Osteoporosis

In the United States today, more than 53 million people either already have osteoporosis or are at high risk due to low bone mass. Osteoporosis can occur at any age, although the risk for developing the disease increases as you get older.

A major contributor to bone loss in women during later life is the reduction in estrogen production that occurs with menopause. Estrogen is a sex hormone that plays a critical role in building and maintaining bone. Decreased estrogen, whether due to natural menopause, surgical removal of the ovaries, or chemotherapy or radiation treatments for cancer, can lead to bone loss and eventually osteoporosis. After menopause, the rate of bone loss speeds up as the amount of estrogen produced by a woman’s ovaries drops dramatically. Bone loss is most rapid in the first few years after menopause but continues into the postmenopausal years.

In men, sex hormone levels also decline after middle age, but the decline is more gradual. These declines probably also contribute to bone loss in men after around age 50.

Risk factors you can or may be able to change:

• Sex hormone deficiencies: The most common manifestation of estrogen deficiency in premenopausal women is amenorrhea, the abnormal absence of menstrual periods. Missed or irregular periods can be caused by various factors, including hormonal disorders as well as extreme levels of physical activity combined with restricted calorie intake—for example, in female marathon runners, ballet dancers, and women who spend a great deal of time and energy working out at the gym. Low estrogen levels in women after menopause and low testosterone levels in men also increase the risk of osteoporosis. Lower than normal estrogen levels in men may also play a role. Low testosterone and estrogen levels are often a cause of osteoporosis in men being treated with certain medications for prostate cancer.
• Medications: Long-term use of certain medications, including glucocorticoids and some anticonvulsants, leads to bone loss and increased risk of osteoporosis. Other drugs that may lead to bone loss include anticlotting drugs, such as heparin; drugs that suppress the immune system, such as cyclosporine; and drugs used to treat prostate cancer.
• Diet: From childhood into old age, a diet low in calcium and vitamin D can increase your risk of osteoporosis and fractures. Excessive dieting or inadequate caloric intake can also be bad for bone health. People who are very thin and do not have much body fat to cushion falls have an increased risk of fracture.

Bone may seem to be stable and unchanging, but in fact, bone is constantly being remodeled. Bone remodeling is triggered by a need for calcium in the extracellular fluid, but it also occurs in response to mechanical stresses on the bone tissue. To understand bone remodeling, and the factors that lead to pathological problems with bone, you need to know about three cell types found in bone:

Osteoblasts are bone-forming cells. They are connective tissue cells found at the surface of bone. They can be stimulated to proliferate and differentiate as osteocytes.

Osteocytes are bone cells. Osteocytes manufacture type I collagen and other substances that make up the bone extracellular matrix. Osteocytes will be found enclosed in bone.

Osteoclasts are bone-resorbing cells (“-clast” means to break; osteoclasts break down bone). They are large, multinucleate cells that form through the fusion of precursor cells. Unlike osteoblasts, which are related to fibroblasts and other connective tissue cells, osteoclasts are descended from stem cells in the bone marrow that also give rise to monocytes (macrophages).