Understanding Osteoporosis

Osteoporosis is a metabolic disease of the skeleton that reduces the amount of bone tissue. Bones weaken as local cells resorb, or take up, bone tissue. Trabecular bone at the core becomes less dense, and cortical bone on the perimeter loses thickness. (See Controlling mineral balance and Bone formation and resorption .)

Normally, the blood absorbs calcium from the digestive system and deposits it into the bones. In osteoporosis, blood levels of calcium are reduced because of dietary calcium deficiency, inability of the intestines to absorb calcium, or postmenopausal estrogen deficiency. To maintain the blood calcium level as close to normal as possible, resorption from the bones increases, causing osteoporosis.

In addition to enhancing bone resorption, low calcium enhances the effects of two other factors: Parathyroid hormone (PTH) and vitamin D. PTH is produced by the parathyroid glands, which are buried in the thyroid gland. Vitamin D is supplied by the diet, produced in the skin as a reaction to sunlight, and processed into a very potent form in the liver and kidneys. Both substances stimulate calcium absorption from the intestine and increase resorption from the bone. This results in an increased sacrifice of bone calcium to maintain normal levels of calcium in the blood.

Bone consists of 30% organic and 70% mineral substances. The organic portion, called osteoid , acts as the matrix or framework for the mineral portion.

Bone cells called osteoblasts produce the osteoid matrix. The mineral portion, which consists of calcium and other minerals, hardens the osteoid matrix.

Osteoclasts are large bone cells that reshape mature bone by resorbing the mineral and organic components. Bone formation and resorption are normal, continuous processes. However, in osteoporosis, osteoblasts continue to produce bone, but resorption by osteoclasts exceeds bone formation.