The RDA for vitamin D was determined based on what consideration?

• A. The RDA was determined based on 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 based on maximal sun exposure.
• C. The RDA was based on bone mineral density alone.
• D. The RDA was determined by dietary intake alone.

Answer: (A)

Vitamin D intake recommendations are set to ensure bone health for people who may have little or no sun-driven vitamin D production. The amount produced in the skin from sunlight varies greatly by factors like latitude, season, skin type, and time outdoors, and relying on sun exposure raises concerns about skin cancer risk. Therefore, the RDA is based on a scenario of minimal sun exposure, so even those with limited endogenous synthesis can meet their vitamin D needs through diet and supplements.

The other ideas don’t fit as well: maximal sun exposure isn’t a safe or reliable basis due to cancer risk; basing the RDA on bone mineral density alone ignores the role of sun-derived production and dietary intake; and basing it on dietary intake alone would neglect the substantial contribution from skin synthesis and the variability in sun exposure.