New data on methylmercury and fetuses.

Eating fish is the main way most people are exposed to methylmercury, a neurotoxicant that in large doses causes mental retardation, seizures, cerebral palsy, and death. Because the fetal brain is particularly sensitive to methylmercury, the Seychelles Child Development Study was started in 1986 to ascertain the effects of low doses of methylmercury among 779 children living in these islands, whose mothers ate an average of 12 fish meals a week while pregnant. The latest report on the children at age 9 finds no detectable risk to the children.

The Seychelles "is a perfect environment for a longitudinal cohort study of child development," says Davidson. "The population is a sentinel for the United States because they eat more fish." A former British colony, the island Republic of Seychelles, located in the Indian Ocean northeast of Madagascar, boasts free prenatal care, a low infant mortality rate, little pollution, and high socioeconomic status. This isn't a society where many children suffer from childhood diseases due to lack of medical care. So the children are basically healthy overall.

The Seychelles children were tested previously at 6, 19, 29, and 66 months of age. In the latest study, described in the 17 May 2003 issue of The Lancet, a battery of 21 tests that measure cognitive ability, neurological function, and social behavior detected no major abnormalities. "We have been unable to confirm any adverse effects of prenatal exposure to mercury from eating fish in the Seychelles," says coauthor Philip Davidson, a professor of pediatrics at New York's University of Rochester Medical Center.

In fact, the study found that increased maternal hair mercury was associated with better scores on a scale of hyperactivity. The researchers speculate that some nutritional components in fish, and not a beneficial effect of mercury, may account for this finding. (Similarly, at 66 months, children with higher mercury had better language skills, which Davidson also thinks is likely due to nutrients in the fish.) The team has launched a new study to explore whether omega-3 fatty acids or other nutrients in fish impact hyperactivity and other developmental parameters.

The Seychelles mothers ate a variety of fish that contain similar concentrations of methylmercury as fish eaten in the United States. Their weekly fish intake was about 10 times greater than that of U.S. women, and the most commonly consumed fish was karang, a species of jack fish. The mothers' hair mercury levels averaged 6.9 parts per million (ppm), compared to 0.5 ppm in U.S. women.

Other groups' findings have not been so sanguine, and, while experts agree that methylmercury can harm the developing fetus, there is controversy regarding what constitutes a safe level of exposure. In studies of Faroe Islands women who ate diets high in whale blubber while pregnant, impairments in language, attention, and memory were found in their children at age 7. In New Zealand, a similar situation occurred with pregnant women eating shark muscle, popular in fish and chips. However, whale blubber and shark muscle contain 5-7 times as much methylmercury as the fish eaten in the Seychelles. Whale blubber is also high in polychlorinated biphenyls and other persistent pollutants. In Minamata, Japan, in the mid-1950s, children were born with mental retardation in a population that consumed fish contaminated with mercury from industrial pollution. The hair mercury of pregnant women in Minamata ranged from 25 to 50 ppm.

"The Seychelles study addresses a very specific issue--the risks to the developing child of a pregnant woman eating mercury in fish," says neuropsychiatrist and epidemiologist Constantine Lyketsos of The Johns Hopkins Hospital in Baltimore, Maryland. So far, the levels of mercury consumed there have proven harmless. However, "higher levels of exposure may be harmful during pregnancy," cautions Lyketsos.