The effects of ambient temperature, humidity and season of year on urine composition in patients with nephrolithiasis

OBJECTIVE To understand the effects of temperature, humidity and season of year on 24-h urine composition in patients with nephrolithiasis. PATIENTS AND METHOD A retrospective review was performed of patients evaluated at four metabolic stone clinics. Multivariate linear regression models examined the relationship between mean temperature, average humidity, season of year and 24-h urine composition. Multivariate models adjusted for known risk factors for stone disease. Mean temperature and average humidity data were obtained from http://www.weatherunderground.com based on patient-provided addresses. RESULTS A total of 599 patients were included in the study, comprising 239 women and 360 men with a mean age of 53.6 years (SD 15.0). Mean temperature was 16.9 degrees C (SD 4.8, range -21.1 to 38.3 degrees C) and average humidity was 58.1% (SD 23.5, range 11 -100%). On multivariate linear regression, increasing temperature was associated with increasing urine calcium (beta = 11.3, 95% CI 2.2-20.0), super-saturation of calcium oxalate (beta = 0.6, 95% CI 0.2-0.9), super-saturation of calcium phosphate (beta = 0.14, 95% CI 0.03-0.2), and decreasing urine sodium (beta = -5.2, 95% CI -10.3 to -0.1). As seasons become warmer (i.e. from winter to autumn to spring to summer), changes were increased urine volume (beta = 0.09, 95% CI 0.01-0.2) and decreased super-saturation of calcium phosphate (beta = -0.2, 95% CI -0.3 to -0.03). There were no associations between quintile of humidity and any 24-h urine constituents. CONCLUSIONS Increasing temperature may increase stone risk by increasing urine excretion of calcium, and the super-saturation of calcium oxalate and calcium phosphate. These findings were independent of humidity and of season of year. This appears to be related to a physiological impact of temperature itself, rather than to geographic location.