Kidney stones are the cause of almost 2 million doctor's office visits and 300,000 trips to the emergency room in the U.S. every year. A new study suggests that just one hour of exercise a week can reduce the risk of getting one by one-third. Researchers found this to be true even when weight, diet, and other known risk factors were taken into account. In the past decade, studies have linked kidney stones to obesity, metabolic syndrome, and diabetes – and experts now see the problem as a systemic issue in the body, not just with the kidney. "That may be one of the reasons [why] exercise has an effect," says study leader Mathew Sorensen, a urologist at the University of Washington. "Activity leads to improvement in metabolic syndrome, which could reduce the risk of stones."

For the study, scientists tracked 85,000 women for an average of eight years each. The women had their height and weight taken at the start of the study, and they filled out surveys detailing their physical activity and their diet, which the researchers could use to calculate how much water, calories, salt, and other nutrients they were getting.

In short, people who exercised were less likely to get kidney stones. "The risk decreased a little bit if you did a little bit of activity, then a bit more if you did more," he says. The effect of exercise leveled off at the equivalent of walking three hours a week or jogging one. How intensely they worked out didn't seem to matter – just that they consistently got this relatively small amount of exercise. "It's a moderate amount of activity for a lot of reduction in risk," says Sorensen."