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What is osteoporosis?

Bone is a living tissue that is constantly being broken down and rebuilt. When the balance between breakdown and rebuilding is disturbed - for example, by hormonal changes or dietary changes - the bone may lose some of the minerals that contribute to its density and strength. A condition of diminished bone density is called osteopenia. When a significant loss in bone density occurs, such that the bone is markedly weakened and susceptible to fracture, the condition is termed osteoporosis (porous bone).

Osteoporosis increases the risk of bone fractures, especially in the hips, spine, and wrists. Although it can affect anyone, the risk of developing osteoporosis increases with age, affects women significantly more often than men, and is most prevalent in Caucasian and Asian women. According to the National Osteoporosis Foundation (NOF), 10 million people in the United States have osteoporosis and another 34 million have low bone mass and are at risk of developing the disease. Of those who have osteoporosis, 80% are women.

Most of the people at risk for osteoporosis are not aware of it. It is called a "silent disease" because there are usually no symptoms until a person has a bone fracture. This breakage, frequently in the hip, the vertebrae of the spine, or in the wrist, can occur with very little pressure and can cause the person significant pain and protracted or permanent disability. If the fracture causes severe debility and affects the person's general health, it may be a contributing factor in their death.

Bones are primarily a combination of type-I collagen protein and calcium phosphate. The protein forms a spongy network that is "mineralized" by the addition of the calcium compound to make the bones both strong and flexible. Bone is living tissue that is slowly but continuously replaced. During a process called bone resorption, cells called osteoclasts dissolve bone on a microscopic scale and enzymes break down the collagen network. This is followed by the formation of new bone by cells called osteoblasts, which secrete osteocalcin and precursors to collagen and create a new protein framework. The framework is then mineralized to create new bone. This on-going process is called bone turnover or bone remodeling and it takes place throughout the body, normally replacing about 8-10% of the body's bone each year.

During childhood, bone formation proceeds at a faster rate than bone resorption, and bone mass increases to peak at about 30 years of age. After this peak, bone formation slows and resorption begins to outpace it, resulting in a decline in bone mass with age. An inadequate intake of calcium and vitamin D during childhood, the use of medications that contain high-dose glucocorticosteroids (although not the usual doses taken as inhalers to control asthma or nasal allergies), anorexia, inactivity, smoking, and excess alcohol consumption can all increase the risk of a person developing osteoporosis later in life. Some diseases, such as thyroid disease, Cushing's disease, rheumatoid arthritis, kidney disease, hyperparathyroidism, and vitamin D deficiency can also have an effect on bone health. Those with a strong family history of osteoporosis may also be at an increased risk of developing it themselves.

Menopausal and postmenopausal women may experience an increased rate of bone mass loss with a decrease in estrogen. Going through menopause early can exacerbate the loss. According to the NOF, some women can lose up to 20% of their bone mass in the first 5 to 7 years following menopause. Men with decreased testosterone levels are also at risk for increased bone loss.

There are two types of osteoporosis:

Primary- or age-related osteoporosis. This refers to osteoporosis developing without any apparent cause. It is much more common in women but can also be seen in men, particularly in older men. As mentioned earlier, in some women the rate of bone loss is very high right after menopause, and fractures can occur at a relatively early age, but in most women osteoporosis does not cause fractures until they are in their 60's or 70's. Changes in lifestyle, calcium, or vitamin D supplements or other medications that decrease bone loss may slow the progression of this type of osteoporosis.

Secondary-osteoporosis. This refers to bone loss due to another disease. It affects both men and women and may be due to several different disorders including rheumatoid arthritis, hyperparathyroidism, Cushing's disease, chronic kidney disease, multiple myeloma, or drugs such as anti-epileptics, glucocorticoids, or lithium. Treatment of the underlying disease or cause may slow the loss of bone density in secondary osteoporosis.

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