No Change in 24-Hour Hydration Status Following a Moderate Increase in Fluid Consumption

Purpose: To investigate changes in 24-hour hydration status when increasing fluid intake. Methods: Thirty-five healthy males (age 23.8 +/- 4.7 years; mass 74.0 +/- 9.4 kg) were divided into 4 treatment groups for 2 weeks of testing. Volumes of 24-hour fluid ingestion (including water from food) for weeks 1 and 2 was 35 and 40 ml/kg body mass, respectively. Each treatment group was given the same proportion of beverages in each week of testing: water only (n = 10), water C caloric cola (n = 7), water C noncaloric cola (n = 10), or water C caloric cola C noncaloric cola C orange juice (n = 8). Serum osmolality (S-osm), total body water (TBW) via bioelectrical impedance, 24-hour urine osmolality (U-osm), and volume (U-vol) were analyzed at the end of each 24-hour intervention. Results: Independent of treatment, total beverage consumption increased 22% from week 1 to 2 (1685 +/- 320 to 2054 +/- 363 ml; p < 0.001). Independent of beverage assignment, the increase in fluid consumption between weeks 1 and 2 did not change TBW (43.4 +/- 5.2 vs 43.0 +/- 4.8 kg), S-osm (292 +/- 5 vs 292 +/- 5 mOsm/kg), 24-hour U-osm (600 +/- 224 vs 571 +/- 212 mOsm/kg), or 24-hour U-vol (1569 +/- 607 vs 1580 +/- 554 ml; all p > 0.05). Conclusions: Regardless of fluid volume or beverage type consumed, measures of 24-hour hydration status did not differ, suggesting that standard measures of hydration status are not sensitive enough to detect a 22% increase in beverage consumption.