Children born to mothers-to-be who ate an average of 12 meals of fish a week - about 10 times the average U.S. citizen eats - showed no harmful symptoms.
The study by scientists at the University of Rochester Medical Center is the latest in a series of updates on children who have been studied since their birth in 1989 and 1990 in the Republic of the Seychelles, an island nation in the Indian Ocean. The children have been evaluated five times since their birth, and no harmful effects from the low levels of mercury obtained by eating seafood have been detected.
"Consumption of fish is generally considered healthy for your heart, yet people are hearing that they should be concerned about eating fish because of mercury levels," says lead author Gary Myers, M.D., a pediatric neurologist. "We've found no evidence that the low levels of mercury in seafood are harmful. In the Seychelles, where the women in our study ate large quantities of fish each week while they were pregnant, the children are healthy."
In a commentary on the research in The Lancet, Johns Hopkins scientist Constantine Lyketsos writes that, "For now, there is no reason for pregnant women to reduce fish consumption below current levels, which are probably safe." He calls the Seychelles study a "methodological advance over previous studies."
Questions about the health effects of mercury often boil down to seafood because fish are the primary source of exposure to mercury for most people. Scientists estimate that about half the mercury in the Earth and its atmosphere originates from natural sources such as volcanoes, and about half comes from man-made sources.
People receive most of their mercury exposure by eating ocean fish like tuna, swordfish and shark. The fish eaten by women in the Seychelles had approximately the same levels of mercury as those eaten by consumers in the United States - but they ate much more fish than most people in the United States. The Seychelles women, however, had an average of six times as much mercury in their bodies, as measured in hair samples, as most people in the US.
"This study indicates that there are no detectable adverse effects in a population consuming large quantities of a wide variety of ocean fish," says Myers, the senior author of the Seychelles study and an internationally recognized authority on mercury. "These are the same fish that end up on the dinner table in the United States and around the world."
In the current study doctors and nurses tested the children in a variety of ways and measured 21 different cognitive, behavioral, and neurological functions such as concentration, attention span, problem-solving abilities, intelligence, and motor skills. Only two functions varied slightly according to mercury level: Children of women with higher mercury levels were slightly less likely to be hyperactive, and sons of such women did slightly worse on a pegboard task. Statistically, both findings are likely due to chance, the researchers say.
The Seychelles findings apply to fish bought and sold commercially, at grocery stores, supermarkets, seafood markets, and restaurants. Those fish are already regulated based on their mercury levels. Consumers should carefully follow advisories about eating fish caught in lakes and rivers, since there are hundreds of polluted waterways whose fish are dangerous to eat in abundance, often because of pollutants like PCBs.
The Seychelles study came about as a result of previous work by the same Rochester team, which put together the first precise data showing that pre-natal exposure to mercury could harm a developing child. Their study of the victims of an accidental mercury poisoning event in Iraq more than 30 years ago spurred them to start the Seychelles study to try to pinpoint the levels at which mercury poses a danger.
Now the team is launching a new study in the Seychelles to compare the levels of nutrients pre-natally to the health of children early in their lives. The study has its roots in a finding in one of the previous Seychelles reports, that children born to mothers with slightly higher mercury levels did better on some neurological and intelligence tests than their counterparts. That may be because those children's mothers with the higher mercury ate more fish. This study, funded by the National Institute of Environmental Health Sciences, is being done with colleagues at the University of Ulster in Northern Ireland and Cornell University.
"There are a lot of good, vital nutrients in fish," says Myers, who is directing the team that is studying 300 children to compare their health with the levels of polyunsaturated fatty acids, selenium, and other nutrients in their mothers during pregnancy.
The Seychelles study, ongoing since 1989 with funding from the National Institute of Environmental Health Sciences, is one of the longest "longitudinal" studies ever done in children. The research has been funded by the NIH, the U.S. Food and Drug Administration, and the Republic of the Seychelles.
"The cooperation from people in the Seychelles and the Ministry of Health has been extraordinary," Myers says. "They recognize the importance of this subject both to their own citizens and to the people around the world who consume fish."
In addition to Clarkson and Myers, the Seychelles team includes Philip Davidson, Ph.D.; Donna Palumbo, Ph.D.; Li-Shan Huang, Ph.D.; Elsa Cernichiari; and Jean Sloane-Reeves, all of the University of Rochester; and Conrad Shamlaye of the Republic of the Seychelles. Christopher Cox, Ph.D., of the National Institutes of Health; Gregory Wilding, Ph.D., of the University at Buffalo; and James Kost, Ph.D., also took part.