More children in the U.S. are being diagnosed with attention deficit hyperactivity disorder (ADHD) than ever before. In the search for a cause, researchers have come to suspect fetal exposure to environmental toxins—mercury in particular. Prenatal mercury exposure has been linked to ADHD, but because omega-3 fatty acids are essential for early brain development, pregnant women shouldn’t cut fish out of their diets.
Mercury in fish
Humans can be exposed to mercury from a variety of sources, which end up accumulating in fish. Power plants, cement plants, chemical manufacturers and other industrial facilities release mercury into the environment. Via air and groundwater, it ends up in lakes, rivers and oceans, where it works its way up the food chain. Predatory fish such as tuna, salmon and mackerel have been found with mercury concentrations 10,000 times higher than what is present in their habitat.
In a new study, researchers report that children who are exposed to higher levels of mercury in the womb by mother’s who eat a lot of fish are more likely to have attention problems, hyperactivity and other ADHD symptoms by the time they’re 8 years old.
Prenatal mercury and ADHD
Researchers at Brigham and Women’s Hospital tested more than 400 women for mercury about 10 days after they gave birth between 1993 and 1998. The women completed a survey about their fish consumption and had hair samples analyzed for mercury levels. When the children were eight, researchers tested their cognitive abilities with a parent questionnaire and other tests.
They found that for children crossing a certain exposure threshold, their risk of having ADHD symptoms (not a clinical diagnosis) increased by 40 to 70 percent. The association was seen primarily in boys, which have been shown to be more sensitive than girls to chemicals that disrupt the endocrine system.
Now the good news
Some of the findings were contradictory, however. Even though prenatal mercury exposure through fish consumption was linked to ADHD, fish in a pregnant woman’s diet also appeared to promote early neurodevelopment. Omega-3 fatty acids mitigated the neurotoxic effects of mercury. The key was level of fish consumption. The data showed that children of mothers who ate two or more servings of fish per week had a lower risk of developing ADHD symptoms.
“How much fish you eat is not equivalent to how much mercury you are exposed to,” said study author Dr. Susan Korrick. “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.”
Low mercury fish
Fish with lower mercury levels, which are also rich in omega-3 fatty acids include salmon, rainbow trout and herring. White fish, including cod, shrimp and haddock, are also low in mercury, as well as fat. White fish are also rich in B vitamins such as niacin (B3), which promotes healthy cells and helps eliminate toxins from the body and pyridoxine (B6), which keeps the skin, nervous system and red blood cells healthy.