Just as muscles grow stronger the more you use them, a bone becomes denser when you place demands on it—certainly until your early 30s. The best bone-building exercise is resistance training—lifting free weights, using the resistance machines at the gym, even Pilates and yoga. But any kind of weight-bearing exercise—where your legs support your body—helps prevent osteoporosis (jogging and dancing count; swimming and stationary bicycling are not weight bearing and therefore are less helpful). And it's never too late to start, according to several studies. In one, postmenopausal women who lifted weights for 45 minutes twice a week for a year increased their bone mass by 1 percent, while the control group, which did no resistance training, lost 2 to 2.5 percent. Experts recommend doing weight-bearing exercise at least four times per week for 30 to 60 minutes and resistance training two or three times, long enough to work each muscle group.

Preventing Osteoporosis

• Calcium
• Drugs
• When to Get Tested