Is there a link between prenatal mercury exposure and fish intake and the development of ADHD? Maybe.
The most common way that we are exposed to mercury is through fish consumption, yet fish is an important part of a healthy diet. Fish contain high quality protein and essential nutrients, including omega-3 fatty acids. Fish is also low in saturated fat. Most fish, however, contain traces of mercury.
The Study: Mercury, Fish Consumption, and Symptoms of ADHD
Researchers at Brigham and Women's Hospital and Boston University School of Public Health investigated the association of prenatal mercury exposure and fish intake with the development of ADHD symptoms. The study included more than 400 children born in New Bedford, Massachusetts (a fishing community where fish consumption is popular) between 1993 and 1998. Shortly after the mothers gave birth, researchers collected hair samples from the mothers and analyzed them for mercury. The mothers were also asked to fill out a survey about their fish consumption during pregnancy.
Eight years later, researchers followed up with the children and administered standardized tests to determine whether the children were exhibiting attention problems, lack of impulse control, hyperactivity, and other ADHD symptoms. They found that the children of mothers whose hair samples showed increased mercury levels tended to be at greater risk for symptoms of inattention and impulsivity/hyperactivity. This was particularly true for the boys in the study.
It is important to note that the research does not show a cause and effect relationship between prenatal mercury exposure and the development of ADHD. The study shows only a statistical association. It is also not clear whether the children in the study actually had ADHD because the study looked only at symptoms, not diagnoses.
A second finding of the study was that children whose mothers reported eating more than two 6-ounce servings of fish per week during pregnancy appeared to develop fewer behavior problems. In other words, fish consumption during pregnancy seemed to provide a protective factor reducing the risk of ADHD symptoms. It is not clear what kinds of fish the mothers ate or what levels of mercury the fish contained.
Higher Levels of Fish Consumption Does Not Necessarily Equal Higher Mercury Exposure
On first glance, the finding -- that mercury exposure may be associated with a higher incidence of ADHD symptoms, while more fish consumption (the main source of mercury exposure) seems to lead to a decreased risk -- might seem quite contradictory.
"It seems a little paradoxical, but fish consumption doesn't necessarily correlate with mercury levels since you could eat a high amount of fish that are low in mercury," explains study co-author, Dr. Susan Korrick. "It's possible to eat fish low in mercury and high in nutritional value, and it's possible to eat fish high in mercury and low in nutritional value. What really matters is the kind of fish you're eating."
Recommendations for Expectant Mothers
Avoid swordfish, shark, king mackerel and tilefish because these types of fish tend to contain higher levels of mercury. Stick with fish and shellfish that are low in mercury including salmon, rainbow trout, herring, shrimp, pollock, catfish, cod, haddock, and canned light tuna. Be aware that albacore tuna has more mercury than canned light tuna.
The U.S. Food and Drug Administration (FDA) recommends that most people eat fish twice a week. For women who are or may become pregnant, the FDA recommends up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury - half of that (a 6 ounce serving) can be albacore tuna.
The study, 'Prenatal exposure to mercury and fish consumption during pregnancy and attention-deficit/hyperactivity disorder-related behavior in children,' is published online in the Archives of Pediatrics and Adolescent Medicine (October 2012).
Visit Pregnancy.About.com for more information.
Sharon K. Sagiv, PhD, MPH; Sally W. Thurston, PhD; David C. Bellinger, PhD, MS; Chitra Amarasiriwardena, PhD; Susan A. Korrick, MD, MPH; 'Prenatal Exposure to Mercury and Fish Consumption During Pregnancy and Attention-Deficit/Hyperactivity Disorder-Related Behavior in Children,' Arch Pediatr Adolesc Med. 2012, October.
Food and Drug Administration, What You Need to Know About Mercury in Fish and Shellfish March 2004.