When the body doesn't make new bone as quickly as it breaks old bone down.

When bones are healthy, Stronger new bone is always replacing old bone. With aging, this process slows. Bone is lost faster than it's made. Besides aging, causes include not enough calcium in the diet, smoking, not moving enough, and certain diseases and medicines. Women are most at risk.

At first, there might be no symptoms. If the condition becomes osteoporosis, spine bones called vertebrae might collapse. That can cause back pain, loss of height and a stooped posture. Other bones also might break more easily than expected.

Treatments include medicines to protect bone mass, better diet and exercise.