A large study found that exercise might help prevent kidney stones in women. Even walking for a couple hours a week can cut the risk of developing kidney stones by about one-third. The leader of the study, Dr. Mathew Sorensen of the University of Washington School of Medicine in Seattle, said, “Every little bit makes a difference,” and the intensity doesn’t matter, as just getting a minimum amount of exercise showed a reduction in the risk of developing kidney stones.

Around 9 percent of people in the United States will develop kidney stones sometime in their life. The problem is a little more common in men, but incidence has risen most rapidly among women over the last 15 years. Many women now take calcium supplements after menopause, increasing their risk of developing kidney stones. A government task force recently advised against healthy older women taking calcium supplements, saying that they don’t do much to keep bones strong but make kidney stones more likely. Obesity also raises the risk of the condition.

The study involved nearly 85,000 women 50 and older. All were examined to measure weight, height, and body mass index and filled out annual surveys on what they ate, so researchers could take into account things known to lower the risk of kidney stones. Participants also reported how much exercise they usually got.

After about eight years, 3 percent of the women had developed a kidney stone. Participants who exercised lightly had a 16 percent lower risk for kidney stones, while those with moderate amounts of exercise had a 22 percent lower risk and those who exercised most often saw a 31 percent reduction in risk. The intensity of the exercise didn’t matter, just the amount of exercise the women got each week.

Dr. Sorensen said, “We’re not asking people to run marathons. This is just a very mild to moderate additional amount of activity.” Exercise changes the way the body handles nutrients and fluids that affect the formation of kidney stones. Salt is sweated out and calcium is retained in the bones, instead of passed into the kidneys and urine where stones form. People who exercise also tend to drink water and fluids afterwards, which is another key to preventing stones.

Dr. Kevin McVary, chairman of urology at Southern Illinois University School of Medicine, said, “There’s something about exercise itself that probably produces things in your urine that prevent stone formation. It’s not just being skinny or not being fat; it’s something about the exercise that protects you.”