EATING fish contaminated with mercury could put people at risk of developing diabetes. That鈥檚 because methyl mercury, the form of the metal that accumulates in fish, can kill the cells in the pancreas that make insulin.
At least, this is what Shing-Hwa Liu and colleagues at the National Taiwan University in Taipei discovered when they exposed beta and islet cells to methyl mercury at levels typically found in contaminated fish (Chemical Research and Toxicology, vol 19, p 1080). Methyl mercury is a powerful oxidant, and this seemed to explain the effect: Lui鈥檚 team was able to protect the cells from damage by adding the antioxidant N-acetyl cysteine.
Diabetes is on the rise in many countries, and has doubled in the US over the past 30 years. While obesity is thought to be the major cause, suspicion is also falling on pollutants. In July, a South Korean team revealed a connection between persistent organic pollutants and diabetes. They found that people with diabetes tended to have elevated concentrations of pollutants including dioxin in their blood, although they have failed to find any link with mercury.
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Vivian Fonseca, a diabetes specialist at Tulane University in New Orleans, points out that very little of the methyl mercury found in fish actually makes it into the human bloodstream, which means the Taiwanese findings could overestimate the risk to human health.
鈥淭here might be many chemicals besides methyl mercury which kill pancreatic cells in experimental settings, but might not cause diabetes in humans,鈥� says Duk-Hee Lee of the Kyungpook National University in Daegu, who led the South Korean study on persistent organic pollutants. Liu agrees that researchers will need to carry out further studies in humans to confirm a link between mercury and diabetes. However, he has already discovered that mice fed low levels of methyl mercury for just a month produce less insulin, have higher blood glucose levels and sustain more oxidative damage to their fat cells.
鈥淭he form of mercury that accumulates in fish may kill the cells in the pancreas that make insulin鈥�
鈥淚 think people should eat less fish,鈥� Liu says, restricting themselves to perhaps two servings of fish a week.
Lee鈥檚 studies have also shown that the links between pollution and diabetes can be subtle and complex. In people in the US, blood concentrations of persistent organic pollutants have declined over the past few decades. However, Lee鈥檚 team found that the association between diabetes and levels of pollutants in the blood was strongest in obese people (Diabetes Care, vol 29, p 1638). This may be because pollutants like dioxin are retained more readily in fatter people.
Fonseca argues that the epidemic of obesity is the main reason why more and more people are developing diabetes, but agrees that pollutants may be a subsidiary factor. 鈥淎dditional factors such as pollutants may be playing a subtle role in some individuals,鈥� he says.
