, gingivitis, and cases of blood-borne halitosis. Diagnostic tests that help differentiate fish odor syndrome (trimethylaminuria) from other causes of body odor are available at specialized laboratories.
• Fish odor syndrome symptoms have been seen temporarily in a small number of premature infants and in healthy women at the start of menstruation, usually in individuals that are heterozygous for the mutant FMO3 gene or due to immaturity of the faulty FMO3 gene expression (FMO3 expression increases with age).
• Fish odor syndrome treatment includes certain dietary restrictions, use of acid lotions and soaps to remove secreted trimethylamine on the skin, use of activated charcoal and copper chlorophyllin, certain antibiotics, laxatives, and riboflavin supplements.

Trimethylaminuria is a disorder in which the body is unable to break down trimethylamine, a compound derived from the diet that has a strong odor of rotting fish.

As trimethylamine builds up in the body, it causes affected people to give off a fish-like odor in their sweat, urine, and breath. The intensity of this odor may vary over time. The strong body odor can interfere with many aspects of daily life, affecting a person's relationships, social life, and career. Some people with trimethylaminuria experience depression and social isolation as a result of this condition.

Although gene mutations account for most cases of trimethylaminuria, the condition can also be caused by other factors. A fish-like body odor may result from an excess of certain proteins in the diet or from an increase in bacteria that normally produce trimethylamine in the digestive system. A few cases of the disorder have been identified in adults with liver or kidney disease. Temporary symptoms of this condition have been reported in a small number of premature infants and in some healthy women at the start of menstruation.

Trimethylaminuria is an uncommon genetic disorder; its incidence is unknown.

Mutations in the FMO3 gene cause trimethylaminuria.

The FMO3 gene provides instructions for making an enzyme that breaks down nitrogen-containing compounds from the diet, including trimethylamine. This compound is produced by bacteria in the intestine as they help digest proteins from eggs, liver, legumes (such as soybeans and peas), certain kinds of fish, and other foods. Normally, the FMO3 enzyme converts fishy-smelling trimethylamine into another molecule that has no odor. If the enzyme is missing or its activity is reduced because of a mutation in the FMO3 gene, trimethylamine is not processed properly and can build up in the body. As excess trimethylamine is released in a person's sweat, urine, and breath, it causes the strong odor characteristic of trimethylaminuria. Researchers believe that stress and diet also play a role in triggering symptoms.

Most cases of trimethylaminuria appear to be inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. Most often, the parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but typically do not show signs and symptoms of the condition. Carriers of an FMO3 mutation, however, may have mild symptoms of trimethylaminuria or experience temporary episodes of fish-like body odor.

These resources address the diagnosis or management of trimethylaminuria and may include treatment providers.

• Gene Review: Trimethylaminuria
• Gene Tests: Trimethylaminuria

You may find the following resources about trimethylaminuria helpful. These materials are written for the general public.

• MedlinePlus - Metabolic Disorders
• Genetic and Rare Diseases Information Center - Information about genetic conditions and rare diseases
• Additional NIH Resources - National Human Genome Research Institute

You may also be interested in these resources, which are designed for healthcare professionals and researchers.

• Gene Reviews Clinical summary
• Gene Tests - DNA tests ordered by healthcare professionals
• OMIM - Genetic disorder catalog

• Fish malodor syndrome
• Fish odor syndrome
• Stale fish syndrome
• TMAU
• TMAuria

Ask the Genetic and Rare Diseases Information Center