Children exposed to low levels of mercury in the womb through their mothers eating fish high in the metal have an increased chance of developing attention deficit-related disorders, according to a study that suggests pregnant women avoid some types of seafood.
The research online today in the Archives of Pediatrics and Adolescent Medicine also showed that pregnant women who ate more than two servings of fish a week, most likely those low in mercury such as shrimp, salmon and canned light tuna, have a 50 percent reduced risk of having children with attention deficit hyperactivity disorder-related behaviors, said senior study author Susan Korrick.
Today’s findings add to previous research that has shown that exposure to environmental toxins, including tobacco and lead, can lead to attention deficit hyperactivity disorder, or ADHD, said Bruce Lanphear, who wrote an accompanying editorial in the journal. Health officials should take greater steps to keep these toxins from pregnant women and children, he said.
“It is time to convene a national scientific advisory panel to evaluate environmental influences of ADHD and make recommendations about what can be done to prevent it,” Lanphear, a professor of health sciences at Simon Fraser University in Vancouver, wrote in the editorial. “This study and a flurry of new evidence linking environmental contaminants with ADHD reinforce the urgency of revising the regulatory framework for environmental contaminants and toxicants.”
Lanphear said lead and tobacco, as well as certain pesticides and other environmental toxins have been linked to ADHD. About 8 to 12 percent of children worldwide have been diagnosed with ADHD, making it one of the most common childhood disorders.
It’s unclear exactly how mercury can lead to ADHD-related behaviors, Lanphear said. It’s thought that the behavioral disorder results from an imbalance or a deficiency of the brain chemical dopamine, and mercury may alter dopamine levels or change the dopamine receptors, he said.
Researchers in the study analyzed data on a group of infants born from 1993 to 1998. Mothers had given hair samples around the time their child was born so their mercury levels could be tested. The researchers followed up to see if the children had any ADHD-related behaviors such as a lack of impulse control or the inability to sit in a classroom at age 8. The study didn’t diagnose the children with ADHD and it didn’t look at which fish the women ate during pregnancy, just the frequency.
The U.S. government recommends pregnant women eat no more than two, six-ounce servings of fish a week.
The researchers found that women with hair mercury levels of 1 microgram per gram, about what the U.S. government says is safe to ingest, had children who were more likely to have attention deficit-related behaviors than those whose hair mercury levels were less, Korrick, an assistant professor of medicine at Brigham and Women’s Hospital and at Harvard Medical School in Boston, said in an Oct. 5 telephone interview.
Of the mothers with hair mercury levels of 1 microgram per gram or greater, every 0.5 microgram per gram increase in mercury was associated with a 50 percent to 60 percent higher risk of having a child with attention deficit-related behavior, she said.
“The fish consumption and hair mercury levels, while related, are not the same, and depending on the type of fish women eat and the mercury levels in the fish, you may see a predominance of beneficial effects from the fish’s nutrient content or deleterious effects from the mercury content,” she said.
Swordfish, shark and king mackerel are among the fish high in mercury, according to the U.S. Environmental Protection Agency.
Korrick said researchers will continue to follow the children from the study.
“It will be important to think about nutritional contributors to ADHD-related behaviors and obviously get a better understanding of the relative role that contaminated exposures may play in ADHD risk,” she said.
The findings also demonstrate for the first time that nutrients, like omega-3 fatty acids, in fish may be beneficial to the developing fetus but only if it’s low in mercury, Lanphear said in an Oct. 5 interview.
The study “reinforces this idea that fish is a good, healthy food for us to eat, and in the long term, we need to find ways to reduce the mercury contamination in fish,” he said.