Vitamin D facilitates the intestinal absorption of calcium and phosphorus and plays an important role in bone mineralization (1). It follows that maintaining optimal 25-hydroxyvitamin D [25(OH)D] [7] concentrations is particularly important during the growth period in children and adolescents, when much of adult bone mass is established (2). Vitamin D is believed to exert physiological effects beyond the skeletal system (3). Indeed, the identification of vitamin D--specific nuclear receptors, in a number of tissues, indicates that this hormone plays a role in several physiological processes including cancer prevention (4), immune regulation (5), and glucose homeostasis (6). Ultraviolet activation of 7-dehydrocholesterol in the epidermis is the predominant source of cholecalciferol or vitamin [D.sub.3] (7). This secosteroid is transported to the liver where it is hydroxylated to yield 25(OH)D. When dietary calcium intake is low, the parathyroid glands respond to minute decreases in ionized serum calcium by releasing parathyroid hormone (PTH). PTH, in turn, regulates the final hydroxylation of vitamin D in the renal mitochondria, yielding the biologically active hormone, 1-[alpha],25-dihydroxycalciferol (8). Active vitamin D has 3 target tissues relevant to calcium metabolism. First, active vitamin D initiates the breakdown of bone tissue, releasing calcium into the serum; second, it increases the absorption of dietary calcium in the gut; finally, active vitamin D increases the re-absorption of calcium in the distal tubule of the kidney (3).
