A persistent fishy smell coming from a man’s body, breath, or groin usually traces back to one of a handful of causes: a metabolic condition called trimethylaminuria, liver or kidney dysfunction, certain dietary patterns or supplements, or a genital infection. The smell itself comes from a molecule called trimethylamine (TMA), which the body normally breaks down into an odorless form, but sometimes that process fails or gets overwhelmed. Less commonly, the odor is not actually present at all, and the person is experiencing a psychiatric condition that makes them believe it is.
Trimethylaminuria and the FMO3 Enzyme
The single most direct explanation for a persistent fishy body odor is trimethylaminuria, sometimes called fish odor syndrome. In this condition, the body cannot properly convert trimethylamine into its odorless counterpart, trimethylamine N-oxide (TMAO). TMA is a byproduct of gut bacteria breaking down certain nutrients in food, and it smells distinctly like rotting fish. Normally, an enzyme in the liver handles the conversion, so you never notice it. When that enzyme is not working, TMA builds up and gets released through sweat, urine, breath, and other bodily secretions.
The enzyme responsible is flavin-containing monooxygenase 3, or FMO3. The primary, inherited form of trimethylaminuria results from mutations in the gene that codes for this enzyme.1JAMA Dermatology. Trimethylaminuria (Fish-Odor Syndrome): A Case Report The condition is inherited recessively, meaning a person needs to carry two defective copies of the FMO3 gene (one from each parent) to fully develop it. Researchers identified specific mutations that completely abolish the enzyme’s ability to do its job, confirming FMO3 as the culprit behind the disorder.2Nature Genetics. Missense mutation in flavin-containing mono-oxygenase 3 gene, FMO3, underlies fish-odour syndrome
Some people carry only one copy of a defective FMO3 gene. They might not smell fishy all the time, but under certain conditions, such as eating a large amount of the wrong foods or during hormonal shifts, they can produce enough TMA to notice. This milder, intermittent version is thought to be more common than the full-blown genetic condition, which is considered rare. That said, the true prevalence is hard to pin down because many people with mild cases never get diagnosed.
Liver Disease and Kidney Failure
You do not need to be born with a genetic defect for your body to lose its ability to handle TMA. Since the conversion of TMA to TMAO happens in the liver, anything that seriously damages liver function can produce the same fishy smell. A case report described a 58-year-old man who developed sudden-onset fish-like body odor. He was diagnosed with secondary trimethylaminuria triggered by liver cirrhosis and hepatitis C, which had compromised his liver’s capacity to metabolize TMA.3PubMed Central. Trimethylaminuria In A 58-Year-Old Male with Liver Cirrhosis and Hepatitis C This “secondary” form of the condition can appear suddenly in adults with no prior history of unusual body odor, which makes it particularly alarming when it shows up.
Kidney failure creates a related problem from the opposite direction. Even if the liver successfully converts TMA to TMAO, the kidneys are responsible for excreting TMAO in the urine. When the kidneys are not filtering properly, as in chronic renal failure, both TMA and other waste products accumulate in the body. Uremia, the general buildup of waste in the blood due to kidney failure, can worsen a fishy smell, and poor control of uremia makes the odor more pronounced.4PubMed Central. Trimethylaminuria (fish malodour syndrome) in chronic renal failure If a man who has been managing kidney disease notices a new fishy odor, it may signal that his renal function has declined further.
Dietary Sources and Supplements
TMA does not appear in the body from nowhere. It is produced when gut bacteria break down foods rich in choline and carnitine, two nutrients found in high concentrations in eggs, organ meats, saltwater fish, and certain legumes.5PubMed Central. Fish odor syndrome In most people, the liver handles the resulting TMA load without any noticeable smell. But if someone eats an unusually large amount of these foods, or if their FMO3 enzyme is slightly underperforming, the system can get overwhelmed. A big choline-heavy meal can push even a person with borderline enzyme activity into noticeable odor territory for a day or two.
