Very low levels of lithium in drinking water may help prevent suicide in the general population, according to a new study.

The study has prompted calls for further research into the possibility of adding lithium to drinking supplies - like water fluoridation to improve dental health.

Researchers at Oita University in Japan measured lithium levels in tap water in 18 municipalities in the Oita region. The lithium levels ranged between 0.7µg/l (micrograms per litre) and 59µg/l.

The researchers then calculated the suicide rate in each of the 18 municipalities. They found that the suicide rate was significantly lower in those areas with the highest levels of lithium in the water.

Writing in the May issue of the British Journal of Psychiatry, the researchers said: "Our study suggests that very low levels of lithium in drinking water can lower the risk of suicide. Very low levels may possess an antisuicidal effect."

Lithium is a naturally occurring metal found in variable amounts in food and water. In medicine, very high doses are used to treat bipolar disorder and mood disorders. But so far the potential benefit of using low levels of lithium to reduce the risk of suicide has not been studied closely.

Vancouver-based psychiatrist Professor Allan Young has described the study as "intriguing".

Commenting on the Japanese study in the same issue of the British Journal of Psychiatry, Professor Young said: "A logical first step would be for the Medical Research Council to convene an expert working party to examine the available evidence and suggest further research.

"Large-scale trials involving the addition of lithium to drinking water supplies may then be feasible, although this would undoubtedly be subject to considerable debate. Following up on these findings will not be straightforward or inexpensive, but the eventual benefits for community mental health may be considerable."