ABSTRACT
This chapter reviews what is known about calcium metabolism and nutrition, inasmuch as calcium is the major mineral component of bone, and relates this to the epidemiology of osteoporosis. Over 99% of the calcium content of the human body, typically 1 to 1.5 kg, is found in the skeleton. In women, at the menopause, the loss of bone calcium is sudden and rapid and does not begin to slow until the seventh decade of life. Intestinal absorption of calcium proceeds by two routes: a saturable, transcellular pathway, regulated almost exclusively by vitamin D, and a nonsaturable, paracellular pathway that is not subject to acute regulation. Attempts to overcome osteoporosis, or at least to slow the course of bone mineral decrease, have, for many years, utilized vitamin D, either by itself or together with calcium supplementation. Typically vitamin D supplementation of women with osteoporosis will increase calcium absorption and urinary calcium output, with no effect on the overall retention of calcium.
