Q. I've read that certain types of exercise help prevent osteoporosis. What types are best? How do they prevent bone loss?
A. Osteoporosis is a disease that weakens bones and makes them more susceptible to breaks. Bones are very dynamic. Inside every bone, some cells are laying down new bone and other cells are chewing up old bone. You want these bone-building and bone-destroying forces to always remain about equal, and for most of our young and early adult lives they are. But as we get older, the bone-destroying forces become slightly stronger than the bone-building forces. That's when osteoporosis begins.
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Weight-bearing exercises and strength-training exercises can help prevent osteoporosis, by boosting the bone-building forces. Weight-bearing exercises force you to support your own body weight. Examples include walking, dancing and climbing stairs. These exercises stress your bones enough to stimulate new bone growth.
Strength (or resistance) training challenges muscle to strain against an opposing force. Resistance can be supplied by your body weight, free weights, elasticized bands or machines. These exercises can strengthen your bones as well as your muscles.
Exercise also offers indirect benefits that can help protect against fractures. Strength training increases muscle mass, which in turn enhances muscle control, strength, balance and coordination. Good balance and coordination can mean the difference between falling -- and suffering a fracture -- and staying on your feet.
Generally, higher-impact activities have a greater effect on bone than lower-impact exercises. Activities such as tennis or running build bone faster than walking or low-impact aerobics. Velocity is also a factor. Jogging or fast-paced aerobics do more to strengthen bone than a leisurely stroll.
Swimming and biking are excellent ways to keep fit, but they aren't weight-bearing, so they won't improve your bone mass or density.
Also, only bone that bears the load of the exercise will benefit. For example, walking or running protects only bones in your lower body. A well-designed strength-training program can target the sites most likely to sustain fractures from osteoporosis: bones of the hip, spine and arms.
Aim to get at least 30 minutes of weight-bearing exercise a day. It doesn't have to be 30 consecutive minutes. Two brisk 15-minute walks -- from where you park your car to your workplace, for instance -- can do the trick. Walking up and down the stairs multiple times a day, at work or at home, also contributes to the daily quota.
If you already have osteoporosis, do weight-bearing and strength-training exercises do any good? You bet they do. But if you have osteoporosis, you need to be careful about the type of exercise you do. Check with your doctor about what's appropriate.
• Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: AskDoctorK.com.