What You Need to Know About Eating Fish

Last Editorial Review: 4/8/2005

With concerns about mercury levels in fish -- a usually healthful food -- how much should women of childbearing age eat?

By Jeanie Lerche Davis
WebMD Weight Loss Clinic - Feature

Reviewed By Charlotte Grayson, MD

Fish and shellfish have gained star status on the dinner menu. Several medical groups now advocate tuna, salmon, and their fishy (and shellfish) cousins as important to a heart-healthy and overall healthy diet.

But for women, the choice has been less clear. The concern: Are fish and shellfish safe -- if pregnancy and children are in the picture? Could mercury in fish put an unborn, newborn, or young child at risk? Should pregnant women eat fish?

Various reports have turned up conflicting results -- some indicating risk, others pooh-poohing all the worry. To clarify this murky issue, WebMD turned to some of the nation's experts.

"[Pregnant] women should be cautious because their unborn fetus is very sensitive to toxicity from mercury," says Robert Goyer, MD, professor emeritus and chairman of pathology at University of Western Ontario. Goyer participated in a National Academy of Sciences (NAS) study evaluating the credibility of the EPA's mercury studies.

"We came up with the same results the EPA did," Goyer tells WebMD. "We don't know which stage of fetal development is more critical -- whether it's the third trimester or the moment of conception, or if it's continuous exposure to mercury during pregnancy. But all this has been factored together in the EPA/FDA advisory."

Government's Advice to Pregnant Women

In their statement issued last year the Environmental Protection Agency (EPA) and FDA -- for the first time -- cited the health benefits of fish.

"Fish and shellfish contain high quality protein and other essential nutrients, are low in saturated fat, and contain omega-3 fatty acids," says their joint statement. "A well-balanced diet that includes a variety of fish and shellfish can contribute to heart health and children's proper growth and development. Thus, women and young children in particular should include fish or shellfish in their diets due to the many nutritional benefits."

However, mercury may be harmful to an unborn child or a young child. Mercury may have damaging effects to a child's developing brain.

"It may be prudent to modify your diet if you are: planning to become pregnant; pregnant; nursing; or a young child," the EPA statement adds.

The EPA and FDA advise pregnant women, young women who may become pregnant, or women who are nursing:

  • Do not eat: Shark, swordfish, king mackerel, or tilefish because they contain high levels of mercury.
  • Eat up to 12 ounces a week: Shrimp, canned light tuna, salmon, pollock, and catfish. (An average can of tuna is 6 ounces.)
  • Buy canned tuna carefully. Light tuna has less mercury than albacore ("white") tuna. However, up to 6 ounces (one average meal) of albacore tuna per week is safe.
  • Check local fish advisories: Locally caught fish should be checked with local EPA departments. If no advice is available, eat up to 6 ounces (one average meal) per week of fish you catch from local waters, but don't consume any other fish during that week.
  • Apply these guidelines to young children: They can eat these low-mercury fish and shellfish. However, feed children smaller portions.
  • Fish sticks: Frozen fish sticks and fast-food fish sandwiches are commonly made from fish that are low in mercury.
  • Tuna steaks generally contain higher levels of mercury than canned light tuna.
Undisputed Benefits of Omega-3 Fats

The omega-3 fats in many fish and seafood are known to lower risk of heart disease and benefit the brain. The American Heart Association advises at least two servings a week of fish like mackerel, lake trout, herring, sardines, albacore tuna, and salmon because of these healthy fats.

In a developing fetus, omega-3 fats promote brain, eye, and motor development, the EPA notes.

The mercury in fish and seafood is indeed the big concern -- although there are other toxins like PCBs that have warranted some worry. Mercury exists naturally in the environment, but more is released into air, land, and water by trash burning, fossil fuel combustion in factories, mining, and the dumping of sewage sludge in croplands.

"It's one of those questions that comes up almost every day? mercury and fish."

Once mercury gets into surface water, it quickly makes its way through the aquatic food chain. In smaller organisms, there is usually an insignificant amount of mercury. But as fish get older or as bigger fish eat smaller ones, the mercury content begins to build.

Fish at the top of the food chain -- pike, bass, older or large tuna, tilefish, king mackerel, shark, and swordfish -- tend to have higher levels of mercury, from one to 1 million times greater than the amount in the waters, according to the EPA.

If you're eating a lot of fish, mercury accumulates in your bloodstream over time. While the body naturally gets rid of mercury, it may take a year for the levels to drop significantly. Thus, it may be present in a woman even before she becomes pregnant. This is the reason why women who are trying to become pregnant -- or pregnant women -- should also avoid eating certain types of fish.

For women wanting to switch to other omega-3 sources, there are options, says Julie Redfern, RD, a registered dietitian in obstetrics at Brigham & Women's Hospital in Boston. She has counseled thousands of pregnant or soon-to-be-pregnant women.

"It's one of those questions that comes up almost every day ? mercury and fish," Redfern tells WebMD. "Some women are very well-read, and they say they are not going to eat any fish. Others say, 'I love fish,' and want to know what's safe. I give them the FDA's list of safe fish. I ask them what fish they usually eat, and look for it on the list. I also talk to them about canned tuna, about the different kinds of tuna -- and what's on the 'avoid' list."

Overall, she says, "I feel very comfortable reassuring them that if they keep it to the 'safe' fish -- and eat no more than two servings a week -- they'll be fine."

But, flaxseed oil, walnuts, canola oil, wheat germ, and omega-3-fortified eggs are excellent food sources for these fats. Also, a couple of new prenatal vitamins -- and a 200 mg supplement -- contain an algae-derived form of omega-3 fats, she adds.

"These are from vegetable plants, so the fats are not quite the same ? the body converts them more slowly. But if someone doesn't want fish, it still works."

Redfern identifies with those who love fish and hate to cast it from their diet entirely. "With them, I advise making sure you're not eating as much as in you once did. You don't want to frighten them, make them not eat fish at all."

Another Issue: PCBs in Farmed Fish

The content of PCBs in fish has not received as much attention. These are synthetic chemicals released into the environment through commercial manufacturing activities. Even though PCB chemicals have been banned in the U.S. since 1979, they continue to be a threat because it does not biodegrade quickly; PCBs continue to "live" in groundwater and soil.

PCBs are a likely cause of cancer, the EPA says. At high levels, PCBs kill lab rats, cause developmental problems, or cause damage to the liver, kidney, and nervous and endocrine systems.

"I think all marine and freshwater fish have PCBs," says Goyer, who has retired to live in Chapel Hill, N.C. "The North Carolina EPA advisory board is more concerned about PCBs than mercury in fish. There is potential for neurological damage." However, in his NAS study, "we determined that mercury was more critical than PCBs. That's not to say that PCBs don't have an affect."

Bottom line: "It's prudent for women of childbearing age to limit their intake fish," he tells WebMD.

Published March 21, 2005.

SOURCES: WebMD Public Information from the Environmental Protection Agency: "What You Need to Know About Mercury in Fish and Shellfish." Robert Goyer, MD, professor emeritus and chairman of pathology, University of Western Ontario. Julie Redfern, RD, registered dietitian, Brigham & Women's Hospital, Boston. WebMD Feature: "Is It Still Safe to Eat Seafood?" WebMD Medical News: "Mercury in Fish No Problem in Pregnancy."

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