October 9, 2012
Prenatal Mercury Exposure Linked To ADHD-related Behaviors In Children
April Flowers for redOrbit.com - Your Universe Online
Globally, about ten percent of children have attention deficit hyperactivity disorder (ADHD). Its causes, however, are not very well understood.
This sounds contradictory, but the duality is possible because many types of fish have low levels of mercury. This makes it possible for a pregnant woman to consume fish and reap the benefits without being exposed to too much mercury.
"These findings underscore the difficulties pregnant women face when trying to balance the nutritional benefits of fish intake with the potential detriments of low-level mercury exposure," said Dr. Susan Korrick of Brigham and Women's Hospital (BWH).
The results provide an important public health message, according to Dr. Sharon Sagiv of Boston University School of Public Health.
"Women need to know that nutrients in fish are good for the brain of a developing fetus, but women need to be aware that high mercury levels in some fish pose a risk."
The research team studied 400 children born in New Bedford, Massachusetts between 1993 and 1998. The scientists collected hair samples from the mothers shortly after the births to analyze for mercury. The mothers also answered a questionnaire designed to determine their fish consumption during pregnancy.
The children were given standardized tests to determine ADHD-related behaviors eight years later.
The research team found a correlation between an increased risk for ADHD behaviors with higher levels of mercury in the maternal hair samples. The levels were lower than levels known to be potentially dangerous in most previous studies.
The mercury exposure was related to inattention and impulsivity/hyperactivity by statistical analysis, and some outcomes had an apparent threshold with associations at 1 Î¼g/g (microgram/per gram) or greater of mercury. As an example, the adjusted risk factor for mild/markedly atypical inattentive and impulsive/hyperactive behaviors were 1.4 and 1.7 respectively, at 1 Î¼g/g or greater.
The team also found a reduced risk of ADHD-related behaviors in the children of mothers who reported eating more than two servings of fish per week. This is a higher consumption rate than the servings recommended by the United States Food and Drug Administration and the Environmental Protection Agency.
Factors the study did not look into included what types of fish are best for a pregnant woman. Previous studies have shown that women should avoid fish high in mercury, such as shark, swordfish, king mackerel and fresh tuna. Fish that are low in mercury and a good source of nutrition include flounder, haddock and salmon.
"In summary, these results suggest that prenatal mercury exposure is associated with a higher risk of ADHD-related behaviors, and fish consumption during pregnancy is associated with a lower risk of these behaviors," the authors conclude. "Although a single estimate combining these beneficial vs. detrimental effects vis-Ã -vis fish intake is not possible with these data, these findings are consistent with a growing literature showing risk of mercury exposure and benefits of maternal consumption of fish on fetal brain development and are important for informing dietary recommendations for pregnant women."
Dr. Bruce P. Lanphear, of Simon Fraser University, commented on the study: "The study by Sagiv et al, which tested whether prenatal exposure to methyl mercury was associated with the development of ADHD-related behaviors, is an important and rigorously conducted prospective birth cohort study."
"What are the implications of the Sagiv et al study and other research on environmental contaminants and ADHD? First, we can take some comfort in recent legislation to reduce mercury contamination, at least from domestic sources. Second, these studies should spur our efforts to enhance the collection of data needed to calculate national estimates and trends in ADHD," Lanphear continues.
"Third, 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. Fourth, 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 concludes.