Your bones are alive and constantly growing -- not static, like you see them drawn in books. Bones continually change throughout your life, with some bone cells dissolving and new bone cells growing back in a process called remodeling. With this lifelong turnover of bone cells, you replace most of your skeleton every 10 years.
But for people with osteoporosis -- a thinning of the bones -- bone loss outpaces the growth of new bone. Bones become porous, brittle, and prone to fracture. Look at an X-ray of a hip with normal bone density, and you see a dense matrix of bone cells. But look at a hip with osteoporosis, and you see mostly air. The bony matrix has all but dissolved, with only a few thin strands left.
Bone density is greatest in your early 20s. But as you age, you can lose bone mass from a variety of factors. Osteoporosis or its early warning sign, osteopenia, signals an imbalance in the remodeling process: Too much bone is broken down, and too little new bone is built back up. Brittle bones result, prone to fracture.
You probably know that you need calcium to build strong bones, but a low-calcium diet isn't the only culprit. There are lesser-known causes of osteoporosis. The experts now believe that a combination of causes is often to blame for bone loss.