Nearly all fish contain trace amounts of methylmercury. The FDA recommends eating no more than 12 ounces of tuna fish a week.
Two new reports suggest the benefits of consuming fish outweigh the risks of environmental contamination from mercury poisoning.
The reports also address the confusion over the health risks and benefits of eating fish.
Harvard cardiologist Dariush Mozaffarian reviewed all available studies documenting the risks of mercury exposure through eating fish. His team also reviewed all studies showing the health benefits of fish consumption. He says the benefits of consuming fish still outweigh the risks.
"When we put all the evidence together [from clinical trials], we found that eating fish reduces the risk of dying from a heart attack by about 35 percent which is a dramatic benefit," he says.
Mozaffarian's paper, published in the Journal of the American Medical Association on Tuesday, suggests that the average consumer has little reason to worry.
"We didn't find any definite evidence that there are health effects of low-level mercury exposure from seafood intake for the general population," he says. "For the average person, we didn't find any health effects for mercury."
Mozaffarian says several studies were conducted in Europe where scientists analyzed whether higher mercury levels in the blood stream correlate to an increased risk of heart attacks.
"In those studies, some find that mercury is related to harm [while] some show a neutral effect, and some show trends toward [a] benefit — where people with higher mercury levels actually had trends to lower risk of a heart attack," he says.
Given the lack of consistency in the findings, Mozaffarian says there seems to be no significant effect on eating fish for adults. Children, however, should still eat limited amounts of seafood. Several studies from New Zealand and the Faroe Islands suggest that higher levels of mercury exposure can negatively affect children's brain development.
The federal government also recommends that pregnant women and women of childbearing age avoid certain types of seafood that are known to contain high amounts of mercury.
The advice has created confusion for many consumers, particularly mothers-to-be. Jonathan Scher is a Professor of Obstetrics and Gynecology at Mount Sinai School of Medicine.
"Working with pregnant women, it's an immediate concern for them when they're deciding to get pregnant," says gynecologist Jonathan Scher. "What do I do about fish?"
Scher, who teaches obstetrics and gynecology at Mount Sinai School of Medicine in New York, says that women should keep eating fish, particularly species low in mercury.
The findings corroborate a second report released today, from the Institute of Medicine. The report concludes that omega-3 fatty acids in seafood contribute to vision and brain development in infants.
Panel Chairman Malden Nesheim says pregnant women should be advised about which species to avoid and how much is safe to eat.
"A reasonable amount would be 2- 3 ounce servings per week," he says "but they can safely consume up to 12 ounces per week."
The Institute of Medicine report concludes that the best way to end seafood-lovers' confusion is to consolidation information. The group Environmental Defense has developed a list that ranks fish species by mercury pollution.
As a rule, the bigger the fish, the higher the mercury content.
Environmental Protection Agency
Environmental Protection Agency
What is mercury?
Mercury is an element that occurs naturally in the environment. It can also be released into the atmosphere through industrial pollution. Because mercury dissolves easily in water, it accumulates at the bottom of bodies of water.
What is methylmercury? How does it enter the food chain?
In environments without oxygen, mercury combines with carbon to become methylmercury, a highly toxic compound. Bottom-dwelling fish consume methylmercury particles on the ocean floor and are then eaten by larger fish. Large fish such as sharks, swordfish and king mackerel contain the largest amounts of methylmercury because they're higher up in the food chain.
Why is methylmercury dangerous?
Methylmercury is toxic. It can harm the immune system, the gastrointestinal system, and even the genetic code. It attacks the central nervous system. High levels of exposure will result in brain damage and eventual death.
Most people already have some levels of methylmercury in their system from eating fish, but effects of mercury exposure depend on factors like age and amount of exposure. Trace amounts of methylmercury are excreted over time.
Should I stop eating fish?
No. The health benefits of fish outweigh the side effects of methylmercury exposure. The FDA recommends the following:
1. Do not eat shark, swordfish, king mackerel or tilefish. These fish typically contain high levels of mercury.
2. It's not risky to eat up to 12 ounces a week of fish and shellfish that are lower in mercury, such as shrimp, canned light tuna, salmon, pollock, and catfish.
3. You may also eat up to 6 ounces of albacore tuna, but the level of mercury is higher in albacore tuna than in light tuna, so substitute other types of fish for your other 6 ounces.
Why should pregnant women avoid methylmercury?
Developing embryos are particularly sensitive to methylmercury, so pregnant women and women trying to become pregnant are advised to avoid all large fish. They should also eat no more than one six-ounce serving of tuna a week, and should limit overall consumption of fish to no more than 12 ounces a week, according to the FDA.
What if I want to catch and eat my own fish?
Contact your local health department about mercury advisories in the waters where you plan on fishing. Fish that is caught recreationally may be OK, depending on whether your local waters have higher or lower mercury levels. If no advice is available, eat up to one meal a week of fish from local waters, but don't consume other fish that week.
Sources: Food and Drug Administration; Environmental Protection Agency; U.S. Geological Survey; and other NPR reports