Vitamin D is a fat-soluble vitamin that regulates calcium homeostasis and is vital for bone health (1). While it can also be obtained from dietary sources or supplements, vitamin D3 (cholecalciferol) is synthesized in the human skin from 7-dehydrocholesterol upon exposure to ultraviolet-B (UVB) radiation from sunlight (see the article on Vitamin D and Skin Health). Vitamin D2 (ergocalciferol) is a vitamin D analog photosynthesized in plants, mushrooms, and yeasts; vitamin D2 is also sometimes used in vitamin D food fortification (2). When vitamin D3 in skin is inadequate due to insufficient exposure to UVB radiation, oral intake of vitamin D is necessary to meet vitamin D requirements.