The adult human body contains approximately 1,200 g of calcium, of which about 99% is present in the skeleton. Bone is constantly turning over, through a continuous process of formation and resorption. In children and adolescents, the rate of formation of bone mineral predominates over the rate of resorption. In later life, resorption predominates over formation. Therefore, in normal aging, there is a gradual loss of bone.
The remaining 1% of total body calcium is found in the soft tissues and plays important roles in such vital functions as nerve conduction, muscle contraction, blood clotting, membrane permeability, and hormonal signaling. Blood calcium levels are carefully maintained within very narrow limits by the interplay of several hormones (1,25-dihydroxycholecalciferol, parathyroid hormone, calcitonin, estrogen, and testosterone) which control calcium absorption and excretion, as well as bone metabolism. Levels of soft tissue calcium are maintained at the expense of bone calcium in the face of inadequate calcium intake or absorption.
Intestinal calcium absorption is generally higher in children than in adults and ranges from 15 to 75% of ingested calcium. Adequate vitamin D status is necessary for normal calcium absorption.
Osteoporosis afflicts a large proportion of the elderly in developed countries. Caucasian and Asian women typically have low peak bone densities, and therefore, are at the greatest risk of developing osteoporosis. It is generally accepted that obtaining enough dietary calcium throughout life can significantly decrease the risk of developing osteoporosis. Among other factors, such as regular exercise, gender and race, calcium supplementation during childhood and adolescence appears to be a prerequisite for maintaining adequate bone density later in life. But even elderly osteoporotic patients can benefit significantly from supplementation with dietary calcium. Calcium citrate is a highly available and well tolerated form of calcium.