Mercury in Fish: Is It Still Safe to Eat Seafood?

How much is too much when it comes to this heart-healthy food?

By Dulce Zamora
WebMD Weight Loss Clinic - Feature

Reviewed By Michael Smith, MD

Many fish aficionados have been using the same gag for years: "Yes, I love seafood. When I see food, I eat it!" Yet, there seems to be nothing funny these days about conflicting reports swimming around on the safety of seafood.

One minute, we're told the fruits of the ocean are filled with harmful chemicals such as mercury. The next moment, we hear that perhaps the mercury in fish isn't as bad for us as previously thought.

Then there's the whole uproar over farmed vs. fresh fish. Some environmental groups have been crying foul about high levels of toxins in pen-raised seafood. Many in the fish-farming (aquaculture) industry, however, insist that what's nurtured is just as safe as what's captured in the wild.

The hullabaloo has been enough to unnerve seafood lovers who are worried about the health consequences of eating fish. It is actually quite a paradox, given that many groups, such as the American Heart Association (AHA), the American Dietetic Association (ADA), and the CDC, fully endorse the health benefits of fish.

Provision and Poison

Seafood is regarded as an important part of a balanced diet, primarily because it contains high-quality protein and omega-3 fatty acids. The latter prevents blood from clotting and protects against irregular heartbeats.

The heart-health benefits of fish are so pronounced that the AHA recommends at least two servings of it a week, particularly fish such as mackerel, lake trout, herring, sardines, albacore tuna, and salmon, since they contain omega-3 fatty acids.

This fish tale would, indeed, be a perfectly happy one, if it weren't for the menacing presence of something else in all fish: mercury. Mercury exists naturally in the environment, and more of it is released into the air, land, and water by activities such as trash burning, fossil fuel combustion in factories, mining, and the dumping of sewage sludge in croplands.

Once mercury is in water, it quickly makes its way through the marine food chain. In smaller organisms, there is usually an insignificant amount of the substance, but as bigger fish eat the smaller ones, the quantity of the element accumulates. Consequently, the fish at the top of the food chain, such as pike, bass, very large tuna, tilefish, king mackerel, shark, and swordfish, tend to have higher levels of methylmercury, approximately 1 to 10 million times greater than the amount in surrounding waters, according to the Environmental Protection Agency (EPA).

There is no disputing that extremely high exposure to mercury can kill people, says Thomas Burke, PhD, member of the National Academy of Science's committee on the health effects of methylmercury. "You can have a seizure and die," he says.

Burke says that a high concentration of the chemical can possibly cause problems in childbirth, the circulatory system (perhaps becoming a risk factor for heart disease), and the nervous system (causing developmental problems, even with low exposure, particularly in kids).

Researchers are still trying to figure out the extent of the negative health effects due to low-level methylmercury exposure, but for now, the FDA, which regulates commercially sold fish, considers safe up to 1 part per million (ppm) of mercury in fish.

The agency reports that, on the average, the goods in U.S. seafood market contain less than 0.3 ppm of methylmercury.

Here's more good news from the FDA: the top 10 seafood species (which make up about 80% of the U.S. seafood market) -- canned tuna, shrimp, pollock, salmon, cod, catfish, clams, flatfish, crabs, and scallops -- generally contain less than 0.2 ppm of methylmercury.

"Don't go AWOL with individual reports...don't become so obsessed as to destroy moderate eating."

A recent report published in the Aug. 29 issue of the journal Science casts doubt on the true dangers of fish eating. In lab tests, Stanford University researchers determined that the mercury in fish may be a different type than previously thought. There are reportedly 26 different known compounds of mercury, and the kind researchers now suspect in fish may be less toxic than the old variety.

Yet this doesn't change the fact that mercury as a substance is, overall, not good for people, says Gail Frank, RD, spokeswoman for the ADA and professor of nutrition at California State University in Long Beach. "We don't want to choose foods because it has mercury," she says. "We also don't want to go around making major changes to our eating pattern just because of one report."

This means people shouldn't be eating much more or much less fish than usual, explains Frank. For good health, she suggests two to four 3-ounce servings of fish per week.