Seafood ChoiCeS

Seafood Choices: Balancing Benefits and Risks
http://www.nap.edu/catalog/11762.html
hEALTh RISKS ASSOCIATED WITh SEAFOOD CONSUMPTION
that are antagonistic to those of methylmercury, it is difficult to obtain
“pure” estimates of methylmercury toxicities. For example, a follow-up
study of the Finnish men reported on by Salonen et al. (1995) showed that
men in the highest quintile of docosapentaenoic acid and docosahexaenoic
acid intake, compared to men in the lowest quintile, had a 44 percent
lower risk of CHD over a 4-year period (Rissanen et al., 2000). Analyses
stratified by hair mercury level suggested, however, that the reduction was
greater (52 percent) for men with hair mercury (Hg) levels 2 ppm (only 24 percent). A similar shift
in the balance of the risks of methylmercury and the benefits of omega-3
fatty acids was found in a study of blood mercury level and blood pressure
among US women (NHANES 1999–2000; Vupputuri et al., 2005). In the
entire cohort of 1240 women aged 16–49 years, blood mercury level was
not significantly associated with either systolic or diastolic blood pressure.
When analyses were stratified by reported fish intake (759 consumers, 481
nonconsumers), systolic blood pressure increased significantly with blood
mercury level among nonconsumers, corresponding to an approximately 5
mmHg difference between the lowest quintile (0.1–0.4 µg/L) and the highest
quintile (2.1–21.4). Among the fish-consumers, systolic blood pressure
declined (nonsignificantly) with increasing blood mercury level. The findings
were similar for changes in diastolic blood pressure with increasing blood
mercury level. Overall, this pattern suggests that increased exposure to
mercury, obtained from sources other than fish consumption, is associated
with higher blood pressure. When mercury exposure occurs in conjunction
with fish consumption, however, the effects on blood pressure are blunted
and, at the levels in most US women, may be counteracted by protective
factors in fish. This interpretation is consistent with the null findings of a
study of hair mercury levels and blood pressure in fish-consuming Indian
tribes of the Amazon rain forest (Dorea et al., 2005).
Methylmercury Reference Dose
A report from the National Research Council of the National Academies
reviewed the US EPA’s process in deriving the RfD (see Box 4-2). It
concluded that the existing RfD of 0.1 µg/kg per day was a “scientifically
justifiable level for the protection of public health,” although it recommended
that it be derived on the basis of the findings of the newer epidemiological
studies rather than of the Iraqi study (NRC, 2000). Such a calculation is
subject to numerous uncertainties, however. Among these are the choice of
the functional form of the statistical model used to identify the methylmercury
dose at which a doubling of the target response occurs (e.g., linear vs.
supralinear vs. sublinear models), the choice of the adverse health effect,
the choice of the point estimate for the excess prevalence to be prevented,
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