A study has found a correlation between mothers' intake of methylmercury from eating fish with relatively high concentrations of the toxicant and delays in the intellectual development of their children, in the first such study conducted on Japanese subjects to show that methymercury amounts below a dangerous concentration to the mother can still have an effect on the fetus.
The study, conducted by a team at Tohoku University, found that pregnant women eating too much of comparatively high-concentration methylmercury fish like tuna or swordfish raised the risk of adverse effects on the athletic and intellectual development of their children.
Since 2002, the study followed around 800 parent-child groups in coastal regions of the Tohoku area who were thought to eat lots of fish. Researchers measured the methylmercury content of the mothers' hair at the time of delivery and checked the children's athletic and intellectual capabilities at age 18 months and 3 1/2 years using tests often administered overseas. They then examined the relationship between these findings.
Among the subjects' hair, the lowest levels of methylmercury were 1 part per million (ppm) or less while the highest levels were in excess of 10 ppm. The World Health Organization and other institutions place the lowest concentration of methylmercury needed to cause nervous system disabilities like Minamata disease at 50 ppm.
Compared to the children of mothers with the lowest concentrations of methylmercury, the children of mothers with the highest concentrations scored about 5 percent lower on the Bayley Scales of Infant Development used to measure athletic ability at age 18 months. The athletic ability of infants is thought to be related to their future intellectual development, and in intelligence tests at the age of 3 1/2 years, a gap of about 10 percent between the scores of the children of mothers with the lowest and highest concentrations of methylmercury was seen in boys only. Foreign research has already shown that boys are more susceptible to the effects of methylmercury.
In 2005, based on foreign research the Japanese government decided that the weekly limit of acceptable methylmercury intake for pregnant mothers was 2 millionths of a gram per kilogram of body weight. Based on this, the Ministry of Health, Labor and Welfare gave a recommendation that pregnant mothers limit their intake of Bluefin tuna to less than 80 grams per week. The Tohoku University team's study included asking the subjects about their food diets, and around 20 percent of the mothers are thought to have exceeded the ministry's recommended limit.
Tohoku University professor of developmental environment medicine Kunihiko Nakai, a member of the study team, says, "We believe that if the recommendation (from the ministry) is followed, there is no need to worry about methylmercury effects. Fish contain precious nutrients, and it would not be good for pregnant women to abstain from fish. It is important to choose which fish to eat, however, such as avoiding fish like tuna that are among the top of the food chain and instead eat fish like saury."