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Chapter: Medical Physiology: Parathyroid Hormone, Calcitonin, Calcium and Phosphate Metabolism, Vitamin D, Bone, and Teeth

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Osteoporosis-Decreased Bone Matrix

Osteoporosis is the most common of all bone diseases in adults, especially in old age.

Osteoporosis-Decreased Bone Matrix

Osteoporosis is the most common of all bone diseases in adults, especially in old age. It is different from osteo- malacia and rickets because it results from diminished organic bone matrix rather than from poor bone calci-fication. In osteoporosis the osteoblastic activity in the bone usually is less than normal, and consequently the rate of bone osteoid deposition is depressed. But occa-sionally, as in hyperparathyroidism, the cause of the diminished bone is excess osteoclastic activity.

The many common causes of osteoporosis are (1) lackof physical stress on the bones because of inactivity; (2) malnutrition to the extent that sufficient protein matrixcannot be formed; (3) lack of vitamin C, which is nec-essary for the secretion of intercellular substances by all cells, including formation of osteoid by the osteoblasts; (4) postmenopausal lack of estrogen secretion because estrogens decrease the number and activity of osteo-clasts; (5) old age, in which growth hormone and other growth factors diminish greatly, plus the fact that many of the protein anabolic functions also deteriorate with age, so that bone matrix cannot be deposited satisfac-torily; and (6) Cushing’s syndrome, because massive quantities of glucocorticoids secreted in this disease cause decreased deposition of protein throughout the body and increased catabolism of protein and have the specific effect of depressing osteoblastic activity. Thus, many diseases of deficiency of protein metabolism can cause osteoporosis.


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