Research
Trimethylaminuria (Fish Odor Syndrome): What Research Shows
Trimethylaminuria, also called fish odor syndrome, is a rare metabolic condition in which the body cannot break down trimethylamine (TMA), a compound with a strong smell of rotting fish. Normally the liver enzyme FMO3 converts TMA into an odorless product; when that enzyme is deficient, TMA builds up and is released in sweat, breath, and urine. It is uncommon and inherited in an autosomal recessive pattern. Carrier frequency of a disease-causing FMO3 variant ranges from about 0.5–1% in white British populations to roughly 11% in New Guinea. This page compiles the verified figures; it is general education, not a diagnosis.
By the Sweat Explained Editorial Team · Published 2026-07-13 · Last reviewed 2026-07-13 · Educational information, not medical advice.
Key statistics at a glance
0.5–1%
carrier frequency of a disease-causing FMO3 variant in white British populations
Mackay 2011
~11%
carrier frequency reported in people of New Guinea origin
Mackay 2011 / StatPearls
1 in 200,000 – 1,000,000
estimated global prevalence of trimethylaminuria
StatPearls 2024
autosomal recessive
usual inheritance pattern: both FMO3 gene copies must be affected
MedlinePlus Genetics
What trimethylaminuria is
Trimethylamine (TMA) is produced by gut bacteria from dietary precursors: mainly choline, lecithin, and trimethylamine N-oxide (TMAO) found in foods such as fish, eggs, and legumes. In most people the liver enzyme FMO3 (flavin-containing monooxygenase 3) oxidizes TMA into odorless TMAO, which is then excreted. In trimethylaminuria, reduced or absent FMO3 activity leaves TMA unprocessed, so it accumulates and is released in sweat, breath, saliva, and urine, producing a fishy or decaying-fish odor.
The odor can vary in intensity over time and may be worse after eating TMA-rich foods, around menstruation, or with stress. The condition is metabolic, not a hygiene problem, and washing alone does not resolve it. Because the smell can be socially and psychologically distressing, clinical references emphasize that affected people may benefit from both dietary guidance and support.
The FMO3 gene and inheritance
Primary (inherited) trimethylaminuria is caused by variants in the FMO3 gene and is usually passed on in an autosomal recessive pattern, meaning a person typically must inherit an affected copy of FMO3 from each parent to show persistent symptoms. Carriers with a single variant are generally unaffected but may occasionally notice transient odor under heavy TMA loads.
There is also a secondary or transient form. Milder, temporary odor can occur with an enormous dietary intake of TMA precursors, with large doses of certain supplements (such as choline or L-carnitine), in some liver or kidney conditions, and transiently in some infants and around menstruation as FMO3 activity fluctuates. This page focuses on the inherited condition, where the figures are best characterized.
How common is it?
Trimethylaminuria is rare, and its true frequency is uncertain because it is under-recognized. The most reliable published figures describe carrier frequency (how often people carry a single FMO3 variant) rather than the disease itself, and these differ markedly by ancestry.
| Population or measure | Estimate | Source |
|---|---|---|
| Carrier frequency, white British adults | 0.5–1% | Mackay 2011 |
| Carrier frequency, New Guinea origin | ~11% | Mackay 2011 / StatPearls |
| Carrier frequency across ethnicities (range) | 0.5–11% | Messenger 2013 review |
| Estimated global disease prevalence | 1 in 200,000 – 1 in 1,000,000 | StatPearls 2024 |
| Cases described in the literature (historic) | 200+ | Messenger 2013 review |
MedlinePlus Genetics states the overall incidence is unknown. Carrier frequency is far higher than disease frequency because carriers usually have no symptoms. Estimates vary widely and are likely underestimates given low awareness and diagnosis rates.
Carrier frequency varies by ancestry
The frequency of FMO3 variant carriers differs substantially between populations, which is why a single global carrier figure is not meaningful. The chart contrasts the two best-cited endpoints of the reported range.
| Group | Value |
|---|---|
| White British | 1% (reported as 0.5–1%) |
| New Guinea origin | 11% |
Source: Mackay RJ, et al., Clin Biochem Rev 2011;32(1):33–43. Chart is an original rendering of the cited data.
Reading the numbers
Two things explain why the figures look so different. First, carrier frequency and disease prevalence are not the same: many people carry one FMO3 variant and never have symptoms, so carrier rates (0.5–11%) are much higher than the estimated disease prevalence (roughly 1 in 200,000 to 1 in 1,000,000). Second, both measures vary by ancestry, so a rate measured in one population should not be assumed to apply globally.
The estimates also carry real uncertainty. MedlinePlus Genetics states plainly that the overall incidence is unknown, and reviews note the true prevalence is likely underestimated because the condition is under-recognized and many people are never formally diagnosed. The honest summary is: trimethylaminuria is rare, carriers are considerably more common than affected individuals, and the exact numbers remain uncertain.
