The truth about tuna

Tuna is about as nutritious as a food can be.

It’s packed with protein, minerals, and vitamins, said Chris Vogliano, a dietitian and research director at the educational nonprofit Food and Planet. It has more selenium than just about any other meat. It’s also low in fat, Vogliano noted — but that means it has fewer omega-3 fatty acids than some other seafood.

There’s not a huge nutritional difference between canned tuna, sushi, and a tuna steak, he added. Cooking the fish might lower its vitamin D, and the canning process might leach out a few nutrients, he said, but its nutritional value is largely the same.

Tuna’s one big health drawback, experts say, is the risk posed by mercury, a neurotoxin. This heavy metal enters the ocean mostly from human activities like burning fossil fuels. It’s absorbed by small organisms and works its way up the food chain and accumulates in bigger, longer-lived species — like sharks, swordfish and, yes, tuna.

In high enough concentrations, mercury can cause serious health problems. Cases of mercury poisoning are rare in the United States, but experts worry about the long-term effects of mercury on the brain — and elevated levels are often more common among urban and coastal populations that eat more seafood.

So what does this mean for tuna eaters? The answer is nuanced because the amount of mercury depends on the species — and there are 15 types of tuna, all of which could end up on a dinner plate. The smallest (and often cheapest), like skipjack, have very little mercury. Albacore and yellowfin can have three times as much; bigeye and bluefin can have far more, Vogliano said.

Because mercury is particularly dangerous for children and pregnant women, the Food and Drug Administration publishes seafood consumption guidelines for them. They recommend no more than three servings (or 12 ounces) a week of canned “light” tuna or one serving of albacore or yellowfin if you’re pregnant, and less for children under 12. Most other developed countries set lower bars, and many experts would recommend that pregnant women and small children avoid the fish altogether.

The FDA doesn’t set limits for the rest of us, but experts say a conservative choice would be to follow the same guidelines. People who eat tuna regularly do tend to have higher mercury levels in their blood than those who don’t, though the metal largely leaves the body after a few months. Even in rare confirmed cases of mercury poisoning from eating seafood, most patients recover after changing their diet.

Experts wrestle with the fundamental tension at play with mercury and seafood, just as consumers do. “There is no level that is risk-free,” cautioned Tracey J. Woodruff, the director of Environmental Research and Translation for Health at the University of California, San Francisco. On the other hand, there’s evidence that eating fish may have brain benefits that outweigh the dangers.

Woodruff summed it up: “You’re a healthy adult, and you eat it every once in a while — it’s probably, maybe, not that big a deal.”

Is eating tuna OK for the planet?

A: If you were a child of the 1980s, you might remember that dolphins were once widely caught in tuna nets. Thanks to years of activism and reforms, this is no longer much of a problem.

However, tuna fishing still takes a toll, especially abroad. Unlike salmon or shrimp, almost all tuna is caught in the wild. Smaller species like skipjack and albacore are netted in vast purse seines that trap other fish as well, ravaging entire ecosystems.

Smaller tuna can be fished sustainably, though, when individual fishing lines are used, as they increasingly are for higher-end products.

Large tuna species pose a different problem: There just aren’t very many fish. The biggest bluefin tuna, often shipped to Japanese markets, can be a dozen feet long, weigh as much as a grand piano, and sell for over $1 million.

Top predators like these will never be as plentiful as the mackerel or sardines they eat, and fishing has threatened them gravely. Pacific bluefin, probably the tuna at greatest risk from overfishing, is at just 10% of its historical population. That’s far better than the 2% it fell to in 2010, but fisheries experts worry there still aren’t enough to support wide consumption.

Buying Better Tuna

To choose the most sustainable and safest tuna, start with the label.

“If I were to tell someone one thing, and they wanted to know how to eat sustainable tuna, go with pole-and-line-caught or troll-caught,” said Andre Boustany, a fisheries biologist with the Monterey Bay Aquarium who advises its seafood guides.

A can of sustainable tuna fish will say something about how the animals were fished — by pole and line, by “troll” or “free school.” This means that steps were taken to catch only the tuna as opposed to everything swimming nearby and that the fisheries were most likely well managed.

If there is no mention of how the tuna was caught, or it says “chunk light,” that probably means it was caught in a purse-seine net and comes at a greater ecological cost.

Sustainably caught canned tuna is generally more expensive — starting around $3-$4, Boustany said, pointing to a stack of tins in his office that he uses for lunch salads. But that’s still pretty cheap for a meal, he added, and the quality tends to be better since nets bruise the fish.

At the fish counter and restaurants, many conservation experts say, to avoid bluefin, no matter how it’s caught. But yellowfin, which also makes for beautiful steaks and sashimi, can be fished sustainably. Look for fish that’s pole-caught in the Pacific, experts say or certified by a group like the Marine Stewardship Council. Avoid fish from the Indian Ocean, where fishing rules are poorly enforced, Boustany said.

If your primary concern is mercury, the tuna species matters even more. Bluefin and bigeye tend to have the most. Skipjack is your safest choice, while albacore (often labeled “white”) and yellowfin probably have more mercury. Cheaper “light” tuna is often a combination of skipjack and albacore, so shop carefully.

Even the experts tuck in occasionally. “I love a good tuna fish sandwich with some pickles and a little bit of mayonnaise or some olive oil and lemon,” said Vogliano. “I personally don’t worry about the mercury, because I’m not eating it every day.”

Woodruff sometimes orders tuna at sushi restaurants, but she tries to buy other fish at her local market.

“Have you had black cod? Freaking amazing,” she said. “It’s small, and they catch it locally out here in California. So it’s like a win-win-win fish.”

This article originally appeared in The New York Times.