vitamin A
The precursor to vitamin D is a prohormone called 7-dehydrocholesterol, which is present in the skin. When exposed to UV rays from sunlight, this precursor is converted into vitamin D3, which then undergoes further processing in the liver and kidneys to become the active form of vitamin D.
No UV light can not be treated as vit D precursor as it only provides the energy in the form of radiation for the reaction to take place. It is the steroid molecule present in dermis which is precursor for vitD synthesis.
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cholesterol
Vitamin A is the precursor for the pigments in the rod and cone cells of the eyes that detect light. With it you can see, a deficiency can cause blindness. However too much vitamin A can be toxic and it builds up in fat. To prevent toxicity supplements do not actually contain vitamin A, instead they contain Beta Carotene a precursor of vitamin A that is nontoxic and that the body converts as it needs vitamin A.
Niacin
Vitamin D production begins when a precursor molecule is exposed to ultraviolet light in the skin.
synthesis of a Vit. D
precursors
Vitamin A, which would usually be taken in the form of Beta Carotene its precursor (which is safer to take than Vitamin A itself).
No. Cholesterol is a lipid and is a precursor to vitamin D and steroid hormones.