Published: May 5, 2013 | Updated: May 6, 2013

By Charles Bankhead , Staff Writer, MedPage Today

---

Action Points

• This study was published as an abstract and presented at a conference. These data and conclusions should be considered to be preliminary until published in a peer-reviewed journal.
• Women who exercised even minimally had a significantly lower risk of kidney stones.
• Point out that the benefit for stone disease occurred with only modest physical activity of just about any intensity.

---

 

SAN DIEGO — Women who exercised even minimally had a significantly lower risk of kidney stones, according to an analysis of a large cohort study.

Overall, physically active women had about a 30% lower risk of kidney stones compared with women who reported no exercise. The risk reduction ranged as high as 80% in one analysis, reported Mathew Sorensen, MD, of the University of Washington in Seattle, and colleagues at the American Urological Association meeting.

The benefit for stone disease kicked in with only modest physical activity of just about any intensity, he added.

“The intensity of exercises does not matter,” Sorensen said during an AUA press briefing. “There was no difference between women who performed primarily mild, moderate, or strenuous activity. The protective effect of increased activity was similar for all exercisers.”

Almost 10% of the adult population in the U.S. will develop a kidney stone at some point. Kidney stone incidence has increased 70% over the past 15 years, and women have accounted for much of the increase, he explained, adding that the increase has been attributed to the rising prevalence of weight gain, obesity, and metabolic syndrome.

The association with obesity is nothing new, he continued, as obesity has a long-established association as a risk factor for kidney stones. Obesity is thought to predispose people to stone formation by altering urinary pH and electrolytes. However, the association involves more than increased solute load resulting from excess caloric intake.

Systemic inflammation fueled by obesity and metabolic syndrome contribute to the risk of stone formation, as do impaired lipid handling and dyslipidemia. Most risk factors for cardiovascular disease also are common to nephrolithiasis.

Body mass index (BMI), a marker of an imbalance between energy intake and expenditure, also contributes to kidney stone risk. Sorensen and colleagues hypothesized that changes in energy intake and expenditure — restricted caloric intake and increased physical activity — might alter the risk.

Adjusting for BMI

To test the hypothesis, investigators analyzed data from the Women’s Health Initiative, which enrolled 93,676 women, ages 50 to 79, from 1993 to 1998 and had a median follow-up of 8 years. Information elicited at enrollment and follow-up included self-reported history of kidney stones, a food frequency questionnaire, and physical activity converted into metabolic equivalents (METs) per week.

After excluding participants with a history of stone disease or incomplete information, the final analysis included 84,225 women, 2,392 (2.8%) of whom developed kidney stones during follow-up. To adjust for bias in self-reported energy intake, investigators made adjustments in subgroup study of 544 participants in 2004 and 2005, which corrected for bias on the basis of measurements involving an energy intake biomarker.

Energy intake was further calibrated according to age, BMI, race/ethnicity, income, and physical activity.

Investigators developed two statistical models for the analysis, one of which adjusted only for BMI and the other which adjusted for activity, energy intake, and BMI.

Sorensen and colleagues performed two exploratory analyses: type and intensity of a participant’s primary form of exercise and total weekly METs and their associations with stone risk.

The participants had a mean age of 64, mean BMI of 27.1, and mean METs/week of 14.0.

“We found that 87% of the women reported some type of physical activity, so this was an active group of people,” Sorensen said.

In the model that adjusted only for BMI, being underweight (BMI <18.5 kg/m2) afforded modest protection against kidney stones (hazard ratio 0.79), and normal-weight women (BMI 18.5 to 24.9 kg/m2) served as the reference.

The BMI range for overweight (25 to 29.9 kg/m2) was associated with a 30% increase in the hazard for stone formation, rising to 62% for moderately obese women (BMI 30 to 34.9 kg/m2) and 81% for severely obese women (BMI ≥35 kg/m2).

As compared with women who reported no physical activity, women who reported any physical activity had a significantly lower (P<0.001) risk of kidney stones:

• 0.1 to 4.9 METs/week: HR 0.84
• 5 to 9.9: HR 0.78
• 10 to 19.9: HR 0.69
• 20 to 29.9: HR 0.70
• ≥30: HR 0.69

 

Adding Energy Intake to the Mix

Sorensen and colleagues also examined the association between daily energy intake in kilocalories (kcal) and stone risk in their model that incorporated physical activity, energy intake, and BMI. Using 1,800 to 1,999 kcal as the reference, they found that the HR for kidney stones increased as follows:

• 1.04 for daily energy intake of 2,000 to 2,199 kcal
• 1.26 for 2,200 to 2,499 kcal
• 1.42 for ≥2,500 kcal

 

An intake <1,800 kcal had no effect on stone risk.

“You don’t have to run a marathon to reduce the risk of kidney stones,” Sorensen said. “Mild to moderate amounts of activity decreased the risk by 16% to 31%. The amount of activity is what mattered, not the intensity of exercise.”

As an example, he said 10 METs/week would require 3 hours of average walking, 4 hours of light gardening, or 1 hour of moderate jogging.

Theories about how exercise might reduce stone risk tend to focus on changes in vitamin and mineral handling induced by physical activity, including reduced sodium excretion, increased fluid intake, decreased sympathetic tone, and improved bone density. Noting a strong association between cardiovascular disease and stone formation, Sorensen said reducing cardiac risk factors favorably affects stone risk, including hypertension, diabetes, obesity, and cholesterol.

The findings require validation in men and in people who have a history of stone formation, said AUA press briefing moderator Tomas Griebling, MD, of the University of Kansas in Kansas City. Nonetheless, the lessons learned from the study can be applied immediately to clinical practice.

“Increasing physical activity and reducing energy intake is good advice for everyone,” Griebling told MedPage Today. “More and more, we’re learning that following common-sense lifestyle practices has a favorable effect on a wide range of conditions, and now we can add kidney stones to the list.”