A fishy smell coming from a guy’s body usually traces back to one of a few causes: bacteria breaking down sweat or skin buildup, diet, an underlying metabolic condition, or less commonly, an infection. The smell can come from the groin, breath, sweat, or urine, and the source matters because it points to very different explanations and next steps.
Bacteria and Hygiene in the Groin Area
The most straightforward explanation is bacterial activity on the skin, particularly around the groin. Sweat from the apocrine glands (concentrated in the groin and armpits) is actually odorless on its own. It only starts to smell after bacteria on the skin’s surface break it down into volatile compounds, including ammonia and short-chain fatty acids. In warm, moist folds of skin, this process accelerates, and the resulting odor can take on a fishy quality.
In uncircumcised men, a buildup of smegma under the foreskin creates an ideal environment for anaerobic bacteria like Bacteroides. These organisms thrive in low-oxygen, moist spaces and are specifically associated with fishy-smelling byproducts. When this buildup leads to inflammation of the head of the penis, a condition called balanitis, the odor can become noticeably stronger. Regular cleaning under the foreskin with mild soap and warm water typically resolves this. If the area is red, swollen, or painful alongside the smell, that points to an active infection that may need treatment.
Foods and Supplements That Trigger the Smell
What you eat can directly produce a fishy body odor. Gut bacteria convert certain nutrients into a compound called trimethylamine, or TMA, which smells like rotting fish. Your liver normally neutralizes TMA into an odorless form before it can reach your sweat, urine, or breath. But when you eat large amounts of TMA precursors, the liver’s capacity can be temporarily overwhelmed, and the fishy-smelling compound escapes through the skin and urine.
The biggest dietary triggers are:
- Eggs, beans, and peas, which are high in choline
- Red meat and fish, which are high in carnitine
- Organ meats like liver, which contain high levels of both
Carnitine supplements deserve special mention. They’re popular among men as a performance-enhancing supplement, and they deliver a concentrated dose of one of the main precursors to TMA. If a guy started smelling fishy around the same time he began a new supplement regimen, that connection is worth investigating. Cutting back on these foods and supplements for a few days is often enough to tell whether diet is the cause.
Fish Odor Syndrome (Trimethylaminuria)
When a fishy smell is persistent, comes from the sweat, breath, and urine simultaneously, and doesn’t go away with good hygiene, the cause may be a metabolic condition called trimethylaminuria, sometimes called fish odor syndrome. This is a genetic disorder where the liver enzyme responsible for converting fishy-smelling TMA into its odorless form is either missing or severely underperforming. The result is that TMA builds up in the body and gets released through sweat, urine, and exhaled air.
The inherited form is autosomal recessive, meaning a person needs to receive the gene variant from both parents. Carriers with only one copy are typically asymptomatic. There’s also a secondary form where the enzyme works but is simply overwhelmed, either by heavy dietary intake of precursors or by changes in gut bacteria that produce excessive TMA. Some people with the secondary form go years without symptoms until something tips the balance.
The odor in trimethylaminuria can be noticed regardless of how well someone maintains personal hygiene, which is one of the most distressing aspects for people living with it. The smell may fluctuate with diet, stress, exercise, and hormonal changes, but for those with the primary genetic form, it rarely disappears entirely without management.
How It’s Diagnosed
Diagnosis involves a urine test that measures the ratio of TMA to its odorless counterpart. In unaffected people, that ratio shows more than 92 percent of TMA has been converted to the odorless form. In someone with two copies of the gene variant, less than 84 percent is converted. The condition often goes undiagnosed for years because many doctors aren’t familiar with it. When suspected, referral to a metabolic specialist is the typical path for proper nutritional support and ongoing management.
Managing the Odor
The primary strategy is dietary modification: reducing intake of foods rich in choline, carnitine, and related compounds. This means cutting back on eggs, red meat, certain fish, legumes, and organ meats. Using body washes with a slightly acidic pH (around 5.5 to 6.5) can also help, because TMA is an alkaline compound and acidic products on the skin can reduce its volatility, making it less likely to become airborne and noticeable. Some people find that careful dietary tracking helps them identify their personal threshold, the point at which their enzyme capacity gets overwhelmed.
Kidney or Liver Problems
Both kidney and liver dysfunction can produce a fishy or ammonia-like smell on the breath. When the kidneys aren’t filtering properly, ammonia accumulates in the blood and gets released through exhaled air. This is sometimes described as “uremic fetor,” and ancient physicians actually used the smell of a patient’s breath as a diagnostic clue for kidney disease. Liver failure produces a similar effect because the liver is responsible for converting ammonia into a harmless compound for excretion. When either organ is compromised, ammonia levels rise.
This type of smell is more often noticed on the breath than on the skin, and it’s typically accompanied by other symptoms: fatigue, swelling, changes in urination, nausea, or confusion. A fishy breath odor on its own isn’t reason to assume organ failure, but if it coincides with these other signs, it warrants a medical evaluation.
Sexually Transmitted Infections
Trichomoniasis, a common STI caused by a parasite, can produce a yellow or green discharge from the penis that sometimes has an unpleasant, fishy-like smell. Most men with trichomoniasis have no symptoms at all, which means the infection can persist and be passed to partners without the man ever knowing. When symptoms do appear, they may include irritation inside the penis, mild discharge, or slight burning after urination or ejaculation.
If the fishy smell is coming specifically from penile discharge rather than from general body odor or sweat, an STI screen is a reasonable step. Trichomoniasis is easily treated once identified.
Telling the Causes Apart
Where the smell is coming from narrows the possibilities considerably. A fishy odor localized to the groin that improves with thorough washing points to bacterial buildup or balanitis. A smell that comes from sweat, breath, and urine all at once, and persists despite good hygiene, suggests trimethylaminuria. A smell that appeared after dietary changes or new supplements is likely diet-related. Fishy breath alongside fatigue or other systemic symptoms raises the possibility of kidney or liver issues. And a smell tied specifically to penile discharge points toward infection.
For persistent, unexplained fishy body odor, a urine test for TMA levels is the most direct way to confirm or rule out a metabolic cause. Many people who eventually receive a trimethylaminuria diagnosis spent years cycling through dermatologists and general practitioners before finding a specialist familiar with the condition.