I can subjectively tell good and bad water apart in double blind tests over a period of several hours, so I know that what's happening to me isn't just in my head. So -- I don't know whether it's fluoride or not, but it's definitely something.
The reason to suspect fluoride specifically is that a working Berkey "black" filter still produces "bad" water for me -- the white filter stage seems to be necessary. Twice now, I've had a white filter go bad on me that I didn't realize had happened, felt like crap, tested the water, found tap levels of fluoride, replaced it, and felt much better. The white filters are advertised to get fluoride and arsenic, so that makes it a prime suspect. I don't know exactly what else they might get that the problem might be, but there are a lot of suspects the black filter would block, and it's none of them. So if the problem isn't fluoride, it's something that that fluoride is a good proxy for in this setting.
As the problem I'm trying to fix is low key dehydration (as opposed to avoiding something medical like fluorosis), I don't expect there would be any medical research pointing to fluoride as a culprit. Neither will there be any saying it's safe for my purposes. I have found that while medicine as a field is excellent for healing injuries and saving lives, it's sort of terrible at optimizing health.
What I do know is that I drink a lot more water than is typical -- about two gallons a day when I am not nursing, and much more when I am. I also know that the studies that proscribe fluoride levels in drinking water are ancient, optimized narrowly for dental health, and based on average intake. I know as well that the difference between an effective dose of fluoride and a dose considered potentially dangerous is about an order of magnitude -- right about the difference between my water intake and what is typical. (And as an aside, it blows my mind that we make formula for babies with water like this, dosed based on evidence like that. They don't even have teeth!)
Hence, it's a prime suspect, but even if it turned out to not be that, I would still filter my water very aggressively because something real is definitely happening to me, and that definitely fixes it.
My choice of Berkey is that I want a maximally aggressive solution. I'm not looking to save money. I'm looking to nuke whatever it is that is causing me problems, and am perfectly fine with paying a bit extra to nuke everything. I bought one actually originally concerned about PFCs; discovering the filtered water made me feel much better was an unanticipated happy accident.
