Metabolic Functions

The most obvious role of calcium is to provide structural integrity and strength in bones and teeth. Approximately 99% of the total body calcium content is used for this purpose. Bone also serves as a reservoir of calcium that can be drawn upon when serum calcium concentrations decline. The remaining 1% of the body's calcium is contained in blood, extracellular space, muscle, and other tissues, in which calcium concentrations are kept relatively constant. The maintenance of constant serum calcium concentrations at approximately 2.5mmol/l is critical for a number of cellular functions.

The extracellular concentration of calcium is in the 10~3M range, whereas in the cytosol it is approximately 10~6M. Almost all of the intracellu-lar calcium is bound within organelles such as the nucleus, endoplasmic reticulum, and vesicles. Cyto-solic calcium concentrations are very low and influenced greatly by release of some calcium from cellular organelles. Therefore, a small change in the release of calcium from intracellular sites or transport across the cell membrane results in a relatively large change in cytosolic calcium concentration. Binding of a hormone or a growth factor to a plasma membrane receptor increases inositol triphos-phate release, which in turn increases the free intra-cellular calcium concentration. The ionized calcium then binds to calmodulin, followed by a conformation change in the protein to trigger cellular events such as muscle contraction, nerve conduction, cell movement and differentiation, cell division, cell-to-cell communication, and secretion of hormones such as insulin. In these roles, calcium acts as an intracel-lular messenger.

Calcium may play a role in energy regulation and risk of obesity. Dietary calcium regulation of circulating 1,25(OH)2D3 in turn regulates the concentration of calcium in adipocytes. When adipocyte intracellular calcium concentration increases, this promotes the expression of lipogenic genes, and fat breakdown is reduced leading to accumulation of lipid in adipocytes. Through this pathway, low-calcium diets appear to promote fat deposition, whereas high-calcium intakes afford some protection from obesity.

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