Which form of vitamin D is produced in the skin as part of the synthesis pathway?

• A. Vitamin D3
• B. Vitamin D2
• C. Calcitriol
• D. Vitamin D1

Answer: (A)

When skin is exposed to UVB light, a cholesterol-related molecule in the epidermis called 7-dehydrocholesterol is converted to previtamin D3 and then quickly becomes vitamin D3 (cholecalciferol). This is the form that the skin synthesizes. Afterward, vitamin D3 is transported to the liver for hydroxylation to 25-hydroxyvitamin D, and then to the kidney for another hydroxylation to the active hormone calcitriol. Vitamin D2 comes from plant sources and fortified foods, not from skin synthesis, and calcitriol is the downstream active form produced after these hydroxylations, not the initial product in the skin. Vitamin D1 is not used in modern physiology.