People who are physically active may reduce their risk of nephrolithiasis by up to nearly one-third, researchers have discovered.

Although obesity is a strong risk factor for the development of kidney stones, the roles of physical activity and caloric intake remain poorly understood, Mathew D. Sorensen, MD, of the University of Washington in Seattle, and colleagues stated in their online report in Journal of the American Society of Nephrology. The group evaluated 84,225 postmenopausal women with no history of nephrolithiasis as part of the Women’s Health Initiative Observational Study. The participants joined the study between 1993 and 1998, and were followed for a median of eight years.

After adjusting for body mass index (BMI) and other nephrolithiasis risk factors, Dr. Sorensen and his team examined the independent association of physical activity (metabolic equivalents, or METs, per week), calibrated dietary energy intake, and BMI with incident kidney stone development. The data revealed that compared with the risk incurred by inactive women, the risk of incident stones decreased by 16% in women with the lowest physical activity level. As activity in general increased, the risk of incident stones continued to decline until leveling off at a decrease of approximately 31% for activity levels of 10 METs per week or higher. Ten METs per week is the equivalent of approximately three hours of average walking (2–3 mph), four hours of light gardening, or one hour of moderate jogging (6 mph), according to an accompanying statement from the American Society of Nephrology (ASN).

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Activity intensity was not associated with the development of stones, prompting Dr. Sorensen to comment in the statement, “Even small amounts of exercise may decrease the risk of kidney stones; it does not need to be marathons, as the intensity of the exercise does not seem to matter.”

Upon analyzing dietary energy intake, the researchers found that the consumption of more than 2,200 calories per day raised the risk of incident stones by up to 42%, yet the intake of fewer than 1,800 calories per day did not protect against stone formation. Higher BMI was associated with increased risk of incident stones.

“In summary, physical activity may reduce the risk of incident kidney stones in postmenopausal women independent of caloric intake and [BMI], primarily because of the amount of activity rather than exercise intensity,” the investigators wrote. “Higher caloric intake further increases the risk of incident stones.”

While acknowledging in a related editorial that a recommendation for moderate physical activity might reasonably be added to dietary counseling for patients with stones, John C. Lieske, MD, of the Mayo Clinic in Rochester, Minn., pointed out that the new study would need to be replicated in populations other than postmenopausal women. Dr. Lieske also noted the possibility that women who exercise regularly could have other healthy habits that reduce nephrolithiasis risk.