Definition
A condition which is characterized by the progressive loss of bone density and thinning of bone tissue.
Causes, incidence, and risk factors
Osteoporosis is the most common type of metabolic bone disease in women. It occurs when the body fails to form enough new bone or when too much of the old bone is reabsorbed by the body, or both. Calcium and phosphate are two minerals that are essential for normal bone formation. Throughout youth, the body uses these minerals to produce bones. If calcium intake is not sufficient, or if the body does not absorb enough calcium from the diet, bone production and bone tissues may suffer. Calcium and phosphate may be reabsorbed back into the body from the bones, in which case the bone tissue is made weaker. Both situations can result in brittle, fragile bones that are subject to fractures.
Usually, the loss occurs gradually over an extended period of time (years) and most of the time, a person will sustain a fracture before becoming aware that the disease is present. By the time this occurs, the disease is in its advanced stages and damage is profound.

There are a number of causes of osteoporosis, however hormone deficiencies (estrogen in women and androgen in men) are the leading cause. Women, especially over the age of 60, are the most frequent sufferers of the disease. This is due to the loss of ovarian function and subsequent reduction in estrogen production that occurs around the time of menopause. Other causes include corticosteroid excess (Cushing's syndrome), hyperthyroidism, hyperparathyroidism, immobilization, bone malignancies, certain genetic disorders, and other miscellaneous problems such as low calcium in diet.
Researchers estimate that about 23% of American women over the age of 50 have osteoporosis and between 40% and 56% of them have osteopenia, which is abnormally low bone density which may eventually deteriorate into osteoporosis if not treated. From these figures, researchers estimate that 50% of women over the age of 50 will suffer a fracture of the hip, wrist or vertebra. The risk of fracture in men of the same age group is about 13%.

Risk factors, in addition to menopause, include genetic and ethnic background. Women who are white or asian, especially those with a family history of osteoperosis have a greater risk of developing osteoperosis than other women. Smoking, eating disorders, low amount of calcium in the diet, heavy alcohol consumption and use of certain medications such as steroids are also risk factors.

Prevention
Throughout life, dietary intake of calcium is essential for bone formation and maintainance. Vitamin D which aids in the absorption of calcium, is also essential. Maintaining a healthy diet as recommended by the FDA, (which includes a sufficient amount of calcium, phosphorous, and vitamin D), is very important.

Regular exercise can reduce the likelihood of bone fractures. Studies show that exercises requiring muscles to pull on bones cause the bones to retain and maybe even gain density.

Post-menopausal women, especially those who have reduced bone density, may consider intervention with estrogen replacement therapy. This may prevent bone loss or fracture in some people. There are a number of side effects, both good and bad, associated with estrogen replacement therapy and the option should be discussed thoroughly with a health care provider.