Fish and Heart Disease
Fatty ﬁsh are the most important sources of long-chain omega-3 fatty acids. In the early 1970s Danish investigators observed surprisingly low frequencies of heart disease among indigenous populations in Greenland that typically ate fatty fish, seals and whales. The researchers attributed the protective effect to the foods’ content of omega-3 fatty acids. Some subsequent studies — but by no means all — conﬁrm this idea.
Because large, fatty ﬁsh are likely to have accumulated methylmercury and other toxins through predation, however, eating them raises questions about the balance between beneﬁts and risks. Understandably, the ﬁsh industry is eager to prove that the health beneﬁts of omega-3s outweigh any risks from eating ﬁsh. Even independent studies on omega-3 fats can be interpreted differently. In 2004 the National Oceanic and Atmospheric Administration — for fish, the agency equivalent to the USDA — asked the Institute of Medicine (IOM) to review studies of the beneﬁts and risks of consuming seafood. The ensuing review of the research on heart disease risk illustrates the challenge such work poses for interpretation.
The IOM’s October 2006 report concluded that eating seafood reduces the risk of heart disease but judged the studies too inconsistent to decide if omega-3 fats were responsible. In contrast, investigators from the Harvard School of Public Health published a much more positive report in the Journal of the American Medical Association that same month. Even modest consumption of ﬁsh omega-3s, they stated, would cut coronary deaths by 36 percent and total mortality by 17 percent, meaning that not eating ﬁsh would constitute a health risk.
Differences in interpretation explain how distinguished scientists could arrive at such different conclusions after considering the same studies. The two groups, for example, had conflicting views of earlier work published in March 2006 in the British Medical Journal. That study found no overall effect of omega-3s on heart disease risk or mortality, although a subset of the original studies displayed a 14 percent reduction in total mortality that did not reach statistical signiﬁcance. The IOM team interpreted the “nonsignificant” result as evidence for the need for caution, whereas the Harvard group saw the data as consistent with studies reporting the beneﬁts of omega-3s. When studies present inconsistent results, both interpretations are plausible. I favor caution in such situations, but not everyone agrees.
Because ﬁndings are inconsistent, so is dietary advice about eating ﬁsh. The American Heart Association recommends that adults eat fatty ﬁsh at least twice a week, but U.S. dietary guidelines say: “Limited evidence suggests an association between consumption of fatty acids in ﬁsh and reduced risks of mortality from cardiovascular disease for the general population ... however, more research is needed.” Whether or not ﬁsh uniquely protects against heart disease, seafood is a delicious source of many nutrients, and two small servings per week of the less predatory classes of ﬁsh are unlikely to cause harm.
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Marion Nestle is Paulette Goddard Professor in the department of nutrition, food studies and public health and professor of sociology at New York University. She received a Ph.D. in molecular biology and an M.P.H. in public health nutrition from the University of California, Berkeley. Nestle’s research focuses on scientiﬁc and social factors that inﬂuence food choices and recommendations. She is author of Food Politics (2002, revised 2007), Safe Food (2003) and What to Eat (2006). She also writes a popular nutrition blog, Food Politics.
Reprinted with permission. Copyright © 2007 by Scientific American, a division of Nature America, Inc. All rights reserved. For more information about the issue, go to: http://www.scientificamerican.com/article.cfm?id=eating-made-simple