Fishy-smelling diarrhea usually points to one of a few specific causes: a gut infection, a problem with fat absorption, or more rarely, a metabolic condition that prevents your body from breaking down certain compounds. The smell itself comes from specific chemicals produced either by bacteria in your gut or by undigested nutrients passing through your system. While it’s often temporary and tied to something you ate or a short-lived infection, a persistent fishy odor can signal something worth investigating.
Gut Infections That Cause Fishy-Smelling Stool
Several infections are known to produce stool with a distinctly fishy odor. Cholera, caused by the bacterium Vibrio cholerae, is one of the few infections where a fishy smell is a textbook feature. The Mayo Clinic describes cholera-related diarrhea as pale and milky, resembling rice water, and notes that it “often smells fishy.” Cholera is rare in developed countries but worth knowing about if you’ve recently traveled to areas with limited water sanitation.
Giardia, a parasite spread through contaminated water, produces diarrhea that the CDC describes as “smelly, greasy poop that can float.” The greasiness happens because Giardia interferes with fat absorption in your small intestine, and undigested fats passing through the gut generate strong, sometimes fishy odors. Giardia is one of the most common waterborne parasites worldwide, and you can pick it up from untreated drinking water, swimming in lakes or streams, or contact with an infected person.
C. difficile, a bacterial infection that often follows antibiotic use, produces its own characteristic smell, typically described by clinicians as a “horse barn odor” rather than fishy. That distinction matters if you’re trying to narrow down what’s going on. If you’ve taken antibiotics recently and your diarrhea smells intensely foul but not specifically fishy, C. diff is a more likely suspect.
Fat Malabsorption and Bile Acid Problems
When your body can’t properly digest fats, they pass through your intestines and get broken down by gut bacteria in ways that produce strong, unusual odors. This is called steatorrhea, and the resulting stool tends to be pale, oily, and powerfully smelly, sometimes with a fishy quality. It may also float and be difficult to flush.
Bile acid diarrhea is one common cause. Bile acids are released by your liver to help digest fats, and normally most of them get reabsorbed before reaching your colon. When that recycling system fails, excess bile acids irritate the colon and cause watery, foul-smelling diarrhea. This can happen after gallbladder removal, after intestinal surgery, or alongside conditions like Crohn’s disease. It’s frequently underdiagnosed because the symptoms overlap with irritable bowel syndrome. A trial of bile acid-binding medication is often used as both a diagnostic test and treatment.
Conditions like celiac disease and chronic pancreatitis can also impair fat digestion and produce greasy, malodorous stool. If the fishy smell comes alongside oily or floating stools, fat malabsorption is a strong possibility.
Trimethylaminuria: The “Fish Odor” Condition
There is a genetic condition specifically associated with a fishy smell from the body. Trimethylaminuria, sometimes called fish odor syndrome, happens when your body can’t break down a compound called trimethylamine. This chemical is produced by gut bacteria during the digestion of certain foods, particularly eggs, legumes, organ meats, and some types of fish. Normally, a liver enzyme converts trimethylamine into an odorless form. In people with trimethylaminuria, that enzyme is missing or underactive, so the compound builds up and gets released through sweat, urine, breath, and stool.
Trimethylaminuria is uncommon, and its exact prevalence isn’t known. The fishy odor it produces isn’t limited to stool. It tends to affect the whole body, so you’d likely notice the smell in your sweat or urine as well. If the fishy smell only shows up in your diarrhea and not elsewhere, this condition is less likely.
Foods That Can Cause Fishy-Smelling Stool
Even without a genetic condition, the foods you eat directly influence stool odor through their effect on gut bacteria. Foods rich in choline, lecithin, and L-carnitine are all converted into trimethylamine by intestinal bacteria. The biggest dietary sources are red meat, eggs, fish, and dairy. Research in healthy adults shows that chronic red meat consumption leads to significantly higher levels of trimethylamine-related compounds in the blood and urine compared to other animal proteins. When participants in one study stopped eating red meat for four weeks, their levels dropped significantly.
Plant-based diets consistently produce lower levels of these fishy-smelling compounds. Omnivorous diets high in animal protein generate more trimethylamine than vegetarian or vegan diets. If you’ve recently increased your intake of eggs, seafood, or red meat, that alone could explain the change in stool odor, especially if you’re also dealing with diarrhea from another cause that’s speeding food through your system before it’s fully digested.
Reducing the Smell Through Diet
If the fishy odor seems tied to what you’re eating rather than an infection, adjusting your diet is the most direct approach. Cutting back on red meat has the strongest evidence behind it for lowering trimethylamine production. Eggs, liver, and certain fish are also significant contributors. Shifting toward more plant-based meals, even temporarily, can noticeably reduce the odor within a few weeks.
For people with confirmed trimethylaminuria, dietary management is the primary strategy. This means limiting high-choline foods like egg yolks, soybeans, kidney beans, and organ meats. It’s a balancing act, though, because choline is an essential nutrient, so working with a dietitian helps ensure you’re not creating deficiencies while managing the odor.
Signs That Something More Serious Is Happening
Fishy-smelling diarrhea that lasts a day or two after a dietary indulgence or a mild stomach bug is usually not concerning. But certain accompanying symptoms change the picture. A fever above 101°F (38.3°C) alongside foul-smelling diarrhea suggests an active infection that may need treatment. Blood or black color in your stool warrants prompt medical attention regardless of odor. Unintentional weight loss, persistent greasy stools, or diarrhea lasting more than a couple of weeks all point toward malabsorption or chronic infection that benefits from testing.
Diagnosis typically starts with a medical history and basic blood work, followed by stool tests. If a parasitic infection like Giardia is suspected, a stool sample examined under a microscope can confirm it. For bacterial infections, stool cultures identify the organism. If malabsorption is the concern, stool fat tests, inflammatory markers like fecal calprotectin, and sometimes imaging or endoscopy help pinpoint the cause. The diagnostic path depends heavily on your other symptoms, travel history, and how long the problem has been going on.