Most people know lithium as the “drug” given to bipolar patients. In reality it is not a drug but a mineral, and in bipolar disorder it’s given in high doses: the target dose is usually 900 to 1,800 mg a day.
However, lithium is a required nutrient. “The available experimental evidence now appears to be sufficient to accept lithium as essential; a provisional RDA for a 70 kg adult of 1000 μg/day is suggested.” (1000 μg = 1 mg.)
This is, as can be seen, much lower than the dose given in bipolar.
Furthermore, low levels of lithium in drinking water have been associated with violence, suicide, and homicide. See, for example, Lithium in Tap Water and Suicide Mortality in Japan.
A study looked at epidemiological evidence of an effect of lithium on human lifespan, and found it. Low-dose lithium uptake promotes longevity in humans and metazoans. Mortality rates were inversely associated with lithium concentrations in tap water; furthermore, the lower mortality rate remained after adjusting for suicide, showing that lithium provides some other health benefit not strictly related to mental health.
Since this association does not show causality, the same authors used low-level lithium, at about the same concentration found in the tap water, and tested it on the worm C. elegans. It extended their lifespan, showing causality.
Lithium seems to extend lifespan by promoting autophagy, the cellular self-cleaning process that rids cells of junk and is crucial to lifespan extension. It does this by an mTOR-independent mechanism, meaning that it does not depend on fasting. Through autophagy, lithium has been found to delay progression of amyotrophic lateral sclerosis.
As stated above, about 1 mg a day is a suggested RDA for lithium. Dose for bipolar patients are hundreds or thousands times higher, but there’s considerable risk of toxicity at those doses, while there appears to be little for low doses. A common formulation, lithium orotate, provides 5 mg lithium.
I generally take one 5 mg tablet of lithium orotate once every few days.