Jun 20 2005
For years medical experts have argued, as fish can be harmful as well as healthy, and what advice to give people, particularly pregnant women, about how much is safe to eat.
Now a new study by Harvard University doctors has to some extent settled the debate by concluding that pregnant women can boost their baby's intelligence by eating fish a couple of times a week, but only if they avoid varieties with large concentrations of mercury.
Fish which is full of omega-3 fatty acids, helps young brains develop and seems to protect against heart disease. But some of it is tainted by mercury, a potent neurotoxin that interferes with the building of brains.
Almost all fish contain traces of mercury, but larger marine species such as swordfish, shark and albacore tuna accumulate the highest levels.
Although mercury can harm adults by affecting the memory, causing the hair to fall out and possibly increasing the risk of heart disease, fetuses are the most vulnerable because neurological effects have been found at low levels.
This new study of 135 Boston-area babies is important because it quantifies and compares the risks and benefits of a fish diet.
The researchers concluded that pregnant women should eat fish because their babies are likely to score higher on intelligence tests, but the benefits of the nutrients disappear and the babies' intelligence scores drop substantially if the fish contains high levels of mercury.
The U.S. Environmental Protection Agency (EPA) estimates that about 630,000 babies a year are born with mercury exposure that could reduce their mental abilities.
Dr. Philippe Grandjean, an environmental epidemiologist at the Syddansk Universitet (University of Southern Denmark) and Harvard University who has studied the effects of mercury on children for 20 years, says the study adds to the mounting evidence that women should eat fish but follow warnings to limit the types and amounts they consume.
Grandjean and others have previously presented similar findings for school-age children, he says the study suggests that the effects on infants are quite strong.
According to the report, the women in the study ate fish on average once a week during the second trimester of their pregnancy. The highest intelligence scores were among the babies whose mothers had consumed more than two helpings of fish per week but whose mercury levels remained under 1.2 parts per million.
Dr. Emily Oken of Harvard Medical School, the study's lead researcher, Oken, who specializes in pregnancy and nutrition, says that for each additional weekly serving of fish, the babies' intelligence scores increased by 4 points, or an average of almost 7%. But for every increase of 1 part per million of mercury, the babies' intelligence scores dropped by 7.5 points, or 12.5%. A woman could raise her mercury level by 1 ppm if she ate an average-sized serving of swordfish once a week.
The study findings support the U.S. Food and Drug Administration's guidelines, issued in 2004, which recommend that pregnant and nursing women and those who might become pregnant eat up to two meals, or 12 ounces, of fish a week and that they avoid certain types of fish entirely.
Young children are advised to follow the same guidelines because their brains are still developing.
In it's guidelines the FDA entirely rules out swordfish and shark as well as king mackerel and tilefish, found on the Atlantic Coast and Gulf of Mexico, for pregnant and nursing women and young children. Some white and albacore tuna, canned and fresh, also have high mercury levels. The darker the fish meat, the higher the mercury content.
Sardines, herring, canned light tuna, cod, haddock, tilapia, sea bass and shrimp are considered good, low-mercury choices. Small fatty fishes such as sardines and herring are especially beneficial to babies because they contain a lot of fatty acids.
Salmon is generally low in mercury and high in fatty acids, but some farmed salmon contains high concentrations of other contaminants, PCBs, which are also risky for babies.
In California, grocery stores and restaurants selling fish are required to post mercury warnings for women and young children. The EPA also has issued localized advisories for some species caught by recreational fishermen, particularly in the Great Lakes and the San Francisco Bay.
Oken, who practices medicine at Harvard Pilgrim Health Care in Boston, say that many doctors are as confused as patients about this issue.
Many are women are unaware they should avoid swordfish and limit tuna and other fish. Others stop eating all fish during pregnancy, which means their babies do not get its brain-enhancing effects.
To add to the confusion, scientists disagree on how much mercury is safe.
In the study the babies were subjected to tests designed by neuropsychologists to study early signs of intelligence and memory. The babies were shown photographs of new faces and ones they had been shown before, and the researchers recorded how much time they spent studying each one. Babies score higher on the test if they move quickly from the familiar face, indicating recognition, to exploring the new face.
Dr. Jane Hightower, a San Francisco internist says consumers might have to resort to omega-3 supplements to get the benefits of fish without the risks, she has apparently detected excessive mercury levels in many of her patients, particularly those who eat swordfish.
Mercury, a natural element found in the Earth's crust, when released into the air through smokestacks, spreads globally and accumulates in tissues of animals, particularly fish. The largest sources of man-made mercury emissions are coal-fired power plants, mainly in Asia.
The report is published online in the journal Environmental Health Perspectives.
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