Because fish can be healthful as well as hazardous, medical experts have grappled for years with what advice to give people, particularly pregnant women, about how much is safe to eat.
A new study by Harvard University doctors concludes that pregnant women can boost their baby's intelligence by eating fish a couple of times a week, but only if they avoid varieties with large concentrations of mercury.
Fish is full of omega-3 fatty acids, which help young brains develop and seem to protect against heart disease. But it also is tainted by mercury, a potent neurotoxin that interferes with the building of brains.
The new study of 135 Boston-area babies is considered important because it quantifies and compares the risks and benefits of a fish diet.
The researchers concluded that pregnant women should eat fish because their babies are likely to score higher on intelligence tests. But they also reported that the benefits of the nutrients disappear and the babies' intelligence scores drop substantially if the fish contains high levels of mercury.
Nearly all fish contains traces of mercury, but large marine species such as swordfish, shark and albacore tuna accumulate the highest levels.
About 630,000 babies a year are born with mercury exposure that could reduce their mental abilities, the U.S. Environmental Protection Agency estimates.
Mercury can harm adults -- hampering memories, causing hair to fall out and perhaps raising the risk of heart disease -- but fetuses are considered the most vulnerable because neurological effects have been found at low levels.
Dr. Philippe Grandjean, an environmental epidemiologist at the University of Southern Denmark and Harvard University who has studied the effects of mercury on children for 20 years, said the new findings added to the mounting evidence that women should eat fish but follow warnings to limit the types and amounts they consume.
Previously, Grandjean and others presented similar findings for school-age children, reporting that their mental skills, particularly memory, vocabulary and attention, were reduced if they had been exposed in the womb to relatively low levels of mercury.
Grandjean, who was not involved in the latest study, said infant intelligence was highly variable so it was "surprising that the authors were able to detect both a positive effect of fish intake and an adverse effect of mercury. That would suggest that these effects [on the infants] are quite strong."
