Evidence justifying a high fluid intake in treatment of nephrolithiasis.

We quantitatively assessed the effect of urinary dilution on the crystallization of calcium salts. Urinary dilution was achieved in vitro (1 to 2 L/d) by addition of water to urine from six patients with renal stones and two normal subjects, and in vivo (1.023 to 2.383 L/d) by an increased ingestion of distilled water in four patients with nephrolithiasis and three normal subjects. Both forms of urinary dilution significantly reduced the urinary activity product ratio (state of saturation) of calcium phosphate (brushite), calcium oxalate, and monosodium urate. Moreover, the formation product ratio (limit of metastability or minimum supersaturation needed to elicit spontaneous nucleation) of calcium oxalate significantly increased, although that for brushite did not change significantly. Thus, there was a reduced propensity for crystallization of calcium salts. The results provide objective evidence for the beneficial role of an increased fluid intake in the management of nephrolithiasis.