(FORTUNE Magazine) – "Break a leg" may be good luck in the theater but not when it comes to osteoporosis. The disease causes bones to become more porous, gradually making them brittle--and it causes 1.5 million fractures each year. Half of all women over age 50 experience a fracture due to osteoporosis, and one-fifth of the people who break a hip die within a year.

Building a dense bone mass is the key to prevention. If you have more bone when you're young, you'll have more when you're old, so it's never too early to start. (Teenagers, girls in particular, should be as active as possible.) It can be done through weight-bearing exercise--walking as opposed to swimming. Getting enough calcium and vitamin D is also important. Milk, yogurt, cheese, fish, shellfish, fortified orange juice, and green vegetables are good calcium sources, as are calcium supplements. Cutting down on caffeine may preserve bone in some people.

We build bone until the mid-30s, after which it's slowly downhill. Once bones are brittle, a fracture can occur, most commonly in the hip, wrist, or spine. Women have lower bone density than men, and at menopause there's an accelerated phase of bone loss that lasts five to seven years. Thus, it's no surprise that women are affected by osteoporosis and experience fractures more often than men. It does occur in men, though--about one-third develop it by 75--and it's becoming more common.

Factors that increase the risk of osteoporosis include race (Caucasians and Asians are more susceptible), family history of osteoporosis, smoking, excess alcohol use, physical inactivity, low body weight, and medical conditions such as Cushing's disease. Certain drugs can also predispose to osteoporosis, including corticosteroids, seizure medications, and excess thyroid hormone.

Screening should be considered for women over the age of 65, those over 50 who have at least one risk factor, and those who've experienced early menopause. (No recommendations exist for men, but anyone with risk factors may want to consider it.) While ultrasound can measure bone density in the heel, a more accurate test is dual-energy X-ray absorptiometry (DEXA). Neither test is invasive or painful.

Once osteoporosis is diagnosed, there are a number of treatment options. You should perform whatever weight-bearing exercises you can manage and minimize other risk factors, such as smoking. Calcium and vitamin D supplements are usually a good idea. It matters less which type of calcium you use than whether you take it regularly; I urge patients to take two or three calcium carbonate antacids daily, along with dietary sources of calcium.

As for prescription medications, estrogen has been proven to increase bone density and is a first-line agent for menopausal women. Fosamax, Actonel, and Didronel are other medications that will increase bone density and decrease the risk of fractures. Additional options: a selective estrogen receptor modulator (SERM), such as Evista, and calcitonin nasal spray.

For the elderly, it's important to decrease the risk of falls. Lifting weights improves strength and balance. Adding railings to stairways, wearing good shoes, and switching to stronger glasses can also help.

For more information on this topic: Go to mayoclinic.com. Mayo Clinic offers Executive Health programs at Mayo Clinics in Jacksonville, Fla.; Rochester, Minn.; and Scottsdale, Ariz.