Children's risk of developing attention deficit hyperactivity disorder (ADHD) later in life may be tied to how much fish their mothers ate while pregnant, according to a U.S. study.
Researchers writing in the Archives of Pediatrics & Adolescent Medicine found that eating at least two servings of fish per week was linked to about a 60-percent lower risk of kids developing certain ADHD-like symptoms.
But elevated mercury levels, which can also come from eating more fish - depending on the fish - were tied to a higher risk of developing the symptoms, such as hyperactivity, impulsiveness and inattentiveness.
Though the study did not prove cause and effect, and did not use a formal diagnosis of ADHD, it may offer insights into a condition that's estimated to impact one in 10 children in the United States, researchers say.
"The really important message is to eat fish," said Sharon Sagiv, the lead author from the Boston University School of Public Health. "Just stay away from mercury-containing fish, because these protective effects are pretty important."
She said it's best to stay away from "big fishes," such as tuna and swordfish, which typically have the most mercury. Instead, eat fish such as haddock and salmon.
Past studies looking at the link between mercury and ADHD had conflicting results. For the new study, the researchers followed 788 children who were born near New Bedford, Mass., between 1993 and 1998. They used hair samples taken from the mothers after delivery to test their mercury levels, and food diaries to see how much fish they ate.
Then, once the children were eight, the researchers asked the teachers to evaluate the kids' behaviours to see how many exhibited ADHD-like symptoms.
After taking all the information into account, researchers found one microgram of mercury per gram of a mother's hair - about eight times the average levels found in similar women's hair in another analysis - was tied to about a 60-per-cent increase in the risk of their child having ADHD-like behaviours.
Sagiv added the negative effects from lower levels of mercury may be cancelled out by the benefits from eating fish.
In an editorial with the study, Bruce Lanphear at Simon Fraser University, echoed advice on avoiding "big fishes."
"In the long term, we have to really find ways to fight contamination levels in fish so years from now we don't have to give this advice."