Abstract
Calcium is the principal cation of bone, making up almost 20% of its dry weight. Bone constitutes a very large nutrient reserve for calcium in terrestrial vertebrates, a reserve that has acquired a major mechanical function. A major mechanism by which calcium is recognized to influence bone strength is through its effect on bone mass. Because bone functions as the calcium nutrient reserve, it follows inexorably that any depletion of that reserve (or failure to produce the genetically programmed skeletal mass during growth) would carry with it a corresponding decrease in bone strength. Calcium needed for critical cell metabolic functions is, in most cells, derived from intracellular stores of the mineral. The requirement for calcium is related to the protection of this mechanical function, not to the metabolic actions of calcium, which could be adequately protected by a reserve several orders of magnitude smaller. Lack of calcium is a key factor for occurrence of osteoporosis. Effective osteoporosis treatments involve a combination of pharmacotherapy designed to reverse the negative remodeling balance prevailing in the skeleton and physical therapy designed both to increase function and to increase mechanical loading on the skeleton.