Oct. 18 (Bloomberg) -- U.S. government warnings against eating mercury-contaminated fish may cause more harm than good because fish oils boost health and mental development more than mercury harms fetuses, Harvard University researchers reported.

Pregnant women who cut their fish intake by one-sixth following a 2001 government advisory about avoiding mercury in fish experienced an 80 percent decline in the intelligence benefit that their developing children otherwise would have gained from maternal fish consumption, according to studies led by Harvard researcher Joshua Cohen.

A similar reduction in fish eating by all Americans in reaction to the mercury warning would produce about 8,000 additional deaths per year from heart disease and stroke, said Cohen, a senior research associate at the Harvard Center for Risk Analysis at the Harvard School of Public Health.

``It's important to evaluate the pros and cons,'' Cohen said. ``Because when you do evaluate the pros and cons, if you make certain plausible assumptions about how people react, you can get some serious negative results.''

Cohen and colleagues published their findings in a series of studies the November issue of the American Journal of Preventive Medicine. Also, the University of Maryland announced the establishment of a Web-based campaign to clarify warnings on mercury and encourage more consumption of fish.

Balanced Diet

The U.S. Food and Drug Administration, in its warnings on mercury in fish, describes seafood as ``an important part of a balanced diet for pregnant women,'' calling it a low-fat, high quality source of protein and other nutrients.

Yet the FDA notes that mercury that can harm an unborn child's developing nervous system and suggests pregnant women avoid eating types of fish found to have high mercury levels, including shark, swordfish, king mackerel and tilefish.

In practice, however, people tend to overreact to such warnings, Cohen said. The degree of overreaction is difficult to gauge, though one study found pregnant women cut their fish consumption by a sixth following the 2001 dvisory, he said.

One analysis of data led by Cohen found that if pregnant women properly observed the advisory and kept eating fish but switched to low-mercury varieties, their children would gain an average of one-tenth of an IQ point.

Omega-3 Fats

Yet if women instead cut their fish consumption by one- sixth and lost some of the benefits of omega-3 fatty acids in fish, the child would only gain an average of 0.02 IQ points, Cohen said.

And if the federal advisory prompted other people to cut their fish intake by a similar amount, it would cause thousands of additional deaths from coronary heart disease and stroke, he said. Among men aged 65 to 74, the annual mortality risk would increase by almost 1 in 10,000, one of his studies found.

The findings show that public health officials ``need to quantitatively evaluate its benefits and countervailing risks'' before issuing any general public advisory, Cohen said. ``I can find no evidence that they did'' make that assessment in the case of fish and mercury, he said.

Spokesmen for the FDA and the U.S. Environmental Protection Agency, which issue advisories on mercury, had no immediate response to the report.

To contact the reporter on this story: Paul Basken in Washington at pbasken@bloomberg.net

Last Updated: October 19, 2005 00:12 EDT