BOSTON, MASSACHUSETTS. The benefits of eating fish are related to the content of the long-chain omega-3 fatty acids EPA and DHA. The risk is associated with a whole host of contaminants such as mercury, PCBs etc. Thus the question—do the benefits outweigh the risks? A detailed examination of this question appeared in the October 18th issue of the Journal of the American Medical Association. Mozaffarian and Rimm from Harvard Medical School first review the cardiovascular and neurological developmental benefits of fish and in particular the fatty acids EPA and DHA for which fish provide a significant dietary source. Numerous studies relating to the cardiovascular benefits of EPA and DHA are presented in tables and impressive graphs. DHA is also critical in the neurological development during gestation and the first 2 years of infancy. A number of studies are quoted to support the importance of maternal intake of DHA during pregnancy and while nursing. The other aspect of the question, i.e. the risks associated with contaminants, is much more difficult to address. Medical scientists obviously do not conduct experiments on humans where the dose dependence of toxicity from the contaminants in question is investigated by giving the participants toxic chemicals and observing the results. Thus data for high doses must come from accidental or occupational exposure but these levels are irrelevant in terms of the levels found in most fish. At the other end of the dose spectrum, making an association between the intake of traces of toxic materials and adverse health outcomes is very difficult and fraught with uncertainty and confounding.
This paper quotes a number of studies that relate to undesirable intakes of organic mercury for women of childbearing age, nursing mothers, and young children, but it is hard to believe that the limits are more than a rough estimate. The situation with regard to health effects of trace amounts of mercury on adults is even less clear-cut. There are reports of adverse effects of mercury on cardiovascular and neurologic health. As regards the former, the authors simply pose the question as to whether the expected benefits from fish consumption would merely be greater if mercury were not present. As regards the neurologic aspect, the evidence as to adverse effects is unclear but there is a growing body of evidence that fish consumption may favorably influence clinical neurologic outcomes in adults, including benefits associated with ischemic stroke, cognitive decline and dementia, depression and other neuropsychiatric disorders. Translated into a useful guideline, the end result is advice not to eat shark, swordfish, golden bass and king mackerel since they typically contain more than 50 micrograms of organic mercury (methylmercury) per serving. Levels of dioxins and PCBs are low in fish and the authors take the position, based on what evidence is available, that any adverse effects from these contaminants are outweighed by the benefits of eating fish. Fish are also a rich dietary source of selenium and the authors mention evidence that some of the adverse effects of organic mercury may be mitigated by adequate intake of selenium, which incidentally is an essential dietary trace element. The bottom line is that the following fish pass muster as being good sources of EPA and DHA and are on average low in mercury: anchovies, Atlantic herring, wild and farmed salmon, sardines, and trout. These fish provide between 600 and 4500 mg per serving of EPA and DHA and contain on average 0.7 or less parts per million of mercury.
The authors also discuss plant sources of EPA and DHA. These two long-chain omega-3 fatty acids in fact do not occur in plants, but must be made in the body from alpha-linolenic acid which is present in flaxseed, canola, soybeans and walnuts. Only small amounts are converted to EPA and further conversion to DHA limited. Thus fish represent by far the best source, and the authors raise no objections to getting these fatty acids from fish oil capsules, which typically contain 20% DHA and 80% EPA with little or no mercury, and taking even 1-3 grams of fish oil, according to the authors, results in low intake of PCB and dioxins.
Mozzaffarian, D and Rimm, E. B. Fish Intake, Contaminants, and Human Health. Evaluating the Risks and Benefits. Journal of the American Medical Association, 2006, Vol 296, No. 15, pp. 1885-99