Comparison of the risks and benefits of fish consumption

A comparison of the risks and benefits of fish consumption suggests that government advisories warning women of childbearing age about mercury exposure should be issued with caution.

The study warns that if advisories cause fish consumption in the general public to drop out of fear about the effects of mercury, substantial nutritional benefits could be lost. The study will appear as a series of five articles in the November issue of the American Journal of Preventive Medicine.

"Fish are an excellent source of omega-3 fatty acids, which may protect against coronary heart disease and stroke, and are thought to aid in the neurological development of unborn babies," said Joshua Cohen, lead author and senior research associate at the Harvard Center for Risk Analysis at HSPH. "If that information gets lost in how the public perceives this issue, then people may inappropriately curtail fish consumption and increase their risk for adverse health outcomes."

Fish are a major source of mercury exposure, a neurotoxin that may cause subtle developmental effects in utero, like the loss of a fraction of an IQ point, even at the modest exposure levels typical of the American population. As a result, the U.S. Food and Drug Administration (FDA) and Environmental Protection Agency (EPA) have issued advisories warning women of childbearing age about mercury in fish.

Because fish are also a good source of omega-3 fatty acids, the advisories have had to walk a fine line. The most recent U.S. government advisories emphasize that other adults need not worry about mercury in fish. They even advise women of childbearing age to keep eating fish, although they caution that group to keep away from some species (shark, swordfish, king mackerel, and tilefish) likely to contain more mercury and to limit total fish intake to about two meals a week.

The Harvard project looked at whether the benefits of lower mercury exposure to pregnant women justified the loss of omega-3 fatty acids from decreased fish consumption. The project also went one step further, asking what would happen if the public did not follow the government's recommendations exactly as they were intended. Although evidence on how people actually react to advisories is limited, one study found that pregnant women cut their fish consumption by one-sixth following a 2001 government advisory. Nor is it difficult to imagine that other adults, not targeted by the advisory, cut back on fish based on misperceptions about the risks.

In order to synthesize the available evidence, the Harvard project convened a panel of experts, chaired by Steven Teutsch, a medical epidemiologist formerly with the U.S. Centers for Disease Control and Prevention and now at Merck and Company. Other panel members included David Bellinger (Harvard University), William Connor (Oregon Health Sciences University), Penny Kris-Etherton (Pennsylvania State University), Robert Lawrence (Johns Hopkins University), David Savitz (University of North Carolina), and Bennett Shaywitz (Yale University). The panel identified important health effects to consider, assessed the dose-response relationships between fish consumption (or its constituents) and health outcomes, and developed an overall health effects model. In addition to Joshua Cohen, Harvard scientific staff included Colleen Bouzan and Ariane König, and principal investigator, George Gray, executive director of the Harvard Center for Risk Analysis.

The study found that if pregnant women were to eat the same amount of fish but replace fish high in mercury with fish low in mercury, cognitive development benefits, amounting to about 0.1 IQ points per newborn baby, could be achieved with virtually no nutritional losses. However, if pregnant women were to decrease their fish consumption by one-sixth, the loss of omega-3 fatty acids during pregnancy would cut the nutritional benefit by 80%. If other adults were to also decrease their fish intake by one-sixth, then risks from coronary heart disease and stroke would increase. For example, among 65 to 74 year old men, the annual mortality risk would increase by nearly 1 in 10,000.

The study also found that increasing fish consumption among individuals who were not going to become pregnant would substantially decrease stroke and coronary heart disease risks. Much of this benefit appears to be associated with getting people to eat at least some fish (e.g., one meal a week), rather than no fish at all.

Cohen explained that the problem with fish advisories is that we do not know what their overall impact on the population might be. "Depending on how the population reacts, that impact could very well be negative." Because of the potential downside, Cohen urges the government to carefully evaluate the pros and cons. He concluded, "Before the government issues advisories, it needs to gather data on how people actually will react, how those changes in behavior will influence nutrient intake and exposure to contaminants, and how those changes in intake and exposure will translate into changes in health. In other words, before we put an intervention into action, we need to estimate its real world impacts – both its benefits and its countervailing risks."

The work was funded by a grant from the National Food Processors Association Research Foundation (now the Food Products Association Research Foundation) and the Fisheries Scholarship Fund.