So says a new study of women and children in the Seychelle Islands of East Africa, where ocean fish are a daily staple. The study, appearing in the May 17 issue of The Lancet, found no solid evidence that babies exposed to mercury in the womb suffered neurological deficits later in life.
"To date we have not found evidence to support" a link between fish consumption by pregnant women and developmental problems in their babies, says Dr. Gary Myers, a neurologist at the University of Rochester Medical Center and leader of the research.
But not for lack of trying. Myers and colleagues have been looking for such an association for the last 30 years. They became curious about the idea after investigating an outbreak of mercury poising in Iraq during the early 1970s, caused when people ate seed grain coated with methyl mercury to deter fungus.
The vast majority of American women have relatively minor contact with methyl mercury, the organic form of the element and the version believed to be most toxic. However, about 8 percent of women have blood levels of the metal above the acceptable limit set by the U.S. Environmental Protection Agency, a recent study found.
People are exposed to mercury through coal burning, the incineration of medical waste, dental fillings, and in various occupations. But the principal route of exposure is the diet, through fish and seafood that accumulate the toxin in their own food chain. Mercury levels are almost four times as high in women who eat at least three servings of fish a week, compared to those who eat no fish.
Certain ocean fish are higher in mercury than others. The U.S. Food and Drug Administration recommends that pregnant women avoid eating swordfish, shark, tilefish and king mackerel altogether. Other fish and shellfish should be limited to no more than 12 ounces per week, or between two and four servings. However, most people in this country eat only one fish dish a week.
Myers says the latest study suggests the FDA's recommendations are reasonable.
The researchers tracked neurological development in 779 children and their mothers, who ate an average of 12 fish meals a week. Mercury exposure during pregnancy was measured by sampling the women's hair, which stores the toxin and can be used to estimate the amount a fetus would encounter in the womb.
The typical woman in the study had mercury levels of 6.9 parts per million in her hair, or about seven times the average U.S. exposure.
When the children were 9, the researchers ran them through 21 mental and motor tests to evaluate their language skills, memory, and other important developmental benchmarks. In only one case -- a peg-board test -- increased exposure to mercury predicted a worse score, and only in boys. The researchers attribute this result to chance. Higher mercury exposure also was associated with lower scores on a test for hyperactivity, which again the scientists consider a fluke.
Fish are a good source of important brain-building nutrients, like fatty acids, and it's possible that loading up on these overcomes any deleterious effect of mercury, Myers says. His group is now looking for such an effect in their Seychelles subjects.
Dr. Constantine Lyketsos, a psychiatrist at Johns Hopkins Hospital in Baltimore and author of an editorial accompanying the study, says pregnant women should heed the FDA's caution, though he doubts most are aware of the guidelines. "I think in general fish consumption is probably fine if the concern is the neurodevelopment of the children," he says.
Still, much about mercury remains a mystery, Lyketsos says. In high doses, mercury is certainly toxic, and even deadly. Yet scientists don't know the lower boundary for how much exposure can harm the brain. Nor do they know if exposure over time or a single, high dose is more dangerous. "There's a range of exposure which for some people is harmful and for others is not," he says.