Washington, DC - Two scientific reviews of the health effects of dietary fish intake probably made bigger splashes than expected by being released on the same day.

Earlier this week, the Institute of Medicine (IOM) released Seafood Choices: Balancing Benefits and Risks, its summary of current knowledge on the health benefits of eating fish and the potential risks from contaminants such as methylmercury, dioxins, and polychlorinated biphenyls (PCBs) [1]. The same day, a literature review on much the same topic was published by the Journal of the American Medical Association [2].

Both documents offer positive messages about the health effects of the n-3 polyunsaturated fatty acids (PUFAs) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) abundant in fish and other seafood. The IOM report arrives at no firm conclusions about whether the regular consumption of fish will prevent disease. The JAMA report's authors, however, appear certain that its health benefits outweigh any risks.

As covered by heartwire earlier this week, Dr Dariush Mozaffarian (Harvard School of Public Health, Boston, MA) and Dr Eric B Rimm (Brigham and Women's Hospital, Boston, MA) found that intake of EPA and DHA equivalent to one or two servings of fish per week was associated with a 36% decrease in the risk of death from coronary heart disease (p<0.001) and a 17% drop in total mortality (p=0.046). They also concluded that the n-3 PUFAs are likely important to early brain development. Those benefits far outweigh any risks from contaminants, they write.

"Avoidance of modest fish consumption due to confusion regarding risks and benefits could result in thousands of excess CHD deaths annually and suboptimal neurodevelopment in children," they write.

The IOM report's dietary recommendations are more circumspect and less concrete. The document "confirms that eating fish and shellfish may reduce people's overall risk for developing heart disease," according to a press release from the organization [3]. "It is not certain whether this is because substituting the lean protein of seafood for fatty cuts of meat reduces consumers' intake of saturated fat and cholesterol or because of the protective effects of omega-3 fatty acids. . . . The report also found evidence that maternal consumption of omega-3 fatty acids through seafood can contribute to vision and cognitive development in infants and lengthen the duration of gestation."

The report acknowledges potential risks from methylmercury, dioxins, and PCBs but notes, according to the IOM statement, that "reliable data on the distribution of some contaminants is lacking, and there is little evidence on how beneficial effects of seafood might counteract some of the risks from contaminants."

Both reports include caveats about contaminant exposure and fish intake for special populations, particularly women of childbearing age or who are nursing, and cautions for people who fish and eat their own catch from potentially polluted inland waters.

More than 400 pages long and designed to help the US government communicate these issues to the public, the IOM report is noteworthy for observing how little is actually known about either the benefits or the risks of consuming fish and for identifying knowledge gaps that can help guide future research. The IOM, an arm of the National Academy of Sciences (NAS), a nonprofit corporation, describes itself as "an adviser to the federal government on issues of medical care, research, and education."

The differences and similarities of the IOM and JAMA reports earned both more attention than they probably bargained for. In her October 18, 2006 column, New York Times writer Marian Burros highlighted their contrasts and included reactions from scientists, advocacy-group representatives, and the authors themselves [4]. [Burros incorrectly identifies the IOM as being "part of the National Institutes of Health."]

JAMA coauthor Mozaffarian is quoted as saying the risks associated with PCBs and dioxins in fish are "greatly exaggerated" and that "seafood is likely the single most important food one can consume for good health." IOM writing panel member Dr José M. Ordovas (Tufts University, Boston, MA), according to the column, said the JAMA article's claimed 36% reduction in CHD risk "is based on circumstantial evidence that does not provide definite proof."

Burros describes Dr Marion Nestle of New York University as referring skeptically to the "very sunny Harvard study" and equates its emphatically positive take on the health benefits of eating fish with pitches that tout "miracle foods." And Jane Houlihan of the Environmental Working Group, described as an advocacy group, is quoted as saying "the Harvard study reads like an advertisement for the seafood industry."

According to Burros, "Both reports have come under criticism from environmental groups and from the Consumers Union. 'In addition to being concerned about the failure of the JAMA and IOM reports to address the risks of mercury in tuna,' said the consumer organization, 'we are also concerned that both reports dismiss concerns about PCBs in most fish.' "

Another advocacy group representative, Rebecca Goldberg of Environmental Defense, claims in the column that "the high degree of certainty in one report and the extreme caution in the other . . . will make people more confused than ever."

New York University's Nestle apparently agrees. Burros paraphrases her as saying that "the situation is so confusing, no rational person can possibly figure out how to make sense of it."