The authors respond:
We appreciate the opportunity to respond to comments1 regarding the validity of fish intake as a surrogate for mercury exposure and the measurement of mercury in umbilical cord tissue in our paper.2
In a subset from ALSPAC study population, umbilical cord mercury levels were higher among fish eaters, but did not correlate in a dose-response manner with the frequency of fish intake. This may reflect the low levels of mercury in fish consumed by this population. Nonetheless, we agree that more careful consideration should be given to the validity of fish as a surrogate, and umbilical cord as a biomarker, for methylmercury.
The water content of umbilical cord decreases with the duration of gestation. In ALSPAC subjects, the variability surrounding the mean mercury concentration in cord tissue increased with decreasing gestation among preterm infants, which may be attributable to higher water concentration earlier in gestation. The analyses presented in our paper were restricted to term births, among which we would expect less variation due to water content. We appreciate the rationale behind using dry versus wet weight for assessing mercury concentration in cord tissue and will consider this in future investigations. Improved measures of mercury, however, would not have substantively changed our interpretation of the relation between fish and neurodevelopment in these data.
If the imprecision in the measurement of mercury led to an underestimate of the adverse effect of mercury on neurodevelopment, then residual confounding by mercury would have weakened the observed beneficial association between fish and neurodevelopment.
When an exposure has the potential to have both beneficial and harmful effects, as fish does, it is tricky to distinguish the components that contribute to the overall relation with the outcome. It can also be difficult to distinguish (and measure) which factors truly confound the relation, rather than modify the level of exposure.3 We encourage others to continue investigating the complicated relation between fish intake and children's neurodevelopment, considering the potential for both beneficial and harmful properties of fish, in light of the current advisories.4
Julie L. Daniels
University of North Carolina; Chapel Hill, North Carolina; firstname.lastname@example.org
Matthew P. Longnecker
National Institute of Environmental Health Sciences; Research Triangle Park, North Carolina
Andrew S. Rowland
University of New Mexico; Albuquerque, New Mexico
University of Bristol; Bristol, United Kingdom
1. Grandjean P, Jørgensen PJ. Measuring mercury concentration. Epidemiology
2. Daniels JL, Longnecker MP, Rowland AS, Golding J, and the ALSPAC Study Team-University of Bristol Institute of Child Health. Fish intake during pregnancy and early cognitive development of offspring. Epidemiology
3. Bellinger DC. Assessing environmental neurotoxicant exposures and child neurobehavior: confounded by confounding? Epidemiology
4. US Department of Health and Human Services and US Environmental Protection Agency. 2004 EPA and FDA Advice for: Women Who Might Become Pregnant, Women Who Are Pregnant, Nursing Mothers, Young Children
. March 2004, EPA-823-R-04-005.