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Osteoporosis is the thinning of your bones. It affects 1 in 4 women over age 65 and 1 out of every 17 men over age 65, according to the Centers for Disease Control and Prevention. According to the American Academy of Orthopaedic Surgeons, 10 million Americans have osteoporosis and an additional 34 million Americans are at risk of developing osteopenia, which is reduced bone mass. A variety of risk factors can determine your risk for the disease. Some are preventable and some unavoidable. What causes bone thinning?
Bone is living tissue with holes inside. The inside has a honeycomb-like appearance. Bones affected by osteoporosis have larger holes and are more fragile. Understanding osteoporosis begins with understanding how bones are made. You repeatedly place demands on your bones. Because of these demands, your bone is constantly remodeling itself.
Bone remodeling occurs in two phases. First, special bone cells called "osteoclasts" break down bone. Then, other bone cells called "osteoblasts" create new bone. Osteoclasts and osteoblasts can coordinate well for most of your life. Eventually, this coordination can break down, and the osteoclasts begin to remove more bone than the osteoblasts can create.
When you’re young, your body creates a lot of bone. In your mid-20s, your bone mass is at the maximum level. After that, you start to lose bone mass slowly as your body dissolves more bone than it rebuilds.
Parathyroid hormone (PTH) is an important contributor to the bone remodeling process. High levels of PTH can activate osteoclasts and cause excessive bone breakdown. Calcium in your blood triggers the release of PTH. Low calcium levels in the blood, or hypocalcemia, can cause high levels of PTH. It can also cause your body to break down bone to make sure you have enough calcium in your blood.
You need calcium for heart health, blood clotting, and muscle function. Your body will mine your bones for calcium if you don’t have enough in your blood. Getting enough calcium throughout your life is important to prevent bone thinning. In your teens and early adult years, you’re building bones. Sufficient calcium intake at that time ensures healthy bones later on. As you get older, eating enough calcium-rich foods helps reduce the amount of bone breakdown.
Vitamin D is essential for maintaining the calcium in your bones. Vitamin D helps you absorb calcium through your intestines. Many older adults don’t get enough vitamin D. Up to 50 percent of older adults with hip fractures have very low levels of vitamin D, according to the National Institutes of Health. Without enough vitamin D, your bloodstream will not properly take up the calcium in milk, calcium supplements, or other sources. Low levels of vitamin D will also trigger a series of events that lead to activation of osteoclasts. It also leads to increased production of PTH, which creates even more osteoclasts.
Osteoporosis is more likely to affect older women, especially white and Asian women, than men. One reason for this is the impact of falling estrogen levels after menopause. A consistent estrogen level is important for maintaining the rhythm of bone remodeling. If estrogen levels decrease, this changes the levels of certain messaging chemicals that help maintain a healthy balance of bone production and breakdown. Osteoclasts then become more active without estrogen, and your body breaks down more bone.
Certain medical conditions and some medications can speed up the process of osteoporosis. This is called secondary osteoporosis. It occurs most often as a result of taking glucocorticoid steroids. Steroids like cortisol and prednisone directly slow down osteoblasts and speed up osteoclasts. They make it harder for your body to absorb calcium, and they also increase how much calcium you lose in urine.
Taking thyroid hormones can also increase your risk of bone thinning. Thyroid hormones speed up the bone remodeling process. This increase in speed results in an increased risk of imbalance between osteoblasts and osteoclasts.
Abusing alcohol, smoking, and having an eating disorder are additional risk factors for osteoporosis. These interfere with your ability to absorb necessary nutrients, such as calcium and vitamin D.
The complex interactions between PTH, calcium, and vitamin D control the balance of bone-making and bone-destroying cells. Some health conditions and medications can impact the bone remodeling process and lead to bone thinning. Maintaining the necessary levels of calcium and vitamin D is key to reducing the risk of osteoporosis.
Written by: the Healthline Editorial Team
Medically reviewed on: May 26, 2016: William A Morrison MD
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