Yes, my vitamin D was very low (19). After learning of that, I do make a point to go out in the sun during mid-day and sunbathe (most skin exposed), but I don't know if that helps.
But everything else about my kidney function seemed fine in the blood and urine tests (it could be that the other kidney is doing double duty)..
I fully intend to do a ~8-day Water Fast every 3 months or so. But if what I need is one long one (21 days) instead of many 8-day ones, that could be a challenge..