Many people think of osteoarthritis--the most common form of arthritis--as a disease that afflicts only older adults. It's true that people under 40 rarely experience the painful, swollen joints that are its hallmark, but the condition usually develops during middle age. So now's your chance to stop it.

"Most people feel that arthritis is inevitable," says Patience White, chief public-health officer at the Arthritis Foundation. "And it is not inevitable."

Granted, there's often no clear cause of the condition, in which cartilage that cushions the ends of bones wears away, causing bone to rub against bone. Bony spurs may form around the joints, which become swollen and lose range of motion. But in someone carrying extra pounds, losing weight can help reduce the odds of developing the biggie: OA in the knees. (It can also slow progression.) It's not clear exactly what role weight plays, but it may be that the excess poundage increases pressure on the knees, speeding up the breakdown of cartilage. A study funded by the Centers for Disease Control and Prevention that was published last September estimates that a person has a nearly 1 in 2 lifetime risk of developing OA of the knee, and nearly 2 out of 3 obese Americans will end up victims. Since being overweight has also been linked to higher rates of OA of the hands, there may be another systemic effect at work, too.

Aim to keep your body mass index under the "overweight" threshold for your height, White advises. BMI is a measure of body fat based on the relationship between a person's height and weight. Someone who is 5 feet, 5 inches tall and weighs 160 pounds has a BMI of 26.6, for example; overweight begins at 25. (Normal weight range: 18.5 to 24.9.) "You also have to keep yourself physically active," she says. Exercise protects your joints by strengthening the muscles around them; it also increases range of motion, making the knee function better and resulting in less abnormal wear on the cartilage surface, White says.

Since OA often results from repetitive overuse of a joint, however, you need to pick your sport--and train--with care. "It's important that you don't do things that will cause a lot of trauma," White says. Gymnasts, for instance, often end up with OA in their wrists and hands. Weekend runners or occasional golfers who overdo to the point of injury put themselves at higher risk, too. White suggests following the government's new physical activity guidelines (which apply, too, to most people who already have arthritis). The guidelines call for a weekly 2 ½ hours of moderate physical activity or an hour and 15 minutes of vigorous activity a week. Twice-weekly strength training is a minimum requirement, too.

(c) 2009 U.S. News & World Report