Fish is not always "brain food" – eating methylmercury contaminated fish causes problems in adults

Warnings about methylmercury contaminated fish are not just for young children and expectant mothers, according to new research published today in Environmental Health: A Global Access Science Source. Adults who regularly eat contaminated fish could find that their concentration, dexterity and verbal memory are impaired.

The major source of methylmercury is diet, particularly large fish like shark and swordfish. The authors of this new research concluded: “methylmercury exposure at levels often encountered by adults in North America may be inducing adverse effects on neurobehavioral performance.”

Methylmercury damages or destroys nerve tissue. It affects the visual cortex and the cerebellum, the part of the brain responsible for controlling complex movements and maintaining balance. This new research challenges the assumption that adults are much less sensitive to its toxic effects than children.

Ellen Silbergeld, of John Hopkins University, and her colleagues conducted the first major study on the effects of long-term exposure to small amounts of methylmercury in adults using sensitive neuropsychological tests.

The team, which included researchers from The University of Rio de Janeiro and the University of Maryland, studied a group of 129 men and women living in fishing communities of the Pantanal region of Brazil. They took samples of recent hair growth and analysed them for mercury. Because hair grows constantly, these values reflect the exposure of individuals to methylmercury within the previous three months. About one out of four of the participants were found to have mercury levels that exceeded the ’safe’ level set by the World Health Organization (WHO) for women and children. WHO will decide next week whether the current exposure limit should be decreased.

The researchers also used a number of tests to assess mental functions such as learning, attention, memory, accuracy, manual speed and dexterity. Those individuals with higher levels of mercury in their hair fared worse on the tests for motor skills, memory and concentration.

Mercury is used in small-scale gold-mining. A gold-rush began in the Patanal region of Brazil in the early 1980’s, leading to an increased concentration of mercury in the environment in the mid-1980’s. Once mercury gets into the water system, it is converted to methylmercury by bacteria found in sediment. Fish ingest it as they feed, and the concentration increases up the food chain. Larger fish species with long life-spans will contain much higher levels of methylmercury.

Methyl mercury levels are a global concern, with the major source of exposure being fish consumption. Women who have eaten grossly contaminated fish may give birth to children with severe disabilities including cerebral palsy and mental retardation. The United States Food and Drug Administration issued an advisory in 2001, warning women who were pregnant or of child-bearing age to restrict their intake of shark, swordfish, king mackerel and tilefish. Governmental advisories in the United Kingdom and other countries also warn that the diet of children should not include contaminated fish species.

This study suggests that this advice should now be extended to everyone, although the effects seen in adults are less severe than in children exposed to methylmercury before birth.