Vitamin D Biochemistry
Vitamin D is a fat-soluble vitamin that exists in a number of different forms. The most important forms in humans are vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). Both are considered vitamin D, but the source and potency of each is different. Ergocalciferol (D2) is a less potent form of vitamin D found in plants, whereas cholecalciferol (D3) is a more potent form synthesized by sunlight. Dairy products are also commonly fortified with vitamin D, commonly vitamin D3. After ingestion or synthesis, vitamin D (calciferol) undergoes hydroxylation in the liver to form 25-hydorxy vitamin D (calcidiol), the inactive storage form of vitamin D. Vitamin D undergoes further hydroxylation in the kidney, forming the active form: 1,25-dihydroxy vitamin D (calcitriol). Active vitamin D is responsible for increased calcium and phosphate absorption. At low levels, it leads to increased bone mineralization. At high levels, it leads to increased bone resorption, thereby increasing calcium and phosphate levels.