Protective Effect of Fluid Intake Although the amount of urine output is the most important risk factor of all the observed parameters, there are not many studies supporting the inference that adequate fluid intake is useful in preventing stone formation.Frank et al. compared the incidence of stones in two desert towns in Israel. In one town, residents participated in increased fluid intake as a preventive method, while another nearby town served as a control without education in this area. Living conditions were comparable between the two locations, and after 3 years of observational follow-up, it was confirmed that urine output was significantly different, as was the incidence of stones, showing that increasing urine output can prevent stone formation in hot desert areas. A long-term prospective study of idiopathic, recurrent urinary stones confirmed that patients with recurrent stones had lower urine output than patients who remained stone-free during two years of follow-up, and further differential analysis of different assessment variables revealed that changes in urine output were an independent risk factor for stone recurrence. In another prospective cohort study involving 45,619 men with no history of kidney stones, fluid intake was inversely associated with the risk of urinary calculi during a 4-year follow-up period. The only prospective randomized study of fluid intake as a prophylactic measure for urinary stone formation included 101 controls and 199 patients with first-time idiopathic stone formation. After a baseline study, stone-forming patients were randomized into two groups; the first (intervention group) was carefully instructed on how to increase fluid intake to achieve a daily urine output of no less than 2,000 ml without any change in feeding; the other group was given no instruction. The follow-up period was 5 years, with annual clinical, laboratory, and radiologic evaluations to determine the risk of urinary tract stones, recurrence rate, and mean time to recurrence. Baseline results showed that male and female patients with stones had significantly less urine volume than normal. 5-year follow-up results were significantly higher urine volume, 50% lower recurrence rate, and longer time to first recurrence in the intervention group, and the corresponding stone-forming salt saturation was significantly different in the two groups. This study confirms that urine output is a real risk factor for urinary stones and that increasing fluid intake by at least 2 l/day is the initial treatment to prevent stone recurrence.