Higher levels of lithium in drinking water appear to guard people from developing dementia, according to a study by researchers in Denmark.
Tap water contains varying amounts of lithium.
The Danish study involved 800,000 people, but the findings are not clear-cut. The highest levels appeared to cut risk, but moderate levels were worse than low ones.
According to experts, it was an intriguing and encouraging study that hinted at a way of preventing the disease.
The study at the University of Copenhagen, looked at the medical records of 73,731 Danish people with dementia and 733,653 without the disease.
The researchers then tested tap water in 151 areas of the country.
According to the results published in JAMA Psychiatry, moderate lithium levels (between 5.1 and 10 micrograms per litre) increased the risk of dementia by 22 per cent as against low levels (below five micrograms per litre).
But the researchers found a 17-per cent reduction in risk in those drinking water with the highest lithium levels (above 15 micrograms per litre).
The researchers said, "This is the first study, to our knowledge, to investigate the association between lithium in drinking water and the incidence of dementia, BBC reported.
"Higher long-term lithium exposure from drinking water may be associated with a lower incidence of dementia."
Although researchers say that the link might be due to other environmental factors, they point out that it may be worth investigating whether adding lithium to tap water could be a cheap way of protecting large numbers of people from dementia.
Lithium, a metallic element, is found in varying quantities in water in the UK, from around one migrograms per litre to around 21 micrograms per litre. The researchers found benefits in concentrations in excess of 15 micrograms per litre.
Lithium is known for its neuroprotective effects and is a standard treatment for bipolar disorder. Experiments on rats have shown that it boosts memory.