SAN DIEGO, California — Women can reduce their risk for kidney stones by exercising, a new study shows.

Participants enrolled in the Women’s Health Initiative reduced their risk 16% to 31%, depending on how much they exercised in the course of a week, said Mathew Sorensen, MD, from the University of Washington in Seattle.

Even moderate exercise can help, said Dr. Sorenson, who presented the findings here at the American Urological Association (AUA) 2013 Annual Scientific Meeting. “We’re not asking women to run a marathon,” he added.

Dr. Sorenson and his team reached their conclusions by analyzing data from 93,676 women aged 50 to 79 years.

For the current study, the researchers excluded 3904 women with a history of kidney stones at enrollment because these women might have altered their diets in response to this event.

They also excluded 2777 women who never answered questions about kidney stones, and they excluded 3070 who did not complete the food frequency questionnaire or who reported extremes of energy intake (<600 or >5000 kilocalories per day).

That left them with 84,225 women who had been followed for a median of 8 years. Of these, 2392 women reported a kidney stone.

We’re not asking women to run a marathon. Dr. Mathew Sorensen

In their statistical analysis, the researchers tried to measure the importance of each risk factor for kidney stones independent of the others.

They found that women with a body mass index greater than 18.5 kg/m2 had a 1.30-fold (95% confidence interval [CI], 1.17 – 1.44) to 1.81-fold (95% CI, 1.57 – 2.10) increased risk for kidney stones compared with women who had a normal BMI. The results were statistically significant (P < .001).

The relationship of BMI to kidney stones was already known, so to explore the mechanisms behind it, the researchers also compared the weekly exercise reported by the women who had kidney stones with those who did not, using as a metric the metabolic equivalent of task (MET), a measurement of the energy expended in various activities. One MET is roughly the amount of energy expended while sitting quietly.

Women who reported exercising 0.1 to 4.9 METs per week had a 16% reduced risk for kidney stones (adjusted hazard ratio [aHR], 0.84; 95% CI, 0.74 – 0.97).

This rose to 31% in women who reported 10 METs per week, and it plateaued at that level. Ten METs per week is the equivalent of 3 hours of average walking (2 – 3 mph), 4 hours of light gardening, or 1 hour of moderate jogging (6 mph).

The intensity of the exercise did not appear to matter.

High caloric intake appeared to increase the risk for kidney stones by up to 42% (aHR, 1.42; 95% CI, 1.02 – 1.98), with more calories associated with greater risk for kidney stones (P < .001).

But low dietary energy intake (<1800 kilocalories per day) did not protect against incident kidney stones (aHR, 1.03; 95% CI, 0.74 – 1.43).

The researchers also found that a high BMI was associated with an increased risk for kidney stones independent of caloric intake and physical exercise.

“We thought physical activity and calorie intake might explain the effect of BMI, but they didn’t,” Dr. Sorenson told Medscape Medical News.

He theorized that the comorbidities of overweight, such as hypertension, diabetes, and other conditions causing inflammation, might explain some of its association between BMI and kidney stones.

Dr. Sorenson said a variety of mechanisms could explain why exercise reduces the risk for kidney stones:

  • People excrete less sodium in their urine when they exercise.
  • People consume more water when they exercise.
  • Weight-bearing exercise increases calcium density, potentially affecting calcium balance.
  • Exercise may be a marker for other healthy lifestyle behaviors.
  • Exercise can reduce comorbidities that might increase the risk for kidney stones.

As for the increased risk associated with high caloric intake, Dr. Sorenson noted that women who overeat may also make poor choices in their diet, and that a deficit of micronutrients could affect their kidney risk.

Session moderator Thomas Griebling, MD, from the University of Kansas, in Lawrence, asked Dr. Sorenson whether the researchers had adjusted for the effects of hormone replacement therapy.

Dr. Sorenson responded that they had, and in fact another study in this population found an association between kidney stones and hormone replacement therapy.

The hazard ratio was also adjusted for age, race, history of diabetes, use of calcium supplement, income, and geographic region, as well as for dietary calcium and the intake of water, sodium, and animal protein.

“It’s a very nice study and an important public health message,” Dr. Griebling said.

Source: Medscape.