"Researchers examined levels of lithium in drinking water and suicide rates in the prefecture of Oita, which has a population of more than one million.
The suicide rate was significantly lower in those areas with the highest levels of the element, they wrote in the British Journal of Psychiatry.
High doses of lithium are already used to treat serious mood disorders.
But the team from the universities of Oita and Hiroshima found that even relatively low levels appeared to have a positive impact of suicide rates.
Levels ranged from 0.7 to 59 micrograms per litre. The researchers speculated that while these levels were low, there may be a cumulative protective effect on the brain from years of drinking this tap water."