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BACKGROUND: Insulin resistance and hyperinsulinemia are hypothesized to promote the development of urinary stone disease. Metabolic abnormalities favoring stone formation seen with hyperinsulinemia include hypercalciuria and low urinary pH. Fasting serum insulin levels are not routinely measured in stone formers. Our institution recently began a multidisciplinary center for high risk stone patients where we have regularly included measurement of fasting insulin levels to assess its utility in the evaluation of these patients. Our study sought to determine whether insulin levels are elevated in high risk stone formers and whether they are predictive of overall stone burden.
METHODS: All patients seen in our multidisciplinary stone center with fasting insulin levels and computed tomography (CT) imaging were included. Stone burden was assessed by totaling the maximal diameter of all renal and ureteral stones seen on CT. All patients underwent standard preventive metabolic work-up, including calcium profiles and a 24 hour urine collection. Student’s t-test was utilized to determine statistical difference and correlation was evaluated with the Pearson product-moment correlation coefficient.
RESULTS: 27 patients were included in our analysis. 17/27 (62.9%) had fasting insulin levels greater than 20. A significantly higher mean stone burden was noted amongst patients with elevated fasting insulin levels compared to those with normal fasting insulin levels (16.5 mm vs. 7.4 mm, p = 0.02). 22/27 (81.5%) were classified as overweight by BMI however only 1 of these patients (3.7%) was diabetic. Mean stone burden was not significantly different amongst patients classified as overweight by BMI and those who were of normal weight (13.7 mm vs. 7.4 mm, p = 0.22). There was no correlation between BMI and fasting insulin levels (r = -0.006).
CONCLUSIONS: Fasting insulin levels are significantly elevated in non-diabetic high risk stone formers. Insulin levels appear to have a stronger predictive value than BMI for determination of stone burden. While obesity is associated with elevated stone disease risk, fasting serum insulin may offer clearer insight into the risk of significant stone disease and should therefore be considered in the work-up of patients with urinary stone disease. In the future, targeting reduction of fasting insulin levels may represent a key element of stone disease prevention.