Can Eating Too Much Tuna Give You Mercury Poisoning?

I’m poor, and lazy, but also occasionally health conscious, so I eat quite a bit of canned tuna. Not what you’d call an extraordinary amount—maybe two to three cans a week—but enough for it to qualify as a dietary staple. Enough, at the very least, for me to stop and wonder from time to time: is this maybe too much? Can you overdose on tuna? Or, more specifically: what would it take for a person to overdose on tuna?

Researchers at the University of California, Santa Cruz, recently put forward a similar query. In a study published in the Society of Environmental Toxicology and Chemistry journal, scientists surveyed a number of students about A: their tuna consumption, and B: their awareness of the health risks involved. Fifty-four percent of those students reported eating tuna about three times a week, while more than 99 percent reported low knowledge of the potential dangers associated with overconsumption—most notably, mercury poisoning.

It’s no secret that tuna, like most fish and shellfish, contains toxic, heavy metal mercury. The substance is often present in seawater in fairly small and benign doses, before getting absorbed by algae and entering the food chain. The amount of mercury in a single organism typically varies depending on the size of the animal, and increases as you go up the chain: a shrimp eats the algae, a tuna eats the shrimp, and that tuna develops a cumulative build-up of mercury in its blood through a process known as bioaccumulation. Since tuna are pretty big, they typically accumulate more mercury than smaller species of fish like salmon—and when we eat that tuna, we absorb that mercury. So, in theory, eating a certain amount of tunafish could lead to dangerous amounts of mercury in our blood.

How much fish are we talking, though, before things start getting dicey? Food Standards Australia New Zealand (FSANZ) broadly suggests keeping one’s tuna consumption to between two and three 150 gram servings a week—which is roughly the size of a medium-sized tin. The specific amount is dependent on body weight, though. Larger people can usually handle more mercury, while smaller people—as well as pregnant people—should exercise a little more caution. The U.S. Environmental Protection Agency offers a formula that’s slightly more fine grain: recommending that an individual consume no more than 0.1 micrograms (a microgram equalling one millionth of a gram) of methyl mercury, per kilogram of body weight, per day.

“Tinned tuna is a very low source of mercury, so people would have to be eating at least three cans a day for about six months before it really became a concern,” Melanie McGrice, an accredited practising dietitian, tells me over the phone. “Even pregnant women, who are one of the cohorts most at risk of mercury toxicity, can eat a small, 95-gram can on a daily basis throughout their pregnancy without concern of mercury toxicity.”

Over the course of her nearly-20 year career, Melanie claims to have seen “thousands of patients”—only two of whom have presented with mercury toxicity. “It is quite rare, and for both of those people the reasoning was that they were eating barely anything else other than fish and rice due to food allergies,” she says. “So they had a very, very limited diet.”

That’s somewhat reassuring. But it’s also worth noting that this “limited diet” of tuna and rice is a go-to meal for gym bros and college students who want to eat “healthy” without spending too much time or money. Sure, three cans a day for six months does seem like a “humongous” amount, as Melanie puts it. But it’s also not too hard to imagine a situation in which someone would be putting away that much fish on a regular basis—particularly if they’re cracking into the larger cans.

“Say somebody was trying to gain muscle bulk, and tuna’s obviously a rich source of protein, then that person may be eating six cans of tuna a day in an effort to increase muscle,” Melanie suggests. “That could be a problem if they were doing that for months on end. I would certainly think that a blood test to check their mercury levels would be warranted.”

In the University of California study, seven percent of participants reported eating more than 20 tuna meals a week. Tests on some students' hair furthered indicated mercury levels that were above what is considered "a level of concern".

There’s a range of symptoms and health complications that can come about as a result of high mercury levels. These might manifest as itching, burning, or even a sensation that small insects are crawling under one’s skin, as well as more visible symptoms like pink cheeks and swelling in certain parts of the body. In more serious cases, mercury poisoning can cause high blood pressure, low cognitive function, blindness, and lung and kidney dysfunction. For pregnant people, the dangers can be even more acute.

“Because your baby is obviously much smaller than an average person, they can develop mercury toxicity much more easily, and that can then actually lead to things like cosmetic problems or stillbirths,” says Melanie. “But if you’re an average person just living your life, having tuna as a bit of a staple in your diet, then I wouldn’t expect to see too many problems.”

So in short, yes: there is absolutely such a thing as too much tuna—but you’d have to be putting away several hundred grams a day, for a period of several months, before you started seeing any serious problems. The more relevant concern for people who insist on eating that much, Melanie suggests, is that they’d likely be forgoing other foods and vitamins in preference of canned fish.

“Tuna is quite a nutritious food to eat—but you still want to be having dairy, you still want to be having your whole grains, you still want to be having vegetables and fruits and healthy oils as well,” she stresses. “That said, if somebody has such severe food allergies that they’re really restricting their diet—or maybe they’ve just decided to put themselves on some sort of crazy diet for whatever reason—they could suffer from mercury toxicity.”

As with all things in life, then, moderation is key. Tuna’s a great source of iron, zinc, omega 3, and protein, sure—but it’s also a fairly reliable source of mercury. While the health benefits might ultimately outweigh the risks, it’s probably best to keep the habit to a single can a day.

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