As many as 25% of infants in a Hong Kong study have been found to have excessive mercury in their blood, which can cause mental retardation, and doctors have warned expectant mothers against eating too much fish.
Methylmercury is readily absorbed into the blood after ingestion and distributed to all tissues of the body in four days. In pregnant women, it readily crosses the placenta into the fetal circulation system and is deposited in the fetal brain.
In a study of 1,057 infants over a two-year period in Hong Kong, researchers found how much fish mothers consumed directly affected the amount of blood mercury found in their babies. 24.7% of the infants in the study were found to have cord blood mercury concentrations of over 61 nanomol/litre, the upper allowable limit. Three percent had blood mercury concentrations of over 100 nanomol/L.
Eating too much fish by the mother during pregnancy is the major factor in elevating the blood mercury level of the fetus, say researchers at the Chinese University. The major source of human mercury intake is dietary methylmercury present primarily in fish and other seafood.
In adults, mercury poisoning causes personality changes, nervousness, irritability, fatigue, insomnia, headache, loss of memory, hearing and vision and even renal failure. In babies, it may lead to stillbirth, cerebral palsy, mental retardation, speech delay, poor control of chewing, salivation and swallowing.
Curiously, however, fish commonly caught and heavily consumed in Hong Kong were not found to have excessive levels of mercury, the researchers said.
"It isn't the case that our fish are particularly bad, it's because we are eating too much fish," the scientists said.
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