Why Do I Get Diarrhea After Eating Tuna?

Tuna is one of the most common triggers of a reaction called scombroid poisoning, which happens when bacteria convert a naturally occurring amino acid in the fish into histamine before you eat it. But histamine buildup is only one of several reasons tuna can send you to the bathroom. Depending on whether the fish was raw or cooked, fresh or canned, and how it was stored, the cause could range from a genuine food allergy to a parasitic infection to the possibility that what you ate wasn’t actually tuna at all.

Histamine Buildup and Scombroid Poisoning

Tuna flesh is rich in an amino acid called histidine. When tuna sits at warm temperatures for too long, certain bacteria on the fish convert histidine into histamine using an enzyme called histidine decarboxylase. The bacteria responsible include species that thrive on fish surfaces during handling and storage.¹ Once enough histamine accumulates, eating the fish floods your body with it, producing a reaction that looks a lot like an allergic episode but isn’t one. Doctors call this scombroid poisoning, named after the Scombridae fish family that includes tuna and mackerel.

Symptoms usually start within minutes to a couple of hours after eating. A burning or tingling feeling on the tongue often comes first, sometimes with a peppery or metallic taste. Flushing of the face and neck follows, along with a rash that can spread down the upper body. Stomach cramps, nausea, vomiting, and diarrhea are common gastrointestinal symptoms, and many people also report a throbbing headache and palpitations.² Because the reaction mimics a true allergy so closely, scombroid is often misdiagnosed as one.³

The frustrating part is that histamine doesn’t change the way the fish looks, smells, or tastes in any reliable way. You can’t tell by inspecting a piece of tuna whether it has safe levels of histamine or dangerously high ones. Cooking doesn’t help either: one study found that the normal cooking process did not reduce histamine concentrations in contaminated tuna.⁴ Once the histamine is there, heat won’t break it down.

Why Storage Temperature Is the Key Factor

The single biggest factor determining whether your tuna is safe is how cold it stayed between being caught and reaching your plate. That same study measured histamine levels in tuna steaks stored at different temperatures and found dramatic differences. At refrigerator temperature, histamine rose modestly over six days. But at slightly warmer temperatures around 12°C (about 54°F, the kind of temperature you might find in a poorly functioning cooler or on a warm loading dock), histamine concentrations climbed to thousands of milligrams per kilogram within the same timeframe.⁴ For context, regulatory limits in most countries set the safety threshold at 50 to 200 mg/kg, so those warm-stored fish were wildly above what’s considered safe.

This is why scombroid outbreaks tend to cluster around restaurants and retailers where the cold chain was broken at some point, even briefly. A tuna steak left on a counter during busy kitchen prep, a shipment that sat in a warm truck for an extra hour, a fish market where ice melted on a hot day: these are the conditions that let histamine-producing bacteria do their work. A review of the tuna supply chain confirmed that maintaining cold temperatures from the moment of catch through to consumption is the most effective way to prevent histamine poisoning.⁵

Even canned tuna isn’t immune once opened. Research on canned tuna stored after opening found that histamine levels remained low at 4°C but reached considerably higher concentrations at 28°C and 31°C within 48 hours.⁶ So if you open a can, make your sandwich, and leave the rest on the counter through the afternoon, you’re creating the same conditions that produce scombroid in fresh fish.

Histamine Intolerance Without Spoiled Fish

Some people get diarrhea after eating tuna that is perfectly fresh and properly stored. If this describes you, the issue may not be the fish but rather your body’s ability to process histamine. Tuna naturally contains more histamine than most foods simply because of its high histidine content. For most people, an enzyme called diamine oxidase (DAO) breaks down dietary histamine in the gut before it causes problems. But in people who produce less DAO than average, even moderate amounts of histamine can trigger gastrointestinal symptoms.

This condition, broadly called histamine intolerance, produces symptoms that overlap heavily with scombroid: abdominal cramps, diarrhea, flushing, and headache. The key difference is that it can happen every time you eat high-histamine foods, not just when a particular piece of fish was mishandled. Other high-histamine foods include aged cheese, fermented foods, cured meats, and certain wines. If you notice symptoms after eating several of these, the pattern may point toward a histamine-processing issue rather than a one-off case of food poisoning. A doctor can help distinguish between the two, sometimes with the help of DAO blood tests or a careful elimination diet.

