A nation-wide study in Denmark has unearthed a surprising new link between the levels of naturally occurring lithium in drinking water and the incidence of dementia.

Dementia is one of the biggest causes of disability in elderly people worldwide, with 9.9 million new cases happening every year, but there's still a lot we don't know about what causes the syndrome and how to prevent it. These new results suggest that adding lithium to tap water could be a future health intervention.

Lithium, the third element in the periodic table, isn't just the stuff that powers batteries in most of our electronic devices. Trapped in various minerals, this light metal is scattered all throughout our planet's crust, and is also naturally found in our drinking water.

For nearly 50 years, lithium salts have been used in the treatment of bipolar disorder, and studies have shown that long-term exposure to lithium might reduce dementia risk in bipolar patients.

With that in mind, a team led by researchers from the University of Copenhagen set out to investigate whether they could find any such effect on the population of Denmark, if they compared naturally occurring lithium levels in drinking water to dementia incidence rates.

And crazily enough, they did indeed find that higher levels of lithium in tap water correlate with fewer cases of dementia in those areas.

They used 151 drinking water samples from across the country, and used that data to develop a comprehensive map of lithium levels in drinking water for all 275 historical municipalities in Denmark.

Then the team crunched data on some medical records, looking at a total of 73,731 dementia patients and 733,653 controls - people without the illness. Matching up the diagnoses with lithium levels in the municipalities revealed a clear relationship between lithium exposure and dementia.

Compared to people who only got 2-5 micrograms of lithium per litre in their drinking water, the population that got at least 10 micrograms had a 17 percent decrease in their incidence of dementia.

But weirdly, in the middle range of lithium exposure of 5.1-10 micrograms per litre, the incidence of dementia was actually increased by 22 percent.

"The hypothesis is surprising but is mechanistically plausible," states an accompanying editorial in JAMA Psychiatry. "The link between lithium and neuropsychiatry has a venerable history."

Lithium is involved in many of the neural pathways we associate with neurological and psychiatric ailments, but the relationships between those pathways are highly complicated, so we don't know why lithium might have such an effect, especially because its amount in water is relatively low.

Besides, as is always the case with population correlation studies, this new data suggests a link, but it doesn't really tell us whether lithium is the definitive factor at play here.

"This is the first study, to our knowledge, to investigate the association between lithium in drinking water and the incidence of dementia," the team writes in the study.

"However, confounding from other factors associated with municipality of residence cannot be excluded."

So it's not like governments can now go and buy buckets of lithium to dump the stuff into our drinking water supply - there's still plenty we don't know, especially considering that the mid-level dose of 5-10 micrograms per litre actually seemed to increase dementia.

"It's almost too good to be true that something as cheap and plentiful as lithium might have a role in future prevention of Alzheimer's disease," says James Pickett, head of research at Alzheimer's Society in the UK.

"However, more research including clinical trials are needed, and until then we should not consider increasing lithium in drinking water.

"In high doses, or even at low doses in some people, lithium can be toxic so it is important that people consult with their doctor before they consider taking it as a supplement."

Researchers are stressing that right now the most important next step will be to do additional studies to see if these results can be replicated - and to actually investigate the potentially protective properties of lithium in the brain.

When such results do come in, we might just end up with an amazing new tool that could help us eventually cut back on the growing dementia rates.

The study was published in JAMA Psychiatry.