Obesity's impact on stone risk found greater in women

Key Points

San Diego-Obesity not only increases the risk of nephrolithiasis but also increases the risk more in women than in men, results from analyses of inpatient hospital discharge data show.

The study was based on a review of data from the Nationwide Inpatient Sample database for 1998 to 2003. During that period, there were 181 million discharges (weighted estimate), of which 1.5% had a diagnosis of renal calculus or ureteral calculus based on ICD-9 codes (592.0 and 592.1, respectively). Obesity (ICD-9 code 278.0) was present in 5% of the sample.

The data showed that the annual prevalence rates of both nephrolithiasis and obesity increased over the study period, while the proportion of women affected by nephrolithiasis also rose. Among inpatients, the ratio of males to females approached almost a 1:1 ratio by 2003.

"Accumulating evidence suggests that metabolic disease predisposes patients to nephrolithiasis, and patients with metabolic diseases are also often obese," explained Dr. Nowfar, a resident at the University of California, San Diego, working with Roger L. Sur, MD, and Kerrin Palazzi-Churas, MPH. "Given this information and the rising prevalence of obesity in the U.S., we were interested in examining the association between obesity and stone disease.

"Our study shows obesity is a risk factor, but also that while female gender was shown to be protective from stone disease in the past, more recently, gender appears to have less benefit on the risk for nephrolithiasis in this inpatient sample. Perhaps increasing prevalence of obesity in women may account for the latter finding."

Dr. Nowfar noted that while previous research has examined the association between obesity and stone disease, the study presented based on the Nationwide Inpatient Sample represents a much larger dataset. However, despite its large size, the study has several limitations relating to the dataset.

"The Nationwide Inpatient Sample, sponsored by the U.S. Agency for Health Care Research and Quality, represents a sample of about 20% of U.S. hospital inpatient stays, and so is only a subset of the U.S. population and fails to represent the outpatient population of urinary stone disease. It depends on ICD-9 physician coding, and so it may underrepresent the true prevalence of stone disease and obesity," Dr. Nowfar said.