Oct. 9, 2012 -- A new study highlights the difficulty pregnant women face while eating for two, finding that more mercury exposure leads to a higher incidence of ADHD symptoms, while more fish consumption -- the main source of mercury exposure -- leads to a decreased risk.
"How much fish you eat is not equivalent to how much mercury you are exposed to," said study author Dr. Susan Korrick of Brigham and Women's Hospital. "I think the public health conclusion that I would come to is that one can benefit from fish consumption, but it's important to try to consume fish that are low in mercury."
Researchers at Brigham and Women's tested more than 400 women for mercury about 10 days after they gave birth between 1993 and 1998, and asked them to fill out a survey about their fish consumption. They measured the mercury in samples of the mothers' hair. When the children were eight, researchers tested their cognitive abilities with a parent questionnaire and other tests, searching for symptoms of ADHD. (It is important to note these children were not diagnosed with clinical ADHD, but only exhibited some of the symptoms.)
Symptoms of ADHD, or attention deficit hyperactivity disorder, include inattention, hyperactivity and impulsive behavior, according to the National Institutes of Health.
Researchers say cutting fish out of the prenatal diet to avoid mercury exposure entirely is a bad idea, and pregnant women should look for fish that are low in mercury, such as salmon.
"It's elegantly showing the paradoxical paradigm that it's both good for you and bad for you," said Christina Chambers of the Organization of Teratology Information Specialists' Collaborative Research Center in San Diego, who read the study but was not involved in it. Teratology is the study of abnormalities in physical development.
"They're finding the kids are slightly above average in the number of symptoms," Richard Gallegher, a professor of child and adolescent psychiatry at NYU Langone, said of the children born to mothers with higher mercury levels in their hair samples. "They [the ADHD symptoms] can certainly impact how well kids are tending to things in school."
Researchers at Brigham and Women's also learned that pregnant women who ate more than two 6-ounce servings of fish a week were less likely to have children with these symptoms. This is actually more than the Federal Food and Drug Administration recommends, which is only 12 ounces of fish a week.
Because the fish consumption survey was originally designed to look at organic chlorine contaminants, the fish were grouped by how much chlorine they were likely to contain -- not how much mercury they had, Korrick said. As a result, the study could not name which fish increased ADHD symptoms and which did not.
Fish high in mercury include shark, swordfish and fresh tuna, Korrick said. Fish with lower mercury levels, which are also rich in healthy fats, include salmon, rainbow trout and herring. A third group, which has different health benefits but still is low in mercury, includes cod, shrimp and haddock.
Although exposure to some chemicals during pregnancy can lead to birth defects you can see, mercury is neurotoxic and can cause changes to the central nervous system that lead to behavioral problems, Chambers said.
Chambers said the study also does a good job of correcting for other factors that could lead to ADHD, such as lead and PCB exposure, parents' education, and mother's IQ.
"They still come up with this significant increased risk of ADHD, especially among boys, with a higher hair mercury level," Chambers said.
Korrick said she and her co-researcher, Sharon Sagiv of Boston University School of Public Health, looked at ADHD symptoms in part because IQ tests are often so broad that behavioral differences are missed. Looking for ADHD symptoms helped them tease out behavioral and cognitive functions more easily.
ADHD is the "most commonly diagnosed behavioral disorder of childhood" and affects 3 percent to 5 percent of schoolchildren, according to NIH.