July 24, 2013
No Link Between Prenatal Mercury Exposure And Autism, Says Long-Term Study
April Flowers for redOrbit.com - Your Universe Online
Scientists have been concerned about the potential impact of exposure to low levels of mercury on the developing brain - specifically by women consuming fish during pregnancy - for a long time. Some have even suggested mercury might be responsible for behavioral disorders such as autism.
Led by the University of Rochester Medical Center (URMC), a new study claims there is no association between prenatal mercury exposure and autism-like behaviors. Their findings were published online in the journal Epidemiology and draw upon more than 30 years of research in the Republic of Seychelles.
"This study shows no evidence of a correlation between low level mercury exposure and autism spectrum-like behaviors among children whose mothers ate, on average, up to 12 meals of fish each week during pregnancy," said Edwin van Wijngaarden, PhD, an associate professor in the URMC Department of Public Health Sciences. "These findings contribute to the growing body of literature that suggest that exposure to the chemical does not play an important role in the onset of these behaviors."
The long-standing debate over fish consumption has created a dilemma for pregnant women and their doctors. On the one hand, fish are high in beneficial nutrients like selenium, vitamin E, lean protein and omega-3 fatty acids - and the latter in particular are essential to brain development. On the other hand, exposure to high levels of mercury has been shown to lead to developmental problems, leading to the claim mothers are exposing their unborn children to serious neurological impairment by eating fish during pregnancy. Some organizations, including the US Food and Drug Administration (FDA), have recommended pregnant women limit their consumption of fish even though the developmental consequences of low level exposure remain unknown.
Mercury is widespread in the environment, originating from both natural sources like volcanoes and man-made sources such as emissions from coal-fired plants. The majority of mercury, however, is deposited in the world's oceans. Once there, it makes its way into the marine food chain and eventually into fish. The mercury level in any individual fish is relatively low, but concerns have been raised about the cumulative effects of a frequent diet of fish.
Scientists have found the Republic of Seychelles to be the ideal location to examine the potential health impact of persistent low level mercury exposure. The nation's 87,000 residents are spread across an archipelago of islands in the Indian Ocean. Fishing in the Seychelles is both an important industry and a primary source of nutrition - the nation's residents consume fish at a rate 10 times greater than the populations of the US and Europe.
Created in the mid-1980s to specifically study the impact of fish consumption and mercury exposure on childhood development, the Seychelles Child Development Study is a partnership between URMC, the Seychelles Ministries of Health and Education, and the University of Ulster in Ireland - and is one of the largest ongoing epidemiological studies of its kind.
"The Seychelles study was designed to follow a population over a very long period of time and focus on relevant mercury exposure," said Philip Davidson, PhD, principal investigator of the Seychelles Child Development Study and professor emeritus in Pediatrics at URMC. "While the amount of fish consumed in the Seychelles is significantly higher than other countries in the industrialized world, it is still considered low level exposure."
The new autism study examined 1,784 children, adolescents, and young adults and their mothers. By first analyzing hair samples collected from the mothers around the time of birth, the researchers were able to determine the level of prenatal mercury exposure. Such a hair sample analysis can approximate mercury levels found in the rest of the body including the growing fetus.
The scientists then determined whether or not the study participants were exhibiting autism spectrum-like behaviors through the use of two questionnaires. The children's parents completed the Social Communication Questionnaire, while the Social Responsiveness Scale was completed by their teachers. While these questionnaires - which include questions on language skills, social communication and repetitive behaviors - do not provide a definitive diagnosis, they are widely used as an initial autism screening tool in the US. The findings may suggest the need for additional evaluation.
The team then matched the prenatal mercury levels of the mothers to the test scores of the children, finding no correlation between prenatal exposure and evidence of autism spectrum-like behaviors. These findings corroborate previous studies which have measured language skills and intelligence, among other outcomes, and have not observed any adverse developmental effects.
There is an emerging consensus among researchers that the "good" of fish consumption may outweigh the "bad" of modest mercury exposure, and these recent findings seem to support that theory. In other words, if there is an adverse effect on child development at these levels of exposure then the benefits of the nutrients found in the fish may counteract or perhaps even supersede the potential negative effects of the mercury.
"This study shows no consistent association in children with mothers with mercury level that were six to ten times higher than those found in the US and Europe," said Davidson. "This is a sentinel population and if it does not exist here than it probably does not exist."
"NIEHS has been a major supporter of research looking into the human health risks associated with mercury exposure," said Cindy Lawler, PhD, acting branch chief at the National Institute of Environmental Health Sciences, part of National Institutes of Health (NIH).
"The studies conducted in the Seychelles Islands have provided a unique opportunity to better understand the relationship between environmental factors, such as mercury, and the role they may play in the development of diseases like autism. Although more research is needed, this study does present some good news for parents."