Fish warning, again

- Last updated on GMT

A new study challenges the assumption that adults are much less
sensitive to the toxic effects of methylmercury, which contaminates
fish in some regions, than children.

The warnings about methylmercury-contaminated fish seemed to be over with a recent study confirming there was no threat. However this week a study in Environmental Health: A Global Access Science Source​ suggests that all age groups can be affected by methylmercury, challenging the assumption that adults are much less sensitive to its toxic effects than children.

The authors write: "Methylmercury exposure at levels often encountered by adults in North America may be inducing adverse effects on neurobehavioral performance."​ The chemical, which affects people through their diet, particularly large fish such as shark and swordfish, can impair concentration, dexterity and verbal memory by damaging nerve tissue. It affects the visual cortex and the cerebellum, the part of the brain responsible for controlling complex movements and maintaining balance.

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, according to the researchers.

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, reported the team. WHO will decide next week whether the current exposure limit should be decreased.

The news comes shortly after the OMB urged US health authorities to include recommended guidelines on omega-3 intake, strongly associated with heart health.

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.

The study​ warns that 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.

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.

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