Inorganic mercury is a known neurotoxin that accumulates in fish, particularly large predator fish. In a well-documented incident in Minamata, Japan, which came to light in the 1950s, babies exposed to very high levels of mercury when their mothers ate contaminated fish developed brain damage and severe cerebral palsy.
But the question remains whether the benefits of long-chain omega-3 fatty acids might outweigh the risks of eating fish containing a lower level of mercury. (See box.)
Dr. Reece said it is possible for people who are worried about mercury or cannot eat fish to get omega-3 fatty acids from fish oil supplements. However, in its advisory, the HMHB Coalition said “consumption of ocean fish rather than ingestion of fish oil supplements is the best.”
Many pregnant women have reduced their fish intake well below beneficial amounts. Vivian E. Lee/Elsevier Global Medical News
Researchers Net Conflicting Evidence on Mercury Risks
The Maternal Nutrition Group considered evidence that includes a growing body of literature indicating that the selenium present in fish flesh may counteract the potential negative impact of mercury exposure, and that omega-3 fatty acids may help prevent preterm delivery and postpartum depression.
The group also took into account a recent study of about 8,000 British mothers and their children. That study asked the women about their fish intake while they were pregnant, and then followed the children until they were up to 8 years of age.
The study reported that children of mothers who ate less fish were more likely to have suboptimal cognitive and developmental outcomes (Lancet 2007;369:578–85). The largest difference was in the scores on a verbal intelligence test, with only 16% of the 1,330 children of mothers who ate more than 12 ounces of fish per week scoring in the lowest quartile, compared with 31% of the 584 children of mothers who ate none. About 24% of the children whose mothers ate between 0 and 12 ounces scored in the lowest quartile.
The study also found a positive impact from fish consumption in the areas of prosocial, social, and fine motor development.
In a recent article on pregnancy and lactation unrelated to the coalition's new recommendation, Dr. Gideon Koren, of the University of Toronto, reviewed some of the evidence regarding mercury exposure from fish. He noted two studies.
One was conducted in the Seychelles Islands, where the usual diet contains about 10 times more ocean fish than does the average U.S. diet. The researchers set out to see if mercury exposure hurt neurologic development. Contrary to their expectations, they found that when they measured mercury levels in hair samples from the mothers and subjected the children to sophisticated testing, children with the highest mercury exposure tended to have the best scores on many measures of development at 66 months of age.
They speculated that perhaps the beneficial effects of fish consumption exceeded the possible detrimental effects of being exposed to mercury (JAMA 1998;280:701–7).
Subsequent follow-up has not shown that any difference in development was sustained, however.
The second study was conducted in the Faroe Islands, where individuals eat pilot whale, a species with high mercury levels. Those investigators did not find any mercury-associated clinical or neurophysiologic abnormalities in about 900 children at 7 years of age. But they did find some subtle reductions in neurologic function in the children with the highest exposures (Neurotoxicol. Teratol. 1997;19:417–28).
Those investigators subsequently found that electrical signaling in the brains of those with more mercury exposure appears to be somewhat delayed at age 14 years, suggesting that the effect of mercury may be irreversible. They found delayed signaling even in children exposed to amounts of mercury lower than the FDA limits.
“This new statement [by the HMHB Coalition] only adds to the confusion,” said Dr. Philippe Grandjean, of the Harvard School of Public Health, Boston, who conducted the Faroe Islands study.
The coalition's advisory is too simple, he said. People should eat fish every week. But they probably should avoid the types of fish known to contain higher levels of mercury, such as tuna, swordfish, and shark. Safer fish are flounder, cod, mackerel, and salmon.
“Some people think that a pollution scare will prevent people from eating fish,” he said. “I don't think so. The problem, rather, is that the FDA does almost no mercury testing, and it is almost impossible for the average consumer to obtain information on which types of seafood are high in mercury.”