Hydration Assessment Technique Responses Over 24-h Post-run With Low, Moderate, And High Fluid Replacement

PURPOSE: This study assessed the response of urine specific gravity (USG), urine color (UCol), thirst sensation (TS), and fluid retention over a 24-h period following runs in the heat with different recovery fluid intake volumes. METHODS: Male runners (n = 13; VO2max = 62.2 ± 9.9 ml/kg/min) completed 3 evening running sessions (WBGT = 26 ± 3 °C) resulting in 3.2 ± 0.4% loss in body mass (2.2 ± 0.3 L) followed by a standardized diet and total of 3.2 L, (LOW), 4.2 L (MOD) or 5.2 L (HIGH) of fluid intake over the next 24-h (1.6, 2.1, and 2.6 L of fluid intake before bed). All voids were collected in separate containers and assessed for USG, UCol, and volume. RESULTS: Next morning (LOW = 0.5 ± 0.2; MOD = 0.9 ± 0.2; HIGH = 1.4 ± 0.3 L) and 24-h (LOW = 1.1 ± 0.5 L; MOD = 2.3 ± 0.5; HIGH = 3.3 ± 0.6 L) urine production differed between all treatments (P < 0.05) resulting in no main effect for fluid intake treatment on absolute fluid retention at 12 h (LOW = 1.04 ± 0.23; MOD = 1.09 ± 0.23; HIGH =1.11 ± 0.34 L; P = 0.76) or 24 h (LOW = 2.04 ± 0.50; MOD = 1.9 ± 0.49; HIGH = 1.89 ± 0.55 L P = 0.67). USG, UCol, and TS increased from pre- (grand means = 1.008 ± 0.007; 2.6 ± 1.1; 3.3 ± 1.7) to first post-run voids (grand means = 1.017 ± 0.007; 5.9 ± 1.1; 6.1 ± 1.9) but did not differ between treatments at either point. Waking USG and UCol were higher (P < 0.05) for LOW (1.024 ± 0.004; 6.5 ± 0.9), versus MOD (1.015 ± 0.005; 5.7 ± 0.7), and HIGH (1.014 ± 0.005; 4.9 ± 1.1), while TS only differed between LOW (6.8 ± 1.6) and HIGH (5.6 ± 1.4). UCol did not detect differences between LOW and MOD for first post-breakfast and 24-h voids. The last void over the 24-h period differed between all treatments for USG and TS (LOW = 1.017 ± 0.009, 6.2 ± 1.6; MOD = 1.010 ± 0.007, 4.5 ± 1.4; HIGH = 1.006 ± 0.004, 3.1 ± 1.6). UCol only detected differences between LOW (4.2 ± 1.4) and HIGH (2.5 ± 0.9). MOD equaled 97 ± 13% fluid replacement for before bed time consumption and 196 ± 26% fluid replacement at 24 h. When asked to compare the fluid intake volume to what participants normally would have consumed MOD rated 56 ± 10 and 48 ± 16 on a 100 point scale. CONCLUSIONS: When food and fluid intake were tightly controlled, change in body mass was unable to detect low fluid intake. UCol and TS tracked well, but USG was most sensitive at detecting LOW versus MOD intake between running bouts for all key points of assessment. Drinking well beyond thirst (~100 and 200% of sweat losses at 12 and 24 h) is unlikely to result in improved hydration between running bouts. Sponsored by ATAGO U.S.A. Inc.