What Causes Osteoporosis? And Why?

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. 

As many as 10 million Americans have osteoporosis and 34 million more have low bone mass, called osteopenia, says the National Osteoporosis Foundation. Why is bone loss so common? WebMD went to the experts to find out. The causes of osteoporosis may surprise you.

Osteoporosis: The "Silent Thief" As You Age

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.

Causes of Osteoporosis: Low Estrogen in Women

What’s the most common cause of osteoporosis? "In general, it's estrogen deficiency in women," says Paul Mystkowski, MD, an endocrinologist at Virginia Mason Medical Center in Seattle and clinical faculty member of the University of Washington in Seattle. Bone loss accelerates after menopause, when older women have a quick drop in estrogen. Over time, the risk of osteoporosis and fracture increases as older women lose more bone than they replace.

Younger women who stop menstruating -- such as thin athletes or girls with anorexia -- also have compromised bone density, says the U.S. Surgeon General's latest report, "Bone Health and Osteoporosis."

Having both ovaries surgically removed, called a bilateral oophorectomy, may also cause osteoporosis and low bone density. In one study, this surgery caused a 54% increase in hip, spine, and wrist fractures in postmenopausal women.

Causes of Osteoporosis: Low Testosterone in Men

Men need both testosterone and estrogen for bone health. That's because men convert testosterone into bone-preserving estrogen. "There's a clear consensus that when you're evaluating men with osteoporosis," says Mystkowski, "you always evaluate for testosterone deficiency."