Association between Urine Volume, Urine pH, and Urine H+ Excretion

Background: Fluid intake is the cornerstone of nephrolithiasis prevention by increasing 24-hour urine volume. Increasing urine volume could raise urine pH through a dilutional effect on urine [H+] yet not impact H+ excretion rate. We evaluated the association between urine volume, pH, and H+ excretion rate in calcium stone formers. Dietary acids and alkali absorption affect the 24-hour urine pH and modulate nephrolithiasis risk. It is unknown whether fluid intake independently affects urine pH via a dilution effect on the urine H+ excretion rate. Methods: We evaluated cross-sectional associations between 24-hour urine volume, urine pH, and urinary free H+ excretion rate in 165 adult treatment-naïve, calcium stone formers. All urine parameters were measured using laboratory derived 24-hour urine collection data. 24-hour free H+ excretion was calculated based on urine pH and urine volume. Dietary alkali was estimated by calculating GI alkali absorption (GIAA) and urine sulfate was used as an estimate of dietary acid intake. Linear regression models were used to evaluate the association between 24-hour urine volume and 1) urine pH and 2) 24-hour free urine H+ excretion rate. Models were adjusted for GIAA, urine sulfate, age, sex, BMI, urine creatinine, and urine ammonium. Results: Mean age was 51 years, 49% were female. Mean urine pH was 5.9 and mean 24-hour urine volume was 1.8 L/d. Urine volume was not associated with urine pH in the adjusted model. However, each 1 liter increase in 24-hour urine volume was associated with a 0.42 (0.33, 0.51) increase in log 24 hour H+ excretion rate in the adjusted model, (see Figure 1). Conclusion: Contrary to our hypothesis, urine volume was not associated with urine pH but was associated with higher free H+ excretion rate, perhaps due to increased urine flow. Increased H+ excretion rate may offset the expected increase in urine pH from dilution alone.Figure 1.