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Bone Metabolism

Osteoporosis (“porous bone”) is a bone disease that increases the risk for fracture. It is caused by the loss of bone density from losing too much, not making enough, or a combination of both. Bone metabolism is the constant process of the body removing old bone (“bone turnover”) and replacing it (“bone resorption”). These processes take place in the osteoblasts, which form new bone, and osteoclasts, which break down old bone. As long as these processes are in balance, bone mass remains on a constant level.

 

At mid-life, individuals begin to lose bone quicker than they are able to replace it due to calcium metabolism, calcium and vitamin D deficiency, and hormonal factors, such as changes of estrogen level. Measuring proteins produced by the osteoblasts and osteoclasts provides a real-time evaluation of bone turnover, especially in the management of post-menopausal osteoporosis. Bone resorption markers can monitor progress of therapeutic interventions within a few weeks or months, whereas bone formation markers can take 6 to 12 months. This is still an improvement over bone mineral density (BMD), which can take as long as one to two years to determine the effectiveness of treatment.

Bone Turnover Measurement: Osteocalcin and Crosslinks Desoxypyridinoline

Measuring Bone Formation:
Osteocalcin
Measuring Bone Resorption:
Crosslinks Desoxypyridinoline
  • A protein found in the bone and dentin manufactured by the osteoblasts that binds to calcium
  • A highly specific marker of late stage osteoblastic activity
  • Elevated serum levels may occur in ostemalacia, Paget’s disease of the bone, hyperthyroidism, primary hyperparathyroidism and renal osteodystrophy
  • Depressed levels have been reported in hypothyroidism and during long-term corticosteroid therapy
  • A urine test that helps establish baseline bone turnover
  • Significantly elevated in post-menopausal women with osteoporosis than normal post-menopausal women
  • It monitors changes in urinary (deoxyopyridinoline) DPD excretion associated with aminobiphosphonate antiresorptive therapy
  • DPD is excreted unmetabolized in urine and is unaffected by diet, making it suitable for assessing resorption
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