24-h Fluid Kinetics and Perception of Sweat Losses Following a 1-h Run in a Temperate Environment

This study examined 24-h post-run hydration status and sweat loss estimation accuracy in college age runners (men = 12, women = 8) after completing a 1-h self-paced outdoor run (wet bulb globe temperature = 19.9 ± 3.0 °C). Sweat losses (1353 ± 422 mL; 1.9% ± 0.5% of body mass) were significantly greater (p < 0.001) than perceived losses (686 ± 586 mL). Cumulative fluid consumption equaled 3876 ± 1133 mL (218 ± 178 mL during) with 37% of fluid ingested lost through urine voids (1450 ± 678 mL). Fluid balance based on intake and urine production equaled +554 ± 669 mL at 12 h and +1186 ± 735 mL at 24 h. Most runners reported euhydrated (pre-run urine specific gravity (USG) = 1.018 ± 0.008) with no changes (p = 0.33) at hours 12 or 24 when both genders were included. However, USG was higher (p = 0.004) at 12 h post-run for men (1.025 ± 0.0070 vs. 1.014 ± 0.007), who consumed 171% ± 40% of sweat losses at 12 h vs. 268% ± 88% for women. Most runners do not need intervention concerning between bout hydration needs in temperate environments. However, repeated USG measurements were able to identify runners who greatly under or over consumed fluid during recovery. Practitioners can use multiple USG assessments as cheap method to detect runners who need to modify their hydration strategies and should promote assessment of sweat losses by change in body mass, as runners had poor perception of sweat losses.