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."
Causes of Osteoporosis: Other Hormone Imbalances
Several other hormones play a role in regulating your bone density, including parathyroid hormone and growth hormone. They help orchestrate how well your bones use calcium -- and when to build up and break down bone.
But too much parathyroid hormone, called hyperparathyroidism, causes calcium loss in the urine at the expense of bone, says Mystkowski. Less calcium means weaker bones. And as you age, your body produces less growth hormone, which you need to build strong bone.
Causes of Osteoporosis: Lack of Calcium
Without calcium, you can't rebuild new bone during the lifelong process of bone remodeling.
Bones are the reservoir for two minerals -- calcium and phosphorus. You need a constant level of calcium in your blood since many of your organs, especially your heart, muscles, and nerves, depend on calcium. When these organs demand calcium, they'll steal it from the mineral storehouse in your bones. Over time, as you deplete the mineral reservoir in your bones, you end up with thin, brittle bones.
Causes of Osteoporosis: Lack of Vitamin D
Too little vitamin D can lead to weak bones and increased bone loss. Active vitamin D, also called calcitriol, is more like a hormone than a vitamin, says Mystkowski. Among its many benefits, vitamin D helps your body to absorb and use calcium.
Causes of Osteoporosis: A Sedentary Lifestyle
Bones weaken if they aren't worked. Remember the early astronauts? They suffered rapid bone loss from being weightless in space. For people who are sedentary or have a condition like paralysis or muscular dystrophy, bone loss happens quickly. As a cause of osteoporosis, this one's in your hands. You can help "remodel" your bones with weight-bearing exercise, where you're putting gentle stress on bones.
Causes of Osteoporosis: Thyroid Conditions
High levels of thyroid hormone have long been linked to an increase in bone loss. "That's always been a concern of most physicians," says Mystkowski, "but if you look at the long-term bone densities of patients who are on high doses of thyroid pills, they're not dramatically different, and their fracture risk isn't dramatically different."
Still, most doctors would agree: anyone on high doses on thyroid hormone can benefit from getting regular exercise and taking enough calcium and vitamin D. These lifestyle factors are potent ways to manage your overall fracture risk, along with monitoring bone density with testing.
Causes of Osteoporosis: Smoking
Smokers suffer from lower bone density and a higher risk of fracture than non-smokers. Studies on smoking and bone health have turned up a host of other dire effects, from direct toxic effects of nicotine on bone cells to blocking the body's ability to use estrogen, calcium, and vitamin D.
Causes of Osteoporosis: Medications
Taking certain medications may lead to bone loss and an increase in bone fractures. Most common are corticosteroids, also known as cortisone, hydrocortisone, glucocortisoids, and prednisone. These drugs are used to treat asthma, rheumatoid arthritis, psoriasis, colitis, and a wide range of other conditions. Antiseizure drugs are linked to bone loss, as well.
Causes of Osteoporosis: Medical Conditions
A host of medical conditions can lead to bone loss, from genetic diseases like cystic fibrosis to digestive diseases to the tumors called multiple myeloma, which infiltrate bones with abnormal cells. Abnormal calcium excretion also contributes to bone loss. "Some people just don't trap calcium like they should," says Mystkowski, "and they excrete it through the urine at the expense of the bone."
Causes of Osteoporosis: Too Much Alcohol
Alcohol can arrest bone remodeling and increase your calcium loss. Being tipsy increases the risk of falling, and with osteoporosis, that means you're risking a fracture.
The good news in all this? Your bone health is largely in your control. Many of the causes of osteoporosis are lifestyle factors you can change -- like getting plenty of calcium, vitamin D, and weight-bearing exercise to build strong bones. If bone loss is still a problem, ask your doctor about what you can do to correct any hormone imbalances or other medical causes of bone loss.