Mothers’ mercury levels linked to later ADHD behaviors in kids

By Deborah Kotz, Globe Staff

Pregnant women are often faced with a conundrum about whether to consume fish to help aid fetal brain development or to avoid it because certain fish can contain high levels of brain-damaging mercury.

A new study of children from the New Bedford area could help guide pregnant women when it comes to fish consumption. Researchers from Brigham and Women’s Hospital and the Harvard School of Public Health found that children born to women who had high levels of mercury soon after giving birth were more likely to exhibit signs of attention deficit hyperactivity disorder in school by eight years of age.

On the flip side, those children whose mothers consumed the most fish while pregnant were the least likely to exhibit fidgety, distracted, and impulsive behaviors in class, according to the study of 604 children published Monday in the Archives of Pediatrics and Adolescent Medicine.

At first blush, this finding appears contradictory because most of the mercury we consume comes from fish. “It seems a little paradoxical,” said study co-author Dr. Susan Korrick, an assistant professor of medicine at Brigham and Women’s. “But fish consumption doesn’t necessarily correlate with mercury levels since you could eat a high amount of fish that are low in mercury.”

Women in the coastal town of New Bedford tend to eat more fish than the average American. Those in the study ate an average of four servings of fish per week, and the researchers found that mothers who consumed two or more six-ounce servings of fish per week during pregnancy had children who had a 60 percent lower risk of developing ADHD-like behaviors.

Fatty kinds of fish such as salmon, tuna, and sardines have higher levels of omega-3 fatty acids, which are thought to be crucial for cognitive function. All types of fish have a host of nutrients such as vitamin D, B-12, and iodine, which could play a role in brain development as well.

While government agencies have advised pregnant women to limit their fish intake to no more than two six-ounce servings a week, Korrick said they might want to aim for three or four servings of low-mercury fish such as salmon, canned light tuna, haddock, cod, and shrimp. (Albacore tuna has more mercury, so consumption should be limited to six ounces a week.)

The key is for pregnant women to avoid fish known to have high mercury levels, including swordfish, shark, tilefish, and king mackerel, Korrick added.

Women in the study with high mercury levels had a 40- to 70-percent greater likelihood of having kids with ADHD symptoms, with the highest levels associated with the greatest increase in risk.

About 15 percent of the children in the study exhibited signs of ADHD based on teacher surveys, but the researchers didn’t measure actual cases of ADHD diagnosed by physicians. The study also couldn’t prove that mercury and fish consumption actually affected ADHD behaviors, but merely made a statistical association.