Maintaining Your Health on Mackinac
Health Benefits of Eating Fish Outweigh Potential Risks
By Yvan Silva, M.D.

The 1980 publication of studies of Greenland Eskimos and their low rates of coronary heart disease owing to their predominant consumption of fish led to the consideration of fish as healthy food. Since then, there has been mounting evidence on several fronts, experimental and clinical, to further promote this hypothesis that eating fish is beneficial to health. On balance, however, there have been conflicting reports on benefits and risks of fish intake, leading to confusion and controversy in the public domain, on the role of consumption of fish in a healthy diet. The risks have been allied to possibly harmful substances that may be present in some fish.

Fish provides one of the leanest sources of proteins available in the diet, as well as many valuable minerals and vitamins. The positive benefits of fish also derive from the presence of unsaturated fats, the healthy component of total fat in the diet. These unsaturated fats are present as three likely active constituents - specific types of fat known as omega-3 fatty acids - n-3 polyunsaturated fatty acids, eicosapentaenoic acid (EHA), and docasahexaenoic acid (DHA). These have been cited as possibly reducing the risk of developing heart disease. They may also reduce the risk of other medical problems. This is, of course, in addition to the protein and nutrients like vitamin D and selenium. The potential harm can come from contaminants that can accumulate in some fish species like mercury, dioxins, and polychlorinated biphenyls (PCBs).

The consumption of fish or shellfish is most beneficial because of the omega-3 fatty acids found in them, especially in oily fish like salmon, sardines, and herring. These substances help lower the heart rate, the blood pressure, and improve overall heart function. Fish consumption reduces the risk of death from heart disease, the leading cause in both men and women. There is also a positive correlation, a link to lowering the risk of depression, mental decline, and importantly, the incidence of stroke. Fish intake is further important for women of childbearing age, pregnant women, and breastfeeding mothers as DHA, a specific omega-3 fatty acid is beneficial for the brain development of the fetus and infant. This information is, of course, important in the context of an overall healthy diet that is usually recommended as containing less than 30 percent of fat, with the caveat that no more than 10 percent of fat intake should consist of saturated fats with less than 300 milligrams of the total amount of cholesterol.

Mercury is a heavy reactive metal that occurs naturally from emissions from volcanoes and other earth sources. Mercury also results from man-made sources like mining, coal-fired electric power plants, chlorine production, and waste incineration. From the atmosphere, mercury cycles into the rainwater into lakes and oceans, where it is converted by microbial activity into methylmercury. This compound accumulates in the bioaquatic food chains and is readily absorbed into the tissues of aquatic species. Concentrations in fish are influenced by the degree of environmental contamination and the size and lifespan of the fish. Swordfish and shark are likely to have higher concentrations compared to short-lived species like salmon and shellfish. Preparation methods of the fish for consumption have little effect on the impact of methylmercury content. The serious effects of high mercury exposure in industrial and occupational accidents have been well established. However, the health effects of low-level, chronic exposure, such as in fish consumption, are less well established. There is public awareness of potential harm from mercury in fish, but there is lack of clear understanding of who is at risk and which species are likely to be contaminated.

In several studies done prospectively in adults, there is little evidence that eating fish poses significant cancer risk. The relative concentrations of PCBs (polychlorinated biphenyls) and dioxins in fish since prohibition of the former in 1977 and regulatory reduction of the latter by more than 90 percent, are minimal in major dietary sources including meat, fish, dairy products, and vegetables. Fish oil capsules contain 20 to 80 percent of EPA and DHA by weight. These and other supplements, however, lack the potentially beneficial proteins, vitamin D and selenium.

Moderate consumption of fish, for example, one to two servings per week, especially of species higher in omega-3 fatty acids, reduces risk of coronary death and total mortality, and may be beneficial for other health functions. Women of child-bearing age and pregnant mothers should consume two servings per week of seafood, limited to selected species. However, freshwater fish may come from possibly contaminated inland sources, and regional and local advisories should be consulted for levels of PCBs and dioxins. Compared with store-bought fish, locally caught freshwater fish may have higher chemical levels. A variety of seafood should be consumed, but individuals

with high consumption, five or more servings per week, should limit species highest in mercury content. Pregnant women, women who may become pregnant, and those who are breast-feeding should avoid fish higher in mercury content - shark, swordfish, bass, and king mackerel. Consumption of other fish is important to allow for brain development benefits of infants from DHA content. Light tuna has relatively low levels of mercury and wild and farmed salmon and shrimp contain very low levels of mercury.

The health benefits of eating fish greatly outweigh the potential risks. Based on the strength of the evidence and the magnitude of the benefits, modest consumption of one to two servings per week clearly outweighs the risks. Guidelines apply to the selection of the species of fish and the sources. These are particularly important to women of childbearing age. Avoidance of modest fish consumption, because of lack of understanding or confusion of the subject, could lead to increased risks of cardiac death and other vascular complications.

Also, avoidance of the benefits of fish consumption by childbearing women could lead to an increase in the suboptimal neurodevelopment in children.

Dr. Silva is a professor of surgery at Wayne State University and a resident of Woodbluff on Mackinac Island.