The vitamin D RDA was determined based on which consideration?

• A. The RDA for vitamin D was determined based on conditions of minimal sun exposure due to the variability of vitamin D synthesis in the skin from ultraviolet light and the risks of skin cancer.
• B. The RDA was determined by dietary intake alone.
• C. The RDA was determined by genetic variation.
• D. The RDA was determined by maximal sun exposure.

Answer: (D)

Vitamin D needs are set to protect bone health even when sun-driven synthesis is limited, because cutaneous production varies greatly among individuals and there’s a real cancer risk with UV exposure. Because relying on sun exposure is unreliable and sometimes unsafe, the RDA reflects the amount of vitamin D that should be consumed through diet and fortified foods to maintain adequate status when sun exposure cannot be assumed. Maximal sun exposure isn’t used as the basis because it’s unsafe and not realistically achievable for most people, so the dietary amount is what the RDA targets.