Metabolism of Vitamin D and Calcium

Recent progress in the metabolism of vitamin D and calcium is reviewed. Vitamin D is metabolized to 25-hydroxyvitamin D (25-OH-D) in the liver and subsequently to 1α, 25-dihydroxyvitamin D [1α, 25-(OH)2D] or 24, 25-dihydroxyvitamin D [24, 25-(OH)2D] in the kidney when plasma calcium concentrations are lower or higher than normal. 1α, 25-(OH)2D, which is known as an active form of vitamin D, is transported to the target organs (intestine, bone and others) to show the physiological activity. The most important role of 1α, 25-(OH)2D is to keep the calcium homeostasis by accerelating the intestinal calcium absorption and bone remodeling and the mechanism is discussed.