The studies you linked to show an interesting correlation between lithium in water supplies and reduced suicide levels, but you can't judge the net benefit on a single metric alone. Lithium is not without side effects, and it's unreasonable to suggest that the vast majority of the population that is otherwise healthy be subjected to those side effects for the incremental benefit of a select few. The magnitude of these side effects may be relatively small in the levels we're talking about, but the net effect is likely not negligible when applied to entire populations over the course of their lifetimes.
I'm not going to argue with the fact that it results in "better" outcomes in some people's minds, but personally I'd rather accept an increased suicide risk. I'm sure I could take any number of mind-altering substances every day and I'd feel great, but that doesn't mean it's something I want to do.
It doesn't matter if you call it an element or a medication, it has non-negligible medical effects and side effects. If it had no relevant effects, we wouldn't be discussing it here.
> The ones who consume slightly more kill themselves significantly less, with no side effects.
"No side effects" is an impossible claim to make. The studies we do have show a correlation between lithium in the water supply and suicidal frequency in the population. The side effects have not been studied.
For some hard evidence: Lithium in drinking water is correlated with changes of thyroid function: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3114818/
Lithium most certainly does have side effects that are dose- and time-dependent. The side effects may be minimal at low concentrations, but when applied to a population over the course of a lifetime, they would certainly add up. Especially for those who are most susceptible to the negative effects.
It's also not hard to imagine that lithium's suicide rate reduction effects (which are purely correlation at this point, FWIW) are due to it's tendency to mellow people out. The cognitive dulling of even small amounts of lithium orotate in healthy (non-bipolar and non-suicidal) people are palpable. Would you still advocate for lithium in the water if it reduced the average intelligence of the population by, say, 1%? I'm picking hypothetical numbers here, but the reality is that there are certainly downsides to mass-medicating people with small doses of lithium.
Additionally, when you talk about mental "diseases", things get a lot more iffy. We have a terrible understanding of the brain, our diagnoses for those diseases are literally check lists, without any proper tests, many people would rather have their natural mental performance than a modified version of it, which is what something like this would force you into whether you like it or not.
While you can't make people exercise and eat their vegetables, they will usually drink the tap water.
But you then have to provide a way for people to opt out without any de facto penalty, which is the problem. So this would so something more suited for a dedicated drinking water system in an apartment building or complex. The mineralized water on tap goes to sinks, drinking fountains, and refrigerators, and regular old municipal water to sinks, toilets, showers, and laundry machines.
It may be a case of doing it, and warning people not to eat too much of it. But you need actual research to discover that.
That said, there is a form of lithium that is available over the counter: Lithium Orotate. The amount of elemental lithium per dose is lower than you'll find in prescription lithium carbonate treatment, but you wouldn't want to attempt to self-medicate in to the prescription range anyway. Lithium has a notoriously narrow therapeutic window, and it would be too easy to dose yourself into the toxic range.
I've tried lithium orotoate myself out of curiosity for circadian rhythm modification. Although the dose is lower than that of prescription lithium, the effects and side effects were still palpable. Personally, I found no positive benefits but substantial cognitive slowing and general feelings of malaise. It's also worth noting that lithium is moderately toxic to the kidneys over time.
Having tried low-dose lithium orotate myself, I would never advocate for any amount of supplemental lithium in water supplies. Let's leave the medication to those who need it, rather than forcing it upon the entire population.
Would you remove the lithium from other water supplies?