A major source of toxic mercury exposure, fish consumption appears somewhat protective against a widespread neurologic disorder in children

By Janet Raloff

Web edition: October 8, 2012

For pregnant women, diets rich in fish can offer their babies protection against  developing behaviors associated with attention-deficit/hyperactivity disorder, or ADHD, a new study finds. Yet for most Americans, fish consumption is the leading source of exposure to mercury — a potent neurotoxic pollutant that has been linked to a host of health problems, including delays in neural development.

Data from the new study, published online October 8 in Archives of Pediatrics and Adolescent Medicine, demonstrate that low-mercury diets and regular fish consumption are not mutually exclusive, says epidemiologist and study leader Susan Korrick of Brigham and Women’s Hospital in Boston. “It really depends on the type of fish that you’re eating,” she says. In fact, some study participants had been eating more than two servings of fish weekly yet accumulated relatively little mercury.

As part of a long-running study of children born during the 1990s in New Bedford, Mass., 515 women who had just given birth completed a dietary survey. About 420 also provided samples of their hair for mercury testing. About eight years later, Korrick’s team administered a battery of IQ and other tests to assess behaviors associated with ADHD in the children.

The children spanned a continuum running from almost no ADHD-related behaviors to those with outright clinical disease. A mom’s hair-mercury level tended to be associated with where her child fell along this spectrum.

Although this study did not collect data on the species of fish eaten, Korrick points to work by others showing that tuna, swordfish and shark can be quite high in mercury, while salmon and cod tend to pick up relatively little of the toxic metal from their environment.

Among women with less than 1 microgram of mercury per gram of hair, fish consumption was associated with a lower risk of ADHD-type behaviors in their children. Over that threshold, increasing mercury levels were associated with an increased risk of ADHD-type behaviors in the kids, regardless of how much fish their moms ate.

Children of women with hair mercury levels in the top 20 percent of the study population showed a 50 to 60 percent increased risk of ADHD-related behaviors, Korrick says — “which is not trivial.” However, she adds, most children showing ADHD-related traits “were still considered to be within the normal range — and not maladaptive.”

On some tests, boys showed a greater sensitivity to mercury than girls. These tests included components of the IQ assessment related to attention and one computer test of attentiveness (where children had to press a button as quickly as they could when they saw the silhouette of a cat but not other animals).

In an unrelated study, Gina Muckle of Laval University in Quebec and her colleagues found an association between elevated mercury concentrations in children at birth and at school-age and an increased risk of ADHD by around age 11. The study also confirmed earlier evidence suggesting a link between lead and ADHD, the researchers report in the October Environmental Health Perspectives.

Studies have begun pointing to a genetic susceptibility to ADHD in some individuals (SN: 9/10/11, p. 12). However, pollutants are also emerging as risk factors, notes Bruce Lanphear of Simon Fraser University in Vancouver. Besides lead and mercury, these pollutants include tobacco smoke and possibly polychlorinated biphenyls, certain pesticides and bisphenol A, he says.