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| POSTED 12/11/2006 |
Fish Intake, Contaminants, and Human Health
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There has long been a debate regarding the benefits of fish consumption in pregnant women. Now there's no question that it is beneficial, according to two decades of evidence from animal experiments, observations, and clinical studies. Apparently fish contain two long chain n-3 polyunsaturated fatty acids (n-3 PUFAs), eicosapentaenoic acid (EPA) and docosahexanenoic acid (DHA). DHA, in particular, has been found to be important for neural development during gestation and infancy. Conversely, concerns have been raised regarding the potential harm from mercury, dioxins and polychlorinated biphenyls (PCBs) found in some fish species. Moreover, oily fish, such as salmon, herring and sardines, provide a higher benefit of these n-3 fatty acids than lean fish, such as cod, catfish, or halibut.
DHA is preferentially incorporated into the rapidly developing brain during gestation and in the first two years of infancy, concentrating in the gray matter and retinal membranes. Infants can convert short chain n-3 fatty acids to DHA, but it is unknown whether such a conversion is adequate for
the developing brain in the absence of maternal intake of DHA. The existing data also support improved visual clarity in a dose-dependent manner. In pregnant women treated with cod oil from week 18 of gestation to three months postpartum, their DHA level in the cord blood rose by 50 percent, and a raised mental processing score, a measure of intelligence at four years, was observed. These findings are consistent with observational studies showing positive associations between maternal DHA levels or fish intake during pregnancy and behavioral attention scores, visual recognition memory, and language comprehension in infancy.
Pregnant women also are wary of eating fish, despite its benefits, due to its mercury content. Mercury is a reactive heavy metal emitted from natural sources such as volcanoes and human sources. Mercury cycles from rainwater into lakes and oceans where it is converted by microbial activity into organic methylmercury. The concentration of methylmercury in aquatic species depends on the levels of environmental concentration and on the predatory nature and lifespan of the species. Larger longer living predators, such as swordfish and sharks, have high tissue concentrations, while smaller shorter living species, such as shellfish and salmon, have very low concentrations. Preparation methods have little impact on methylmercury content. Methylmercury crosses the placenta and fetal exposure correlates with maternal exposure.
In a British study gestational mercury exposure was not associated with neurodevelopmental scores, but maternal and infant fish intake was associated with improvement of those scores. The Environmental Protection Agency (EPA) took a prudent approach to these findings. It advises a 70 kg (154 pound) woman to consume no more than 50 mcg of methylmercury per week.According to the EPA, this level will not affect the fetus' neurological development in a negative way. The EPA also published a focused advisory for women of childbearing age, nursing mothers, and young children to not eat swordfish, golden bass, and king mackerel, which all contain 50 mcg of methylmercury per serving. They are advised to eat two meals per week of a variety of fish and shellfish lower in mercury, including up to 6 ounces of albacore tuna that contains 30 mcg of methylmercury per serving. They were also advised to consult local advisories for the locally caught freshwater fish.
The good news is that the adverse effects of mercury intake may be reversed by adequate consumption of selenium, a mineral that may reduce tissue accumulation of mercury in both humans and fish. Fish is a rich dietary source of selenium. Other good sources of selenium are nuts, sunflowers, and eggs.
Fish may also contain PCBs, synthetic compounds previously used in industrial and commercial processes, and dioxins, byproducts of waste. Although the manufacture and processing of PCBs was prohibited in 1977 and federal regulatory and industrial efforts have reduced dioxide emission by more than 90 percent since 1987, these contaminants still persist in the environment. While their levels are steadily declining, PCBs and dioxins continue to be present in low concentrations in many foods. Multiple structure variances of PCBs and dioxins exist. The potential toxicity of foods is calculated using toxic equivalents (TEQ). In the U.S. PCBs comprise 28 percent and dioxins comprise 72 percent of total TEQ exposure. Although major sources of exposure to PCBs and dioxins are in meats (34 percent), dairy products (30 percent), vegetables (22 percent), and eggs (5 percent), considerable attention has been given to fish sources, which only accounts for 9 percent of total PCB and dioxin exposure. Farmed and wild salmon also contain substantial levels of n-3 PUFAs, 4,504 and 1,774 mg of EPA and DHA per six ounces, respectively. PCBs common in fish can be reduced by 12 percent to 40 percent by trimming belly and back fat during filleting and by not consuming the skin. Also, contaminant levels are typically measured in unprepared foods, and by contrast to methylmercury, cooking may reduce PCBs and dioxin contents.
Because most exposure generally comes from meat, dairy, and vegetable sources, this concern is not specific to fish consumption, particularly since fish also contains potentially beneficially DHA. However, women consuming one or more servings per day of commercial freshwater fish or consuming locally caught freshwater fish from highly contaminated inland sources may be more greatly exposed to PCBs and dioxins and should consult regional advisories.
Fish oil capsules contain 20 and 80 percent of EPA and DHA by weight, 200 to 800 mg per gram, little to no mercury, and variable levels of PCBs and dioxins. Eating fish instead of taking these supplements, however, is preferable. Fish intake also provides potentially beneficial protein, vitamin D, and selenium. For example, a commercially prepared fried fish meal from fast food restaurants or supermarket frozen section is often made using white meat fish, is lower in n-3 PUFAs, and is prepared with partially hydrogenated oils containing trans fats or from oil reused for multiple frying cycles to lower the PCB/dioxin level.
Also, ecologic and animal studies suggest that linolenic acid, an n-6 fatty acid, may counteract the potential benefits of the n-3 fatty acids. The n-6 fatty acids are widely consumed in cooking oils, salad dressings and prepared foods. Therefore, instead of reducing the intake of n-6 fatty acids, pregnant women should eat one serving of oily fish per week. Alpha-linolenic acid (ALA) is an n-3 fatty acid present in flaxseed oil, canola, soybeans, and walnuts. In humans ALA is converted to EPA in small quantities and in women more so than in men. Further conversion to DHA is very limited.
DHA appears important for early neurological development. Therefore, consumption of flaxseed oil or flax oil does not provide optimum supplementation of n-3 PUFAs especially critical during pregnancy. Seventy-four percent of women of childbearing age and 85 percent of pregnant women consume no seafood. They need to eat 12 ounces per week of fish to ensure the neurological health of their offspring. Pregnant women should avoid shark, swordfish, golden bass, and king mackerel. Locally caught fish should be consumed according to local advisories, and the intake of albacore tuna should be limited to 6 ounces per week to minimize methylmercury exposure. The 12 ounces per week consumption should include other fish and shellfish to provide for a reasonable amount of DHA.
Date Posted: December 11, 2006
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