WASHINGTON (AP) -- As baby boomers get older and fatter, they're also more likely to come down with one of the consequences of age and weight -- arthritis.

Although they can't prevent aging, boomers can reduce the risk from obesity, and diet and exercise can help them achieve it, doctors say.

Osteoarthritis, the most common form, develops as cartilage that cushions the joints wears away, leaving the ends of the bones rubbing against each other. The Arthritis Foundation says X-rays can find signs of osteoarthritis in most people over 60.

The condition commonly begins to show up between ages 40 and 60, so most boomers are "right smack in the target," said Bill Roberts, an associate professor of family medicine at the University of Minnesota and president-elect of the American College of Sports Medicine.

Extra weight on the cartilage brings extra risk of the joint disease. "The simple explanation is, you're putting more biomechanical stress on the joints," said Kevin Fontaine, an assistant professor of rheumatology at the Johns Hopkins School of Medicine.

Fontaine and his colleagues looked at survey data from the Centers for Disease Control and Prevention, and concluded that the odds of joint pain went up as weight did. People ages 60 and older had a 12 percent likelihood of pain if they were underweight but a 60 percent chance if they were very obese, according to the study in the October edition of the journal Obesity Research.

Although the CDC survey did not ask whether the pain came from arthritis, Fontaine believes it did because the people polled were in the right age range.

Losing weight can reduce the risk of developing arthritis, said Dr. David Pisetsky, director of the Duke University Arthritis Center and a medical adviser to the Arthritis Foundation.

"There is good evidence that people who are close to their ideal weight are going to have fewer problems," he said.

Pisetsky also said overweight people ought to give special consideration to physical activity because it offers protection beyond weight loss. "Many tissues just perform better when they move or bear weight," he said.

Cartilage benefits by being well-fed. It gets nutrients from fluids in the joint, and the nutrients can be squeezed into the tissue by movement.

The reduced risk of arthritis by exercising muscle comes about differently.

For instance, muscles that protect the knee, one of the most common sites for osteoarthritis, act as shock absorbers. Strengthening them relieves pressure on the joint, Dr. Nicholas DiNubile, a clinical associate professor of orthopedic surgery at the University of Pennsylvania.

Researchers can't tell yet how much activity is the right amount to reduce risk, Pisetsky said. They do know that at least one half hour a day of moderate activity such as brisk walking, as the Surgeon General advises, is good for cardiovascular health. So Pisetsky's advice is to do at least what the Surgeon General recommends, figuring that some protection against osteoarthritis of the knee should get picked up as well.

If people want to do more, possibly walking an hour and adding some weight training, they might get more benefit, but the research can't prove it, Pisetsky said.

People who already have arthritis also can gain from losing weight and keeping their joints active, although they have to be more careful about their activity.

Exercises that pound joints, such as running, are off-limits, but gliding movements are fine, DiNubile said. He recommended water aerobics designed for people with arthritis, as well as exercise bikes and elliptical trainers.

Activity can make arthritic joints hurt. But people can learn the difference between a little soreness and a pain worth worrying about, and go on to get the improvements that exercise can deliver, DiNubile said.

Copyright 2004 The Associated Press. All rights reserved.