Supplements are a less obvious but well-documented trigger. L-carnitine supplements, popular in fitness and energy contexts, directly increase the pool of carnitine available for gut bacteria to convert into TMA. One case report described a patient who developed a clear fish odor during L-carnitine treatment. The smell resolved completely after the supplement was stopped.6PubMed. L-carnitine treatment and fish odor syndrome: an unwaited adverse effect The authors concluded that the odor appeared when the body’s elimination pathway became saturated. A broader risk assessment of carnitine supplementation confirmed that fishy body or urine odor is among the recognized side effects.7Regulatory Toxicology and Pharmacology. Risk assessment for carnitine So if a guy recently started taking carnitine-based supplements and notices a new smell, that is worth investigating before anything else.
Choline supplements, sometimes marketed for cognitive support, carry the same risk for the same reason. Even certain protein powders or meal-replacement shakes may contain high amounts of choline or carnitine. The connection is not always obvious, because people often do not read the fine print on supplement labels.
Genital and Urinary Tract Infections
A fishy smell localized to the groin or genital area often points to a completely different set of causes than the metabolic ones discussed above. While bacterial vaginosis is the condition most commonly associated with fishy genital odor in women, men are not exempt from the relevant infections.
Trichomoniasis, a sexually transmitted infection caused by a parasite, is one of the most common STIs worldwide. It affects men and women, though in men it often produces fewer symptoms or none at all. When it does cause symptoms, it can present as urethral discharge, which may have a foul or fishy odor.8PubMed. Trichomoniasis: clinical manifestations, diagnosis and management A man might notice the smell most during or after urination, or on his underwear. Because trichomoniasis in men is frequently asymptomatic, a fishy smell coming from the urethra or genitals is worth getting tested for, even if nothing else seems wrong.
Research has also shown that the bacteria associated with bacterial vaginosis can colonize men, particularly in the area under the foreskin and in the urethra. Male partners of women with BV carry higher levels of these organisms in the sub-preputial space and distal urethra, and BV-associated biofilms have been detected in male urine and semen.9PubMed Central. Treating male partners of women with bacterial vaginosis (StepUp): a protocol for a randomised controlled trial to assess the clinical effectiveness of male partner treatment for reducing the risk of BV recurrence This means a man can carry the same bacteria that produce fishy-smelling amines in women, and those bacteria can create a noticeable odor on or around his genitals. The smell might be more apparent in uncircumcised men, where the warm, moist environment under the foreskin is especially favorable for bacterial growth. Good genital hygiene helps, but if the odor persists, a doctor can test for these organisms.
Urinary tract infections can also produce unusual urine odors, though “fishy” is not the most typical description. More often, a UTI makes urine smell strong, foul, or ammonia-like. Some bacteria that cause UTIs produce compounds with a vaguely fishy character, but in men UTIs are relatively uncommon compared to women, and when they occur, they usually come with other symptoms like burning during urination or frequent urges to go.
Getting a Diagnosis
If the smell is coming from the whole body rather than just the genital area, trimethylaminuria is the diagnosis to rule in or out first. The standard test involves measuring the ratio of TMA to TMAO in a urine sample. Both compounds need to be measured because it is the proportion that matters: a person with functional FMO3 will have converted most of their TMA into TMAO, while someone with the condition will show an abnormally high percentage of unconverted TMA.10PubMed Central. Trimethylaminuria: causes and diagnosis of a socially distressing condition In milder or intermittent cases, the urine sample should be collected after eating a meal high in TMA precursors, most simply a marine fish meal, to provoke the system into revealing the deficiency.
The same urine markers, TMA and TMAO, serve both as the diagnostic tool and as the way to monitor whether treatments or dietary changes are working.11PubMed. Rapid screening assay of trimethylaminuria in urine with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry Genetic testing for FMO3 mutations can confirm the inherited form, but the urine test is usually done first because it catches both the genetic and the secondary (liver- or kidney-related) forms.
For genital odors, a doctor will likely test for trichomoniasis and possibly run a urine culture to check for UTI-causing bacteria. If the man’s female partner has recurrent BV, testing him for BV-associated organisms may also be worthwhile, though clinical guidelines on treating male BV carriers are still evolving.
