Osteoporosis

Osteoporosis , which is characterized by a decrease in the mass of otherwise normal bone is the most common metabolic bone disease. Normal bone is made of a hard outer shell (the cortex) and an inner network of spicules (fibers), called trabeculae, that give bone its characteristic strength. Bone mass is maintained at a progressive and then constant level until around the age of thirty-five. This maintenance is accomplished through bone remodeling, a cycle of breaking down and building up of bone. This cycle is controlled by osteoblast cells, which make bone, and osteoclast cells, which destroy bone. Beginning around age forty, the rate at which bone breaks down can exceed that at which it is built, resulting in diminished mass and a diminished amount of calcium in the bone. For women, in addition to this normal age-related bone loss, menopause and its subsequent reduction in female hormone levels (specifically estrogen ) cause a specific loss in cortical and trabecular bone. In those who develop osteoporosis, the reduction in cortical and trabecular bone can be up to 30-40 percent, resulting in fragile bones that are prone to fracture.

Several factors contribute to the development of osteoporosis. Smoking, alcohol, and a sedentary lifestyle have all been shown to increase the risk of developing the disorder. Age and gender are also contributory factors. Women who have low estrogen levels (e.g., after menopause) are more likely to develop osteoporosis than others. Also, men generally maintain a higher bone density than women, making them less susceptible to the condition. Race can also play a role. Africans and people of African descent, for example, have a naturally higher bone density than Europeans and people of European descent and are therefore less likely to develop osteoporosis. A family history of osteoporosis certainly predisposes an individual to the

condition, and research is currently underway to identify genes linked to it. Other risk factors include long-term steroid therapy, Cushing's disease, hyperparathyroidism, and hyperthyroidism.

Traditionally, low intake of calcium and vitamin D , both of which are essential to bone building and maintenance, have been associated with osteoporosis as well. However, the role of dietary calcium remains controversial. Countries in Europe and North America, where the dietary intake of calcium is adequate, still show very high rates of osteoporosis. Studies have shown that high-protein diets, like those found in Europe and North America, raise the body's calcium requirement, thereby creating a calcium deficit in some.

One of the difficulties in understanding and managing osteoporosis is that its signs and symptoms are not apparent until the late stages of the disease, and many people with the osteoporosis are not diagnosed or treated until a fracture occurs. Hip and wrist fractures are very common, and vertebral compression fractures can occur with as little stress as that from sneezing or bending. These compressions can cause chronic backaches or cause patients to seemingly "lose height" as the vertebrae progressively curve into what is known as the "dowager's hump." Fractures also occur in the ribs, pelvis, and humerus (upper arm bone). Hip fractures can be the most devastating, often leading to death or long-term disability.

The most commonly used method to diagnose osteoporosis is to measure bone mineral density using dual energy X-ray absorbitometry (DEXA scans). This test is performed routinely in people who have risk factors or a prior diagnosis of osteoporosis. Density is usually measured in the lower spine or the hip, and the procedure is noninvasive and well tolerated. Quantitative CT (computerized tomography) scans and densitometry are also used, though less commonly. Blood levels of calcium, phosphorus , and parathyroid hormone—three hormones directly involved in bone building and remodeling—are usually normal. A more recent test that measures calcium excretion in urine may prove to be a helpful way of identifying risks for osteoporosis.

Early intervention and treatment of osteoporosis can halt or slow its progress. In some cases treatment can even reverse changes in bone density due to osteoporosis at least to a certain degree. Research regarding primary prevention of osteoporosis is ongoing. Supplements of dietary calcium and vitamin D, as well as weight-bearing exercises for the upper body, have been shown to slow bone loss. The use of supplementary estrogen (hormone replacement therapy) is very controversial. While estrogen has been shown to decrease bone loss and reduce the risks of certain fractures, it may also increase the risk of certain cancers and heart disease . Drugs called bisphosphonates stop osteoclast activity, increase bone density, and decrease the risk of fracture. In addition, supplements of calcitonin, a protein naturally made by the thyroid, can inhibit bone resorption by osteoclasts. It is important to identify those who may be at risk as early as possible, so that a healthy lifestyle, including a diet high in calcium and vitamin D, as well as exercise and early screening can be instituted.

According to the National Osteoporosis Foundation, 10 million people in the United States suffer from osteoporosis, while 34 million have early signs of bone density loss that could lead to osteoporosis (as of 2003). But despite what is known about populations at risk and potential treatments for osteoporosis, some research reports that up to 40 percent of Caucasian women in the postmenopausal age group will sustain an osteoporotic fracture during the course of their lifetime (see Schnitzer). Approximately 20 percent of those women who sustain hip fractures will die within one year of the fracture, and those who survive will most likely require nursing-home care (see Andreoli). As populations around the world live longer, osteoporosis may continue to be an epidemic, and understanding how to identify, diagnose, and treat populations at risk will be of paramount importance.

SEE ALSO Aging and Nutrition ; Calcium ; Osteomalacia ; Osteopenia ; Rickets ; Vitamins, Fat-Soluble ; Women's Nutritional Issues .

Seema P. Kumar Neela Pania

Bibliography

Andreoli T. E., ed. (2001). "Osteoporosis." In Cecil Essentials of Medicine, 5th edition. Philadelphia: W. B. Saunders.

Looker A. C.; Orwell, E. S.; Johnston C. C., Jr.; et al. (1997). "Prevalence of Low Femoral Bone Density in Older U.S. Adults from NHANES III." Journal of Bone Mineral Research 12:1761–1768.

Internet Resources

National Osteoporosis Foundation. <http://www.nof.org>

Schnitzer, T. J. (2002). "Diagnosis and Treatment for Osteoporosis: Current Status and Expectations for the New Millennium." Available from http://www.medscape.com/viewprogram/605 .