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 by Harvard School of Public Health 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.

"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.

Fish are also a major source of mercury exposure, a neurotoxin that may cause subtle developmental effects in utero. As a result, the US Food and Drug Administration (FDA) and Environmental Protection Agency (EPA) have issued advisories warning women of childbearing age about mercury in fish.

Joshua Cohen: "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."

But due to the benefits of omega-3 fatty acids, the advisories have had to walk a fine line. The most recent US 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 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 per cent.

The study also found that increasing fish consumption 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, rather than no fish at all.