A fishy smell coming from your armpits usually points to one of two things: either bacteria on your skin are producing sulfur and acid compounds from your sweat, or your body is releasing a chemical called trimethylamine (TMA) that it can’t properly break down. The first is far more common and often fixable with hygiene changes. The second is a metabolic condition that may need dietary adjustments or medical attention.
How Bacteria Create a Fishy Odor
Your armpits host a dense community of bacteria that feed on the proteins and fats in your sweat. Two species in particular, Staphylococcus hominis and Staphylococcus epidermidis, are the primary drivers of underarm odor. These bacteria break down sweat into a mix of compounds including acetic acid, isovaleric acid (which smells sour), and a sulfur-containing molecule called 3M3SH that carries a pungent, sometimes fishy quality.
This type of odor is called bromhidrosis, and it typically smells like intensified body odor rather than a distinctly “rotting fish” scent. It gets worse with heat, stress, exercise, or wearing synthetic fabrics that trap moisture. If you notice the smell mainly after sweating and it fades after a good shower, bacteria are the most likely explanation.
Trimethylaminuria: The Fish Odor Condition
If the fishy smell persists even after bathing, shows up in your breath or urine too, and seems unrelated to sweating, you may be dealing with trimethylaminuria, sometimes called fish odor syndrome. This is a metabolic condition where your liver can’t efficiently convert trimethylamine into an odorless compound called TMAO. The unconverted TMA builds up in your blood and gets released through sweat, breath, and urine, producing a strong fishy odor.
The primary form is genetic. A mutation in the FMO3 gene reduces or disables the liver enzyme responsible for this conversion. About 0.5% to 1% of the British Caucasian population carries a disease-causing variant of this gene, and because it requires two copies (one from each parent), the estimated incidence of full-blown trimethylaminuria is roughly one in 40,000 people. That number is likely an undercount, and carrier rates vary across populations.
There’s also a secondary form. People with normal FMO3 genes can still experience a fishy smell if they overwhelm the enzyme’s capacity by eating large amounts of foods high in choline or carnitine, which gut bacteria convert into TMA. Liver disease or changes in gut bacteria can also reduce TMA processing.
Foods That Make It Worse
Your gut bacteria convert choline and carnitine from food into TMA, which your liver then has to process. If your liver enzyme is sluggish or you’re eating a lot of these precursors, TMA can accumulate faster than your body clears it. The foods highest in choline include beef liver (356 mg per 100 grams), eggs (147 mg per large egg), prawns (246 mg per 100 grams), and wheat bread (227 mg per 100 grams). For carnitine, the biggest sources are red meat (up to 122 mg per 100 grams of beef steak), mussels (225 mg per 100 grams), and cod (71 mg per 100 grams).
Fish itself is a double contributor: it’s both high in carnitine and already contains TMA and TMAO. If you notice the fishy smell gets noticeably worse a day or two after eating red meat, eggs, or seafood, that dietary connection is a strong clue that TMA metabolism is involved.
How to Tell the Difference
The key distinction is where the smell shows up and how it behaves. Bacterial odor is localized to your armpits (or other sweaty areas), gets worse with physical activity, and disappears after washing. A TMA-related fishy smell tends to be more generalized. You or others might notice it on your breath, in your urine, or even on your clothes after they’ve been worn briefly. It can fluctuate with your diet and hormonal cycle rather than with how much you sweat.
Trimethylaminuria is diagnosed with a urine test that measures the ratio of TMA to TMAO. Healthy individuals convert over 80% of TMA into TMAO, producing a ratio (TMAO divided by the total of TMA plus TMAO) above 0.8. People with the condition typically fall below that threshold. Under normal dietary conditions, the body excretes about 1 mg of TMA and 40 mg of TMAO per day in urine. A dramatically elevated TMA level relative to TMAO confirms the diagnosis.
What You Can Do About It
If bacteria are the culprit, the fix is straightforward. Wash your armpits with a mildly acidic cleanser (around pH 5.5, which matches your skin’s natural acidity) to keep odor-producing bacteria in check. Antiperspirants containing aluminum compounds reduce the amount of sweat bacteria have to work with. Wearing breathable natural fabrics and changing shirts after heavy sweating also helps significantly.
If you suspect trimethylaminuria, dietary changes are the first line of management. Reducing your intake of choline-rich and carnitine-rich foods, particularly eggs, organ meats, red meat, and certain seafood, can lower the amount of TMA your body has to process. The effect is often noticeable within days.
Riboflavin (vitamin B2) has shown promise in boosting the activity of the FMO3 enzyme in people with milder forms of the condition. In at least one clinical case, 200 mg per day of riboflavin produced an immediate reduction in TMA excretion without any dietary changes. This suggests the enzyme was present but underperforming, and the vitamin helped it work more efficiently. Riboflavin is a cofactor for the FMO3 enzyme, so supplementing it can be particularly helpful for people with partial enzyme function.
Chlorophyllin supplements are sometimes marketed for body odor, but studies in patients with odor concerns have not shown a statistically significant improvement. It’s not a reliable option for fishy armpit smell specifically.
When the Smell Comes and Goes
Many people notice the fishy odor isn’t constant. It may intensify around menstruation, during periods of stress, or after specific meals. This pattern is consistent with trimethylaminuria, where hormonal fluctuations and dietary load both affect how much TMA accumulates. Even people who carry only one copy of the FMO3 mutation (carriers, not fully affected) can experience intermittent fishy odor under conditions that temporarily overload their enzyme capacity.
If the smell appeared suddenly and you haven’t changed your diet or hygiene habits, it’s worth considering whether a new medication, supplement (betaine is a known trigger), or shift in gut health could be responsible. Anything that increases TMA production in the gut or decreases your liver’s ability to process it can tip the balance.