When to seek evaluation
A persistent fishy body odor that does not respond to normal hygiene warrants medical evaluation rather than self-diagnosis, because other causes of body odor exist and diagnosis of trimethylaminuria typically involves a specific urine test (measuring the ratio of trimethylamine to its N-oxide), sometimes after a measured choline load, and may include FMO3 genetic testing. Management is generally supportive and centers on reducing dietary TMA precursors under guidance, and addressing the psychological impact.
Nothing on this page is a diagnosis or treatment plan. Anyone concerned about a persistent unexplained body odor should speak with a clinician, who can arrange appropriate testing and referral.
Methodology and limitations
This page compiles a peer-reviewed review (Mackay RJ, et al., Clin Biochem Rev 2011), a clinical reference chapter (StatPearls, 2024), a dermatology review (Messenger J, et al., J Clin Aesthet Dermatol 2013), and the US National Library of Medicine's MedlinePlus Genetics entry. Each figure was traced to its source page and confirmed by direct reading.
Limitations: trimethylaminuria is rare and under-recognized, so prevalence figures are estimates with wide uncertainty: MedlinePlus states the overall incidence is unknown, and StatPearls' global estimate (1 in 200,000 to 1 in 1,000,000) is broad. Carrier frequencies (0.5–11%) describe how often people carry a single FMO3 variant, not how often the disorder occurs, and both measures vary by ancestry. An older, commonly repeated figure of roughly 1 in 40,000 could not be confirmed on a primary source and is not reported here. This is general education, not medical advice.
Frequently asked questions
- What causes trimethylaminuria?
- A deficiency of the liver enzyme FMO3, usually due to inherited variants in the FMO3 gene. FMO3 normally converts strong-smelling trimethylamine (made by gut bacteria from foods like fish, eggs, and legumes) into an odorless compound; without it, trimethylamine builds up and is released in sweat, breath, and urine.
- How rare is it?
- Rare. StatPearls estimates a global prevalence of roughly 1 in 200,000 to 1 in 1,000,000, and MedlinePlus notes the true incidence is unknown. Carriers of a single FMO3 variant are far more common, about 0.5–1% in white British populations and up to around 11% in New Guinea.
- Is it inherited?
- The primary form is usually inherited in an autosomal recessive pattern, meaning a person typically inherits an affected FMO3 gene copy from each parent. A milder secondary form can occur temporarily with very high intake of trimethylamine precursors or certain supplements, and in some liver or kidney conditions.
- Why does the odor come and go?
- Trimethylamine load fluctuates. Odor can worsen after eating trimethylamine-rich foods (fish, eggs, legumes, some supplements), around menstruation, and with stress, and may be milder at other times. It is a metabolic effect, not a hygiene problem, so washing alone does not resolve it.
- Is there a test for it?
- Yes. Diagnosis typically uses a urine test measuring the ratio of trimethylamine to its N-oxide, sometimes after a measured choline load, and may include FMO3 genetic testing. Testing and interpretation should be arranged through a clinician.
- Why do carrier rates and disease rates differ so much?
- Because carriers usually have no symptoms. Carrying a single FMO3 variant (0.5–11% of people, depending on ancestry) is far more common than having the disorder, which generally requires both gene copies to be affected, hence the much lower disease prevalence.
Sources
Primary peer-reviewed studies and official sources first, then reviews and institutional framing (secondary).
- US National Library of Medicine. Trimethylaminuria. MedlinePlus Genetics. Incidence unknown; autosomal recessive; FMO3 gene. MedlinePlus Genetics
- Mackay RJ, McEntyre CJ, Henderson C, Lever M, George PM. Trimethylaminuria: causes and diagnosis of a socially distressing condition. Clin Biochem Rev. 2011;32(1):33–43. PMID: 21451776. Carrier frequency 0.5–1% (white British) to ~11% (New Guinea). Full text
- StatPearls (NCBI Bookshelf). Trimethylaminuria. StatPearls Publishing; 2024. Estimated global prevalence 1 in 200,000 to 1 in 1,000,000. StatPearls
- Messenger J, Clark S, Massick S, Bechtel M. A review of trimethylaminuria: (fish odor syndrome). J Clin Aesthet Dermatol. 2013;6(11):45–48. PMID: 24307925. Carrier estimate 0.5–11% by ethnicity. (secondary) Full text
How to cite this page
Sweat Explained. Trimethylaminuria (Fish Odor Syndrome): What Research Shows. Published 2026-07-13; last reviewed 2026-07-13. Available at: https://sweatexplained.com/research/trimethylaminuria-fish-odor-syndrome
Please cite the original studies for the underlying figures. Journalists are welcome to link to this page; the charts are original renderings of the cited data.