Nicole Schoff, age 12, of Biloxi, Miss., for her question:

WHAT IS OSTEOPOROSIS?

Osteoporosis is a bone disorder characterized by a higher rate of bone tissue loss than of new bone tissue formation. This leads to an increasingly porous, weaker structure of the inner bone. Such bone is more easily fractured than normal bone and body weight can cause compression of the spinal vertebrae and other bones of the skeleton, with resulting back pain and other problems.

The most common form of osteoporosis, primary osteoporosis, is observed in some women in late middle age. A smaller percentage of elderly men, however, also suffer from the disorder.

Secondary osteoporosis is caused by bone disuse as a result of paralysis or other conditions including weightlessness in space. The disorder may also be caused by endocrine and nutritional disorders, including anorexia nervosa.

Osteoporosis is treated with estrogen, calcium, fluoride and exercise programs, but controversy exists over the use of some of these treatments for preventive therapy.