Practical Management Strategies
There is no cure for the genetic form of trimethylaminuria, but the odor can be reduced significantly through a combination of dietary changes and a few specific supplements. The most straightforward approach is reducing intake of choline-rich and carnitine-rich foods: limiting eggs, organ meats like liver, certain legumes, and some types of fish. This reduces the raw material available for gut bacteria to produce TMA in the first place.
Two over-the-counter supplements have shown measurable effects. In a study of Japanese patients with trimethylaminuria, activated charcoal taken at about 1.5 grams per day for ten days reduced the amount of unconverted TMA in urine and brought TMAO levels back to normal. Copper chlorophyllin, taken at about 180 milligrams per day for three weeks, achieved a similar effect, and its benefits appeared to last longer, persisting for several weeks after stopping.12PubMed. Effects of the dietary supplements, activated charcoal and copper chlorophyllin, on urinary excretion of trimethylamine in Japanese trimethylaminuria patients Neither is a miracle fix, but both can meaningfully improve quality of life.
For secondary trimethylaminuria caused by liver or kidney disease, managing the underlying condition is the priority. Better uremic control in kidney patients can reduce odor. In the liver cirrhosis case mentioned earlier, identifying and treating the liver disease itself was part of the approach, along with reviewing the patient’s dietary supplements. If a supplement like L-carnitine is to blame, simply stopping it often resolves the smell entirely.
For infection-related odors, the solution is usually a course of antibiotics or antiparasitic medication. Trichomoniasis responds well to standard treatment, and both partners should be treated simultaneously to prevent reinfection. Improving genital hygiene, particularly thorough washing under the foreskin for uncircumcised men, can reduce bacterial buildup that contributes to odor.
When the Smell Is Not Actually There
There is one cause worth mentioning that has nothing to do with metabolism, infections, or diet. Olfactory reference syndrome is a condition in which a person becomes preoccupied with the belief that they emit a foul body odor that others can perceive, even though no such odor exists. It has been described in medical literature for over a century and causes severe distress.13PubMed Central. Olfactory reference syndrome: issues for DSM-V The perceived smells vary, but among those studied, about a fifth reported smelling something they described in terms like “oily, fishy” alongside more typical body-odor descriptions.14PubMed Central. Olfactory Reference Syndrome: Demographic and Clinical Features of Imagined Body Odor
The distress caused by olfactory reference syndrome is real and can be devastating. In one study, about 40% of people with the condition had been housebound for at least a week due to their symptoms, roughly two-thirds had experienced suicidal thoughts, and about a third had attempted suicide.14PubMed Central. Olfactory Reference Syndrome: Demographic and Clinical Features of Imagined Body Odor This makes it a condition worth knowing about, not as a dismissive “it’s all in your head” explanation, but as a genuine medical condition that responds to treatment. If a man is convinced he smells fishy but nobody around him can detect it, and if urine and medical tests come back normal, a mental health evaluation for olfactory reference syndrome is a reasonable next step. The condition is treated with certain psychiatric medications and therapy, and outcomes tend to be good when it is properly identified.
The Psychological Weight of Smelling Different
Whether the fishy odor is real or perceived, its psychological impact on men is often underappreciated. The original research characterizing fish odor syndrome noted that sufferers display a range of psychosocial reactions, from withdrawing from social situations to clinical depression and suicide attempts.2Nature Genetics. Missense mutation in flavin-containing mono-oxygenase 3 gene, FMO3, underlies fish-odour syndrome Men may be especially reluctant to bring up a fishy body odor with a doctor, partly because the symptom is more culturally associated with women’s health (through its connection to BV) and partly because body-odor concerns in men are sometimes trivialized.
This reluctance can lead to years of undiagnosed suffering. A man might cycle through deodorants, body washes, and cologne without any improvement, growing increasingly isolated and anxious. If you or someone you know is dealing with this, the most productive step is a direct conversation with a doctor, specifically asking about trimethylaminuria testing (a urine test), liver and kidney function panels, and STI screening if the odor is genital. These are straightforward, widely available tests that can either identify the problem or narrow it down quickly. The causes are treatable or at least manageable in almost every case, and no one should have to white-knuckle through years of social embarrassment when a urine sample and a blood draw could point toward answers.