Although there is no straightforward answer to this question, the current recommended daily allowances for adults—600 IU for children and adults, and 800 IU for those older than 70 years— are probably too low.1,2 To provide a little background, vitamin D should be considered a prohormone rather than a vitamin (a substance humans need in small amounts and usually cannot synthesize).3 Animals convert 7-hydrocholesterol to calciferol (D3) in the dermis via a photochemical reaction with ultraviolet B (UVB) sunlight.
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