Mercury can be found throughout the environment, and enters the air during fossil-fuel combustion, mining, smelting, and solid-waste incineration, according to background information in the article. It is converted to methylmercury by microorganisms, enters the food chain, and accumulates in predatory fish. Consumption of certain fish is the primary source of methylmercury exposure in the general population. Methylmercury distributes rapidly throughout the body and easily crosses the blood-brain barrier into the brain, where it may become trapped.
Recent regulations for mercury emissions, the increasing trend in fish-consumption advisories, clinical studies, and heightened media attention have led to the emergence of mercury as a leading public health concern. Fish consumption is frequently recommended for older adults due to its high omega-3 fatty acid content, well-documented cardiovascular benefits, and, more recently, its possible protective association with Alzheimer disease. Since the aging nervous system is more sensitive to neurotoxicants, there is reason for concern about mercury contamination in fish.
Megan Weil, M.H.S., of the Johns Hopkins Bloomberg School of Public Health, Baltimore and colleagues conducted a study to examine associations between mercury exposure and neurobehavioral outcomes in a representative sample of older adults in the United States. The research included 474 randomly selected participants in the Baltimore Memory Study, a longitudinal study of cognitive decline involving 1,140 Baltimore residents aged 50 to 70 years. Total mercury in whole blood samples was measured and the researchers used multiple linear regression to examine its associations with scores from 12 neurobehavioral tests. First-visit data were obtained in 2001-2002. Participants also completed a food frequency questionnaire to determine fish consumption levels
"In summary, the study provided no compelling evidence that blood mercury levels were adversely associated with neurobehavioral test scores. There were some consistent associations across models but because of the large number of comparisons and the observation that statistically significant associations were in different directions (i.e., worse performance on a test of visual memory and better performance on tests of manual dexterity), we cannot exclude the possibility that associations were due to chance," the authors write.
"Since the aging population may be particularly vulnerable to neurotoxicants, this study was an attempt to examine whether this rapidly growing group is sensitive to even lower levels of exposure. Since the blood mercury levels in our study did not appear to be associated with adverse neurobehavioral effects, our results suggest that these levels of exposure may not present a concern for older adults. Studies with more detailed dose assessment are necessary to confirm this conclusion since a single blood-mercury level may not be an optimal estimate of cumulative dose," the researchers conclude.
(JAMA. 2005;293:1875-1882. Available post-embargo at JAMA.com)
Editor's Note: This work was supported by training grants from the National Institute of Environmental Health Sciences and from the National Institute of Occupational Safety and Health and a research grant from the National Institutes of Health.