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Limit Fish While Pregnant? Study Questions Advice

Mercury, Fish, and Attention in Kids

For the study, which is published in the Archives of Pediatric & Adolescent Medicine, researchers at Brigham and Women’s Hospital in Boston followed a group of 788 babies born in the coastal community of New Bedford, Mass. Shortly after the children were born, about 400 mothers agreed to let researchers test their hair for mercury, a heavy metal that is a potent nerve toxin. 

Mercury concentrates in the flesh of larger predator fish like tuna, sharks, mackerel, and swordfish.

About 500 mothers in the study answered detailed questions about their diets, including how much fish they ate. And they ate a lot of fish -- almost four servings a week, on average.

Eight years later, researchers gave kids tests to measure their attention and impulsiveness. They also asked the children’s teachers to rate how distracted and hyperactive they were in class.

Researchers found that mothers with mercury levels over 1 microgram/gram were more likely to have children who showed signs of ADHD than those with lower mercury levels.

Other studies, including one published a few weeks ago about Inuit Eskimos, have shown that children exposed to very high levels of mercury in the womb are more likely to have trouble paying attention in class.

Mercury Linked to ADHD Behaviors, Even at Lower Levels

The new study is first to see the association in children exposed to lower levels of mercury.

“Most of the research has been in highly exposed populations,” says Sagiv. “Our levels were high compared to the U.S. population but not much higher.”

At the same time, women who ate more than two 6-ounce servings of fish each week during pregnancy were less likely to have kids who were inattentive and hyperactive in class. What’s more, those kids were able to solve problems more quickly on a computer test, and they were less likely to be distracted while they were taking the test.

Those findings held even after researchers massaged their data, trying to remove the influence of other things that are known to be risk factors for attention problems and hyperactivity, like a mother's age, her education, smoking during pregnancy, and other kinds of drug use.

And surprisingly, even though women who ate a lot of fish also had high levels of mercury, the findings didn’t change when researchers separated fish consumption from mercury exposure. More fish still lowered the risk of hyperactivity and wandering attention, while more mercury raised the risk for those behaviors.

What could be at play here, according to researcher Susan A. Korrick, MD, MPH, an associate physician at Brigham and Women’s Hospital in Boston, is that it's possible that a woman could eat a lot of fish low in mercury, and her children could "reap the benefits of the nutritional content of the fish" instead of the harm from the mercury.

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