Objective: to calculate the minimum amount of water to be ingested in order to find significant differences in one-hour urine volume between euhydrated and dehydrated humans. Methods: Five participants (22.6±2.9 years old, 63.70±13.18 kg; mean±standard deviation) were evaluated following an overnight fast, on eight different non-consecutive days, using a factorial block design. For the euhydration condition (EuA, EuB, EuC and EuD), they remained seated for 45 minutes. For the dehydration condition (DeA, DeB, DeC and DeD), they exercised intermittently at 33±4° C, 65±6% relative humidity until they were dehydrated by 1% of body mass (BM). The order of treatments was randomized. Next, they ingested a water volume equivalent to 0.5% BM, 0.72% BM, 1.07% BM or 1.43% BM in 30 minutes, under both the euhydration and the dehydration conditions. Urine volumes were collected 0, 30, and 60 minutes after water ingestion. Results: baseline values were consistent among conditions (P > 0.05), and there was no difference in water intake volume between euhydration and dehydration (P > 0.05). There was a clear association between the volume of water in-take and urine volume (R2 = 0.64, R2a = 0.58; P = 0.001); in addition, this tendency was different between euhydration and dehydration (interaction P = 0.005). Finally, ingestion of water equivalent to 1.07% BM or more resulted in a 95% CI for the urine volume difference between euhydration and dehydration greater than 100 mL. Conclusion: the minimum volume of water to be ingested to detect a 100 mL difference in one-hour urine volume between euhydrated and dehydrated humans is 1.07% BM.
