Environmental Engineering Reference
In-Depth Information
cause severe neurologic damage or death (USEPA, 1997;
Mozaffarian and Rimm, 2006). The estimated lethal dose of
methylmercury is 10 to 60 mg/kg of body weight (ATSDR,
1999). High-dose exposures in utero can result in delayed
cognitive and neuromuscular development in children
(USEPA, 1997; Mozaffarian and Rimm, 2006).
The health effects of long-term exposure to mercury at
lower doses are less well defi ned. Studies of adults with
low-dose mercury exposures have shown both positive
and negative associations between mercury levels and
various neurologic outcomes (Mozaffarian and Rimm, 2006;
Mergler et al., 2007). Although some evidence has suggested
an association between low-dose mercury exposure in
adults and an increased risk of cardiovascular disease, the
overall body of research on adult cardiovascular outcomes is
inconclusive (Mozaffarian and Rimm, 2006; Mergler et al.,
2007). Mercury's carcinogenicity is also questionable. The
USEPA and the World Health Organization's International
Agency for Research on Cancer both classify methylmer-
cury as a “possible carcinogen,” based on inadequate data in
humans and limited evidence of carcinogenicity in animals
(National Research Council [NRC], 2000; USEPA, 2008).
Studies of nonadults with low-dose, prenatal, or postnatal
mercury exposures—including a well-known study in the
Faroe Islands—have shown an association between mercury
and adverse outcomes (NRC, 2000), including neurobehav-
ioral defi cits (Grandjean et al., 1997) and decreased heart
rate variability (Grandjean et al., 2004). However, another
major study in the Seychelles found no adverse effects on IQ
or neurodevelopment in young children from prenatal or
postnatal mercury exposure (Davidson et al., 1998).
acid (DHA). Omega-3 PUFAs reduce triglycerides and the for-
mation of plaque in blood vessels and have repeatedly dem-
onstrated cardiovascular health benefi ts. Numerous studies
have linked omega-3 PUFA intake with a decreased risk of
death from coronary heart disease (Mozaffarian and Rimm,
2006). In addition, some evidence suggests that DHA, which
accumulates in the developing brain, is benefi cial for cog-
nitive and neurologic development (Daniels et al., 2004;
Mozaffarian and Rimm, 2006; Mergler, et al., 2007).
Like omega-3 PUFAs, selenium is an essential nutrient. It
also is a trace mineral, required by humans in only small
amounts, that plays a role in protecting tissues from oxida-
tive damage (ATSDR, 2003).
Potential Complicating Factors
The challenge of developing guidelines that account for
both the risks of mercury in fi sh and the benefi ts of nutri-
ents in fi sh can be complicated further by dietary and other
factors. Such factors introduce complexity by attenuating,
enhancing, or confounding the effects of mercury (NRC,
2000).
For example, mercury and selenium, both found in fi sh,
have a high affi nity for each other and are known to bind
together to form mercury selenides. In doing so, selenium
makes mercury less bio-available, and thus may protect
against mercury toxicity, an effect that has been observed
in various animal studies (USEPA, 1997; Raymond and
Ralston, 2004). However, as Raymond and Ralston (2004)
note, the binding of these two elements may also make
the converse true: mercury may make selenium less bio-
available. This may lead to a disruption in the formation
and activity of selenoproteins, which are essential for nor-
mal neurologic development, an effect hinted at by some
animal studies (Raymond and Ralston, 2004). The health
effect ultimately experienced from consuming both mer-
cury and selenium, then, may depend on which element
is more abundant. If this indeed is the case, it may in part
explain the confl icting results in health outcomes noted
previously between the people of the Faroe Island popula-
tion (who consume whale, which may contain more mer-
cury than selenium) and the Seychelles population (who
consume fi sh, which may contain more selenium than
mercury) (Raymond and Ralston, 2004). It would also
underscore the importance of understanding the specifi c
consumption behaviors and nutrient intake of a particular
population when developing fi sh-consumption guidelines.
Likewise, the health effects associated with mercury and
omega-3 PUFAs may compete with each other, although
not through a direct chemical interaction like that of mer-
cury and selenium. As noted earlier, substantial evidence
suggests that omega-3 PUFAs decrease the risk of death
from coronary heart disease. Meanwhile, limited evidence
suggests that mercury may increase the risk of cardiovas-
cular disease and even death from coronary heart disease,
possibly by promoting lipid peroxidation and therefore
Benefi ts of Nutrients in Fish
Although fi sh as a food source may harbor mercury, it
also offers many dietary benefi ts. Fish contains important
nutrients, including protein, vitamins, omega-3 polyunsat-
urated fatty acids (omega-3 PUFAs), and selenium.
Like the protein in other animal food sources, the protein
in fi sh is complete, meaning it contains all of the essential
amino acids required by the human body (Medline Plus,
2008a). However, fi sh is lower in saturated fats than many
other animal foods.
Oily fi sh in particular are a good source of the fat-soluble
vitamins A, D, and E. Vitamin A is an antioxidant that is
needed for vision, promotes cell growth, and is important
for embryo development. Vitamin D promotes the uptake
of calcium and phosphorous by the body and may help pre-
vent bone fractures. Vitamin E is an antioxidant and helps
repair body tissues. Almost all fi sh are a source of some B
vitamins, which aid metabolism (Medline Plus, 2008b).
Omega-3 PUFAs are essential nutrients, meaning they
cannot be manufactured by the body, and therefore must
be obtained through food. Highly oily fi sh such as salmon
and sardines are especially high in omega-3 PUFAs, most
notably eicosapentaenoic acid (EPA) and docosahexaenoic
