Osteoporosis is a disease in which the bones become fragile. In severe osteoporosis, bone loss can be so bad that bones fracture with very little stress.

The most common bones to fracture in osteoporosis are the hip, vertebrae in the spine, forearm, upper arm, pelvis, and ribs.

Bone is composed primarily of calcium, phosphate, and a porous material called collagen. Though bone seems stable, it is constantly being formed and broken down, or re¬absorbed. In childhood, adolescence, and young adulthood, more bone is formed than is broken down.

In normal adult bones, these two processes are in balance—when one of these activities increases or decreases, so does the other—and the amount of bone remains constant.

Then, sometime be¬tween ages 35 and 45, bone starts to resorb faster than it forms, so that the amount of bone begins to decrease. In most cases, the difference is very small, less than one-half percent each year.

Women have a special problem, however. For about 8 to 10 years immediately before and after menopause, bone resorbs some 10 times faster than at younger ages. During that period, women can lose between 2 and 5 percent of their bone each year.

About 15 to 20 million people in the United States, the majority of them elderly women, have osteoporosis. People with osteoporosis suffer an estimated 1.3 million bone frac¬tures each year.