We've all heard the advice on how to improve our diets: Eat more fruit, vegetables, and whole grains, less red meat -- and more fish. Even the American Heart Association recommends eating fish at least twice a week. Fish is low in saturated fat and high in heart-friendly omega-3 fatty acids. So fish is good for us, right? The answer is yes, but with a caution: Seafood is the only significant source of mercury in our food, and some fish have levels of mercury so high that it may be harmful to a developing baby.

Heavy metals like mercury and lead are toxic to the brain and nervous system. Though acute poisoning is rare, organic mercury compounds like methylmercury (found in seafood) may have subtle effects on fetal and childhood development. Methylmercury easily passes through the placenta to the fetus and deposits in fetal tissues. The toxin also passes through breast milk, but not as easily as through the placenta.

A tragic occurrence in Japan in the 1950s originally clued researchers in to the effect of methylmercury on the developing brains of fetuses. Women who ate fish from waters that had been heavily contaminated by industrial mercury waste gave birth to babies with disabilities ranging from cerebral palsy to mental retardation. Unfortunately, researchers were unable to determine how much methylmercury the mothers ingested, so they couldn't establish a safe level of exposure.

No, it just means you need to choose your fish more carefully, especially if you eat a lot. In general, the larger and longer-lived the species, the more mercury the fish will accumulate by the time it's caught and eaten. Methylmercury binds to the muscles of fish, so it can't be removed by cutting it out or by cooking the fish.

Fish and shellfish are important sources of lean protein and omega-3 fatty acids, and you don't have to give them up entirely. According to the U.S. Food and Drug Administration (FDA), it's safe for pregnant and nursing women to eat up to 12 ounces of low-mercury seafood per week, OR up to 6 ounces of albacore (white) tuna per week. The FDA also lists four types of high-mercury fish to avoid entirely while pregnant: shark, swordfish, king mackerel, and tilefish from the Gulf of Mexico (see below for more details).

In general, smaller fish of a given species are lower in mercury. Most types of shellfish also have low levels of mercury, although North American lobster is an exception. Scientists consider fish with less than 0.1 milligrams of mercury per kilogram to have low levels of mercury; moderate to high levels range from 0.16 to .5 mg/kg or higher. The chart below indicates average mercury levels for some common seafood.

(Note: Many experts feel that wild-caught salmon from the West Coast is better, both from a health standpoint and for the environment, than farmed salmon. Most Atlantic salmon on the market is farmed, and the flesh is colored with dyes. Some farmed salmon also have high levels of PCBs, chemical compounds linked to a variety of ills.)

Certain large, long-lived species of fish are known to have high levels of mercury, and it's best to limit them to one serving a week. The FDA advises that young children, pregnant and nursing women, and women who may become pregnant in the near future avoid the following four species entirely.

(Note: Marlins are billfish similar to swordfish and often have higher than normal mercury levels as well. And while the mackerel species includes Hawaiian ono or wahoo, average levels in ono are only half as high, 0.4 mg/kg, putting it in the moderate-to-high-level category.)

The FDA warning does not specifically mention bluefin tuna (maguro), which is severely overfished and is sold as steaks and sashimi. Bluefin is a large species with reported mercury levels at least as high as the four listed above (average levels reported in Japan are 1.3 mg/kg, ranging as high as 6.1 mg/kg).

Many species have moderate to high levels of mercury (between 0.2 and 1.0 mg/kg), and it's not always easy to tell if it's safe to eat large amounts or not. This moderate category includes most tuna. Keep in mind that a particular fish at the grocery store may have a higher or lower amount of mercury than average for that species, depending on its age, size, and where it's caught. Some average figures:

Yes and no. It's a little complicated, but canned chunk light tuna is the safest choice. The new FDA guidelines state that it's safe for women who are pregnant or nursing (or thinking of becoming pregnant soon) to eat up to 12 ounces of seafood low in mercury, OR up to 6 ounces of albacore (white or tombo) tuna per week. Albacore tuna is sold as steaks and canned (solid and chunk) and is the only kind that can be sold as "white" tuna.

The reason that the FDA singled out albacore is because it tends to have higher levels of mercury than other tuna varieties commonly eaten in the U.S. However, some experts feel that even 6 ounces of albacore a week is too much for pregnant women; to be on the safe side, some advise pregnant women and young children to avoid albacore, bluefin, and other large tuna species altogether.

Other canned tuna -- sold as "light" tuna (solid and chunk) -- can be from different species of tuna. The most common canned species are skipjack (aku), albacore, and yellowfin (ahi); some canners also use tongol. In general canned chunks are made from smaller fish and generally have half to one-third the level of mercury found in albacore (most canned tuna averages 0.12 mg/kg). Keep in mind that if you eat any tuna as steaks or sushi they're likely to be from larger fish, so you should apply the albacore guidelines to be on the safe side.

If you stick to safe choices, you don't need to give up seafood. If you're still too nervous about eating fish, giving it up will reduce your baby's exposure as much as possible, but it may not eliminate it entirely. That's because mercury stays in the body for many months. Once you stop eating seafood, the level of methylmercury in your blood will almost certainly drop slowly back down to a safe level or zero.

So if you decide to give up fish for a while (or if the smell just turns your stomach while you're pregnant), make sure you include other sources of lean protein such as poultry, dairy, or beans like soy. To make up for the healthy omega-3 fatty acids you'll miss out on, try a fish oil supplement or adding walnuts to your cereal.

-- Deepi Brar is the senior multimedia editor of Consumer Health Interactive. As a research associate in a gene therapy lab for three years at the City of Hope National Medical Center in Los Angeles, she has co-published scientific papers in peer-reviewed journals such as Cancer Gene Therapy. A member of the Association of Health Care Journalists, she earned a B.S. in biology and literature from the California Institute of Technology and a master's degree in journalism from the University of California at Berkeley.

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First published August 15, 2005
Last updated November 30, 2007
Copyright © 2005 Consumer Health Interactive

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