True Fish Allergy

A genuine allergy to fish is a different mechanism entirely. In fish allergy, the immune system produces antibodies called IgE against specific proteins in the fish. The primary culprits are a family of proteins called parvalbumins, which are present in most fish species and are highly cross-reactive, meaning that if you’re allergic to one type of fish, you often react to others too.⁷

Symptoms of fish allergy can include hives, swelling, vomiting, diarrhea, and in severe cases, anaphylaxis. Unlike scombroid, which is tied to a specific piece of fish that accumulated histamine, a true allergy will produce a reaction every time you eat the allergenic fish regardless of how fresh it is. If your diarrhea happens consistently after tuna and also after other types of fish, allergy testing is worth pursuing. Tuna does have somewhat lower parvalbumin content than some other fish species, so it’s possible to tolerate tuna while reacting to, say, cod or salmon, but the cross-reactivity among parvalbumins means this isn’t guaranteed.

Parasites in Raw or Undercooked Tuna

If you’re eating raw tuna, whether as sushi, sashimi, poke bowls, or tartare, parasites are another possible explanation. The most relevant are Anisakis nematodes (roundworms), which infect marine fish and can cause a condition called anisakiasis when their larvae are swallowed alive. Reports of gastrointestinal infections and allergic reactions caused by these parasites have been increasing.⁸

Symptoms of parasitic infection from fish can range from mild stomach pain and diarrhea to severe abdominal distress and allergic reactions.⁹ The timeline is different from scombroid: parasitic symptoms can take hours to days to develop, whereas histamine reactions are usually fast. Anisakis larvae are killed by freezing (the FDA recommends fish intended for raw consumption be frozen at specific temperatures for specific durations) and by thorough cooking. Reputable sushi restaurants freeze their fish before serving, but not every restaurant or home cook follows this practice consistently.

Contamination from Handling

Sometimes the tuna itself is fine, but bacteria or viruses were introduced during preparation. Vibrio parahaemolyticus is a bacterium naturally found in warm ocean waters that can contaminate raw seafood, including tuna. It’s one of the leading causes of seafood-associated gastroenteritis, producing watery diarrhea, cramps, and sometimes nausea and vomiting within 12 to 24 hours of eating contaminated food.

Viruses can also hitch a ride. An investigation of foodborne outbreaks in Tokyo found norovirus on raw tuna used as a sushi ingredient, among other foods. In most of those outbreaks, the virus was traced not to the fish itself but to infected food handlers who contaminated the product during preparation.¹⁰ Because raw tuna is eaten without further cooking, any contamination introduced by hands, cutting boards, or utensils goes straight to your stomach. This is why norovirus outbreaks in restaurants often involve sushi and similar uncooked items.

When the “Tuna” Isn’t Actually Tuna

This is a surprisingly common scenario, particularly in restaurants and with cheaper sushi. Fish mislabeling has been documented worldwide, and one of the species most frequently substituted for tuna is escolar (sometimes called “white tuna” on sushi menus). Escolar and its close relative oilfish contain flesh whose oil is made up of more than 90% indigestible wax esters. Your body simply cannot absorb these fats, so they pass through your digestive tract and come out the other end as an oily, orange-to-brown discharge. The resulting condition, called keriorrhea, involves oily diarrhea without the kind of fluid loss seen in ordinary diarrhea, along with abdominal cramps and nausea.¹¹

Keriorrhea is distinctive enough that you can often identify it: the stool has an oily sheen and an orange tint that you won’t see with normal diarrhea. It’s not dangerous in a medical sense, but it’s unpleasant and can be alarming if you don’t know what’s happening. Several countries have banned the sale of escolar or require it to be clearly labeled, but enforcement varies widely. If you get oily diarrhea specifically after eating “tuna” at a restaurant, escolar substitution is a strong possibility.

