In a major new finding almost a decade in the making, researchers at Harvard Medical School say they’ve found a key that may unlock many of the mysteries of Alzheimer’s disease and brain aging — the humble metal lithium.

Lithium is best known to medicine as a mood stabilizer given to people who have bipolar disorder and depression. It was approved by the US Food and Drug Administration in 1970, but it was used by doctors to treat mood disorders for nearly a century beforehand.

Now, for the first time, researchers have shown that lithium is naturally present in the body in tiny amounts and that cells require it to function normally — much like vitamin C or iron. It also appears to play a critical role in maintaining brain health.