[trevorkoob]

One interesting aspect of Vitamin D that is not regularly discussed is the photo-isomerization of Vitamin D into inactive isomers in the skin.

I think of it similarly to Beta-Carotene, in that consuming the "natural source" has no known toxic amount, while consuming processed vitamin A can be toxic in large doses.

Increasing sun exposure will not raise blood vitamin D metabolite concentration indefinitely, and we don't have good studies that accurately measure the amounts of Vitamin D and its photo-isomers and their concentration in relation to large doses of UV exposure.

Therefore, through sun exposure, people may be storing large doses of Vitamin D isomers in their skin, keeping their levels much higher than typically prescribed oral supplementation levels.