By Brenda Goodman, MA
WebMD Health News
Reviewed by Louise Chang, MD
Oct. 8, 2012 -- Pregnant women are told to limit how much fish they eat because many fish are tainted by mercury, which may harm a baby's brain.
But a new study suggests that advice may be flawed.
The study found that children born to women who ate more than two servings of fish a week during pregnancy -- more than federal guidelines recommend -- were about half as likely as kids born to women who ate less fish to have trouble with attention and hyperactivity at school.
The study can't prove more fish was the only reason kids could function better at school. But fish are rich sources of omega-3 fatty acids, which are important for healthy brain development.
"We saw dramatic protection against these behaviors," says researcher Sharon K. Sagiv, PhD, MPH, an assistant professor of environmental health at Boston University.
"This is only one study. More studies should look at this. But if indeed eating more fish does seem to be protective across different studies, that's an important public health message," Sagiv says.
But the good news about fish comes with a big catch.
Where does the mercury come from? Mostly from fish in mom's diet.
"Eating fish is good for brain development," Sagiv says. "But eating fish high in mercury is a risk for brain development."
What that means, Sagiv says, is that pregnant women should eat fish, but should try to stick to species that are lowest in mercury.
Good choices include catfish, mullet, trout, sardines, sole, tilapia, and wild-caught salmon, according to the Natural Resources Defense Council, a nonprofit environmental group that publishes a guide to mercury in fish.
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.
Conversely, women who eat less fish that's high in mercury could have the opposite effect, making their children more prone to ADHD.
"Given that fish is a major source of mercury, it seems a little counterintuitive that both of these things could be observed simultaneously," Korrick says. "Fish consumption and mercury exposure are related, but they aren't identical."
"It's a complicated message, but the part that's most important from a public health perspective is that fish is a very healthful food for women to eat during pregnancy. It's healthy for women to eat fish during pregnancy as long as it's low in mercury."
Experts who were not involved in the study agree, saying that telling women not to eat fish while they're pregnant to avoid mercury may be throwing the baby out with the bath water.
"The beneficial effects of fish intake are [confused] by the presence of mercury in fish. If you don't take both of them into account, you tend to underestimate the beneficial effects of fish, and you underestimate the adverse consequences of mercury," says Bruce P. Lanphear, MD, MPH, who studies the effect of environmental exposures on brain function at Simon Fraser University in Vancouver, Canada. Lanphear wrote an editorial on the new research.
"The message is that yes, we should be eating fish. Not only is it beneficial for learning abilities, it is protective against ADHD," Lanphear says. "Just eat the fish that's low in mercury."
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