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A provitamin is a substance that may be converted within the body to a vitamin. The term "previtamin" is a synonym. For example, "Provitamin B5" is a name for Panthenol, which may be converted in the body to "Vitamin B5" a.k.a. Pantothenic Acid.
The term "provitamin" is used when it is desirable to label a substance with little or no vitamin activity, but which can be converted to an active form by normal metabolic processes. For example, "Provitamin A" is a name for Beta-Carotene, which has only about 1/6 the biological activity of Retinol (Vitamin A) -- the body converts Beta-Carotene to Retinol. In other contexts, both Beta-Carotene and Retinol are simply considered to be different forms (vitamers) of Vitamin A.