A study that examined the benefits and risks of eating fish found that current advice may be undermining consumer confidence, preventing people gaining the undisputed nutritional benefits of regular fish consumption.

A study that will be published as a series of five articles in the November issue of the American Journal of Preventive Medicine suggests that government advice warning women of childbearing age about mercury may do more harm than good.

Lead researcher Joshua Cohen of the Harvard Center for Risk Analysis commented: "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 […] 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."

The government does not advise women of childbearing age to stop eating fish altogether; on the contrary, it advises them to continue to do so, but to limit total fish consumption to two meals a week and to steer clear of those species most likely to contain mercury, namely shark, swordfish, king mackerel and tilefish. However, there are concerns that the detail of the advice will be ignored as consumers decide instead to 'play it safe' by avoiding fish altogether, thus foregoing a key source of valuable nutrients.

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 behaviour 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.