Additives in Canned and Processed Tuna Products

If your symptoms come specifically after eating canned tuna or tuna salad rather than fresh tuna, the culprit may be something mixed in rather than the fish itself. An outbreak in the 1970s demonstrated that a soy protein extender marketed for tuna salad caused nausea in about 91% of those who ate it, along with abdominal cramps in about 71% and diarrhea in roughly half. Testing confirmed that the textured soy protein was the cause, and the effect was worse in people with a history of food sensitivities.¹²

While that specific product is no longer on the market, the principle holds. Canned and premade tuna products often contain added ingredients like soy, various vegetable broths, preservatives, and flavorings. Mayonnaise-based tuna salads introduce eggs and oils that can go bad quickly if left unrefrigerated. If you tolerate fresh tuna steak but consistently react to canned or prepared versions, start reading labels and consider whether one of the added ingredients is the real trigger.

How to Tell Which Cause Applies to You

With so many possible explanations, the pattern of your symptoms is the best clue. Here are some practical distinctions:

• Fast onset, flushing, headache: Scombroid poisoning. Symptoms within minutes to two hours, especially if others who ate the same fish are also sick. This usually resolves on its own within several hours, though antihistamines can help.
• Happens every time with tuna and other fish: True fish allergy. Consistent reactions regardless of preparation method or freshness. An allergist can confirm with skin-prick or blood testing.
• Happens with multiple high-histamine foods: Histamine intolerance. Reactions to aged cheeses, fermented foods, and cured meats in addition to tuna suggest a DAO processing issue.
• Oily, orange-tinged stool: Keriorrhea from escolar or oilfish mislabeled as tuna, especially from sushi restaurants.
• Delayed onset, watery diarrhea: Bacterial or viral contamination. Symptoms 12 to 48 hours after eating, often with fever, point toward an infectious cause rather than a chemical one.
• Specific to canned or prepared tuna: Likely an additive or a storage issue with the opened product rather than the fish itself.

If you’ve had a single episode, it was most likely scombroid or a handling issue, and you don’t need to swear off tuna permanently. If it happens repeatedly, tracking when and how you eat tuna, whether raw or cooked, canned or fresh, and what else was in the meal, will narrow the field considerably.

Reducing Your Risk at Home and When Eating Out

Most tuna-related diarrhea comes down to temperature abuse at some point in the chain between the ocean and your mouth. When buying fresh tuna, look for fish displayed on abundant ice and buy from vendors you trust. Once you get it home, refrigerate it immediately and use it within a day or two. If you’re not cooking it the same day, freeze it. For raw preparations at home, freeze the tuna to kill potential parasites before thawing and serving.

For canned tuna, the contents are commercially sterile before opening, so the risk starts when you pop the lid. Transfer any unused portion to a sealed container in the refrigerator and use it within a day. Leaving an open can on the counter at room temperature accelerates bacterial growth and histamine production in ways that can catch people off guard, since the fish was “safe” when the can was sealed.⁶

At restaurants, you have less control, but a few signals can help. Sushi bars that freeze their fish and maintain visible cleanliness standards are lower risk. If “white tuna” is on a sushi menu at a very low price, treat it with skepticism, since genuine albacore tuna costs more than escolar. And if you’ve been told you have a fish allergy, carry your prescribed medication: unlike scombroid, which is uncomfortable but self-limiting, true anaphylactic fish allergy can be life-threatening and requires immediate treatment.

Mercury and Tuna: Does It Cause Diarrhea?

Mercury in tuna gets a lot of attention, and it’s reasonable to wonder whether it could be behind your digestive symptoms. Tuna, particularly larger species like bigeye and albacore, accumulates more mercury than most seafood because of its position in the food chain. However, the levels found in commercially sold tuna typically do not cause acute gastrointestinal symptoms. Mercury’s health concerns are mainly about chronic exposure over months and years, and the symptoms of mercury toxicity are primarily neurological: numbness, tremor, cognitive changes, and vision problems.

That said, there is some emerging research suggesting that chronic dietary mercury exposure can alter gut bacteria in ways that affect intestinal health. Animal studies have found changes in gut microbial communities and in the short-chain fatty acids that those bacteria produce after mercury exposure, though the mechanisms behind these shifts are still being worked out.¹³ This is early-stage science, and it would be premature to blame a single bout of diarrhea after a tuna sandwich on mercury. If you eat tuna frequently and have ongoing, low-grade digestive issues that don’t fit any of the patterns described above, it might be worth discussing your mercury intake with a doctor, but it’s far more likely that one of the other causes explains an acute episode.