As anyone with basic nutrition knowledge is aware, humans can synthesize Vitamin D from sunlight.
Specifically, UVB rays convert a cholesterol precursor on the skin known as 7-dehydrocholesterol into previtamin D3.
Previtamin D3 is then converted into Vitamin D3 -- the active form -- with the simple "application" of heat (luckily, sunlight takes care of that!)
As with other vitamins and minerals, there is an tolerable upper limit for the "sunshine vitamin."
The latest data considers daily intakes higher than 10,000 International Units a day to be excessive, as they can result in nausea, fatigue, high calcium levels in blood, and calcinosis (a condition in which calcium deposits form in soft tissue).
Which begs the question: do humans run the risk of making too much Vitamin D if they are exposed to sunlight for hours with no protection?
Not at all, thanks to our incredibly smart bodies.
Turns out the cutaneous photoactivation and synthesis of vitamin D (that's a lofty way of saying "the process by which we create vitamin D from sunlight") is self-regulating.
Consistent exposure to heat photodegrades, or destroys, previtamin D3.
Therefore, once sufficient amounts of D3 have been created, subsequent amounts of previtamin D3 turn into inactive metabolites that are not converted into vitamin D3.