Bone and calcium
1Professore Associato MED46 , Scienze e tecnologie di Medicina di Laboratorio, Universita’ Degli Studi di Milano.
Bone is a living tissue undergoing continuous remodeling throughout life, allowing the adaptation of form and organized structure of bones to biomechanical forces, maintaining the integrity of bone (repair microtraumas) and homeostasis of calcium and phosphates. Bone mass is determined by a balance of endogenous (genetic, hormonal) and exogenous factors (nutrition, exercise). Nutrition plays an important role and in particular, calcium and vitamin D are essential in bone metabolism and turnover. Calcium acts structurally as supporting material in bones as calcium hydroxyapatite (Ca10(PO4)6(OH)2. Calcium intake affects peak bone mass by influencing calcium retention during bone growth and preventing bone loss and osteoporotic risk in older age (1). Indeed , reduced supplies of calcium are associated with reduced bone mass (2). Conversely, calcium supplements reduced the rate of bone loss in osteoporotic patients (2, 3). Plasma calcium levels is highly controlled and maintained within narrow limits (1.3–1.5 mmol/L ). Alteration of this values lead to hypocalcemia (insufficient calcium or loss of calcium), or hypercalcemia (calcium excess in the blood), often as result of malignancy or primary hyperthyroidism. Bone remodeling in hypercalcemia results from increased net bone resorption as occurs in osteoclastic metastatic bone cancer, primary hyperparathyroidism, and vitamin D poisoning. In CKD patients with adynamic bone disease, hypercalcemia is readily produced because the bone is unable to take up calcium by formation (4, 5). Bone remodeling in hypocalcemia results from increased net bone formation as occurs in post-parathyroidectomy “hungry bone syndrome” and osteoblastic metastatic bone cancer. It has been hypothesized that bone can release to, and remove calcium from, the circulation by active mechanisms separate from the remodeling system (6). Calcium and bone disorders involve abnormalities of bone metabolism. The most common of these disorders is osteoporosis, as well as hyperparathyroidism and Paget’s disease of bone.
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