CHAPTER 6:Vitamin D and Impact on Total-Body Calcium

Calcium is an essential daily nutrient. Critical functions of life are dependent on narrow concentration limits for plasma calcium, which is subject to obligatory losses from the body requiring a complex homeostatic mechanism and a daily nutrient intake. Stringent regulation of plasma calcium homeostasis is maintained by an intricate pathway of agents for which the biologically active metabolite of vitamin D, 1,25-diydroxyvitamin D, is one component. Ninety nine per cent of total body calcium is present in the skeleton. One of its functions is as a store of body calcium. Vitamin D exerts activities as an endocrine agent at the intestine, kidney and bone tissues and as an autocrine/paracrine agent within bone tissue to both stimulate bone mineral resorption and inhibit bone formation or to inhibit bone resorption and stimulate bone formation. These activities are coordinated by a range of regulatory factors to maintain plasma calcium homeostasis with secondary effects on bone-mineral status.