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    What Causes Osteoporosis? And Why?

    Think you know what causes osteoporosis? Think again -- some of the causes may surprise you.

    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.

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