A persistent fishy smell coming from someone’s body is almost always caused by trimethylamine, a compound that the body normally converts into an odorless form before excreting it. When that conversion process fails or gets overwhelmed, trimethylamine escapes through sweat, breath, and urine, producing a distinct fish-like odor. The causes range from simple dietary triggers to a rare genetic condition, and most of them are manageable once identified.
The Enzyme That Normally Prevents the Smell
Your body produces an enzyme called FMO3 in the liver. Its job is to convert trimethylamine (TMA), a naturally occurring byproduct of digesting certain foods, into an odorless molecule that leaves the body through urine without anyone noticing. When this enzyme is working properly, TMA never builds up enough to produce a smell.
When FMO3 activity is reduced or absent, TMA accumulates and gets released through sweat, breath, and urine. This is the core mechanism behind almost every case of unexplained fishy body odor, whether the root cause is genetic, dietary, or related to another health condition.
Trimethylaminuria: Fish Odor Syndrome
The most direct explanation is a condition called trimethylaminuria, sometimes called fish odor syndrome. People with the inherited form carry mutations in the gene that produces FMO3, leaving them with a severely reduced ability to neutralize TMA. The odor can be constant or come and go depending on what they eat, how much they sweat, and hormonal fluctuations.
Primary trimethylaminuria is rare. Global estimates put it at roughly 1 in 200,000 to 1 in 1,000,000 people, though about 1% of the white British population carries at least one copy of a disease-causing gene variant. Carriers may never develop noticeable symptoms, or they may experience mild, intermittent odor that only surfaces under certain conditions. The first case was documented around 1970, and as of 2020, only a few hundred cases had appeared in medical literature. That low number likely reflects underdiagnosis rather than true rarity, since many people with mild cases never seek medical evaluation.
There is also a secondary form that appears in adulthood without a genetic cause. This can be triggered by medical conditions that alter gut bacteria, by hormonal changes, or by medications and supplements that flood the body with TMA precursors.
Foods That Can Trigger Fishy Odor
Even in people with fully functional FMO3, eating large amounts of certain foods can temporarily overwhelm the enzyme’s capacity and produce a noticeable smell. The key dietary compounds that gut bacteria convert into TMA are choline, carnitine, and lecithin.
The highest-choline foods are animal-based. A 3-ounce serving of pan-fried beef liver contains 356 mg of choline. A single hard-boiled egg has 147 mg. Braised beef top round provides 117 mg per 3 ounces, and roasted soybeans deliver 107 mg per half cup. Chicken breast, ground beef, and cod each provide around 71 to 72 mg per serving. Cruciferous vegetables, beans, nuts, seeds, and whole grains also contribute choline to the diet.
Lecithin, which is rich in a choline-containing compound, shows up as an additive in many processed foods: gravies, salad dressings, margarine, and other emulsified products. Someone eating a diet heavy in eggs, red meat, and processed foods could be consuming enough TMA precursors to produce a detectable odor, especially if their FMO3 enzyme is even slightly less efficient than average.
Supplements and Medications
Certain supplements can push TMA production past what the body can handle. L-carnitine, commonly taken for exercise performance and energy, is converted to TMA by bacteria in the colon. Large supplemental doses have been known to provoke a fishy odor in susceptible individuals, even though the carnitine naturally present in meat typically does not cause problems at normal dietary levels.
High-dose choline supplements (8 to 20 grams per day) used in medical settings have triggered trimethylaminuria in patients being treated for neurological conditions. Glycine betaine, another supplement sometimes prescribed for a genetic condition called homocystinuria, can also be converted to methylamines by gut bacteria and produce the same effect. If your husband recently started any new supplement, particularly one marketed for fitness or heart health, that could be the explanation.
Gut Bacteria and Bacterial Overgrowth
The bacteria living in the gut are the ones actually producing TMA from food. The composition of someone’s gut microbiome determines how much TMA gets generated in the first place. An overgrowth of certain bacterial species, particularly in the small intestine where they don’t normally thrive in large numbers, can dramatically increase TMA production. This means the same diet that caused no odor a year ago could start causing one if the gut microbiome shifts due to illness, antibiotics, or changes in eating patterns.
Kidney Disease and Liver Problems
Chronic kidney failure can produce a fishy smell. When the kidneys lose their ability to filter waste products effectively, compounds like dimethylamine and trimethylamine build up in the body and escape through the skin and breath. This is distinct from the ammonia-like “uremic” breath that also accompanies kidney disease; the two smells often coexist.
Severe liver disease produces a different odor profile, typically described as sweet and musty rather than fishy. But because the liver is where FMO3 does its work, significant liver damage can impair the conversion of TMA to its odorless form, potentially contributing to a fishy component in someone’s overall body odor. If the smell appeared alongside other symptoms like fatigue, swelling, changes in urination, or yellowing skin, an underlying organ issue is worth investigating.
Skin Bacteria and Sweat
Sometimes the odor originates not from inside the body but from the interaction between sweat and bacteria on the skin’s surface. Apocrine sweat glands, concentrated in the armpits and groin, secrete a fluid that is initially odorless. When skin bacteria break down the organic compounds in that sweat, they produce short-chain fatty acids and ammonia that can carry strong, sometimes fishy, odors. This condition, called bromhidrosis, tends to be worse in areas where moisture gets trapped and bacteria flourish. It’s more of a surface-level problem than a metabolic one, and it responds well to improved hygiene practices and antibacterial cleansers.
Getting a Diagnosis
The standard test for trimethylaminuria is a urine test that measures the ratio of TMA to its odorless byproduct. A high proportion of TMA relative to the odorless form suggests impaired enzyme activity. Some testing centers use a choline challenge, where the person eats a high-choline meal before providing the sample, to see how their body handles a TMA load under stress. Genetic testing can confirm whether specific FMO3 mutations are present.
Because the smell can be intermittent, a single normal test result doesn’t always rule out the condition. If your husband’s odor comes and goes, it may help to collect the sample on a day when the smell is noticeable.
Managing the Odor
The most effective first step is dietary. Reducing intake of high-choline foods, particularly eggs, organ meats, and red meat, often produces a noticeable improvement within days. This doesn’t mean eliminating these foods entirely, but scaling back and paying attention to which meals seem to precede the odor.
Low-pH soaps and body washes can help neutralize TMA on the skin’s surface, since trimethylamine is a base that becomes less volatile in an acidic environment. Cleveland Clinic recommends discussing specific pH levels with a healthcare provider to find the right product.
Several supplements have shown promise in reducing odor. Activated charcoal at 750 mg twice daily for ten days can help bind TMA in the gut before it’s absorbed. Copper chlorophyllin, taken at 60 mg three times daily after meals for three weeks, has also been used to reduce odor. Riboflavin (vitamin B2) may enhance whatever residual FMO3 enzyme activity exists; the recommended approach is 30 to 40 mg taken three to five times per day with food. These are not cures, but they can meaningfully reduce the intensity of the smell when combined with dietary changes.
Clothing choices matter too. Synthetic fabrics trap odor-causing compounds more than natural fibers like cotton. Changing clothes after sweating and washing workout gear promptly can prevent TMA from building up in fabric, where it continues to smell even after the skin has been cleaned.