Ever since studies demonstrated lower rates of dying from heart disease among Greenland Inuit, North Americans have been told repeatedly to eat more fish. But concerns over potential harm from chemical contaminants in fish have left many people confused about what type of fish is safe to eat -- or if they should steer clear of it all together.
According to a report published today in the Journal of the American Medical Association, the benefits of eating fish (including shellfish) are far greater than the potential risks. One to two servings of fish per week is enough to reduce the risk of dying from a heart attack by 36 per cent. Researchers from Harvard School of Public Health and Harvard Medical School based their conclusions on a review of hundreds of studies about fish and health. Fish is an excellent source of protein that's low in artery-clogging saturated fat. Cold water fish, such as salmon, trout, sardines, mackerel and herring, contain two omega-3 fatty acids called DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid). Plenty of evidence suggests that DHA and EPA guard against heart attack and stroke.
A steady intake of omega-3 fats might even reduce the risk of Alzheimer's disease, dementia and rheumatoid arthritis. DHA is also needed for the development and maintenance of the brain, eye and nervous tissue throughout life, beginning in the final trimester of pregnancy.
The cardio-protective benefits of eating fish are so clear, the Heart and Stroke Foundation of Canada advises people to consume fish, especially fatty fish, at least two times per week. Omega-3 fats in oily fish make the blood less likely to form clots, lower blood fat levels and protect against irregular heartbeats that cause sudden cardiac death.
Yet concerns about harmful chemicals in fish -- methylmercury, polychlorinated biphenyls (PCBs) and dioxins -- have caused some people to question the wisdom of this advice. In May of 2002, Health Canada issued an advisory, warning pregnant women, women of childbearing age, and children 4 and younger to limit their intake of high-mercury fish -- swordfish, tuna steaks and shark -- to no more than once a month. Other people were told to consume these fish no more than once a week. (Health Canada is currently in the process of updating its fish consumption advice.)
Last month, Toronto Public Health released a report outlining stricter consumption advice. The report advises women who are pregnant or breastfeeding, women who could become pregnant, and young kids to completely lay off high-mercury fish.
Mercury is naturally occurring at very low levels in the air, soil, lakes and oceans. It also makes its way into the environment from industry, such as pulp and paper processing and mining operations. When mercury enters streams and water, it's converted by bacteria to methylmercury, which is then absorbed by fish. Larger, longer-living predators (e.g. shark, swordfish and tuna) have higher concentrations of methylmercury than smaller fish (e.g. shellfish, salmon). Cooking has little impact on mercury content.
The concern is that mercury can accumulate in the body and affect the developing nervous system, especially the brain, of infants and young children. If women consume too much mercury before and during pregnancy, it may increase the risk of birth defects and learning disabilities. Some, but not all, studies have found associations between a women's mercury exposure during pregnancy and cognitive test scores during childhood.
The link between high-mercury intakes and the risk of heart disease remains inconclusive. According to the Journal of the American Medical Association review, mercury in fish may lessen the natural benefits from fish, but the overall benefit is still positive.
The review turned up similar results for PCBs and dioxins in fish. PCBs were widely used in industry until the chemicals were banned in North America in 1977. Dioxins are byproducts of waste incineration, paper bleaching and the production of certain plastics.
