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

Abstract

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.

Comment on: “The Utility of Thirst as a Measure of Hydration Status Following Exercise-Induced Dehydration”

“…The answer is no, in that following the recovery period, heart rate, core temperature, urine specific gravity, and thirst pleasantness had all returned to baseline levels despite the persistence of hypohydration. Slowly but assuredly then, with no health issues, and driven by the changes in blood tonicity, participants likely regained their lost water within the next 12-24 h, just in time for another training session, as previously demonstrated by the authors themselves [10] and others [11].…”

Post-Exercise Sweat Loss Estimation Accuracy of Athletes and Physically Active Adults: A Review

“…Competitive runners and endurance athletes are also apt to train more than once in a 24-h period. For these reasons our laboratories have made multiple efforts to examine ad libitum hydration behavior, fluid dynamics and practical hydration marker validity for~12 h following runs that produce significant sweat losses [13,18,37,38]. Change in absolute body mass from one training session to the next is commonly promoted in field settings as method to assess hydration status.…”

“…No additional published articles were discovered. Data from nine published investigations [10][11][12][13][14][15][16][17][18] in which sweat loss was estimated were shared by current authors through personal communication. A new spreadsheet database was created for comprehensive analysis.…”

The Hydration Sweet Spot: Importance of Aquaporins

“…42 However, during exercise, the use of body weight as a marker of hydration status and guide for fluid replacement is confounded by exercise-induced substrate utilization, metabolic water production, and variable electrolyte composition of sweat. 43 Furthermore, because body weight is not a physiologically regulated variable, runners greatly underestimate sweat water losses 44,45 and cannot subjectively gauge how much fluid they need to ingest to prevent dehydration during exercise. 44 Thus, it appears that the use of body weight as a marker of hydration status may be more useful under steady-state (non-exercise, resting) conditions and less useful as a guide for monitoring plasma osmolality or volume during exercise.…”

“…43 Furthermore, because body weight is not a physiologically regulated variable, runners greatly underestimate sweat water losses 44,45 and cannot subjectively gauge how much fluid they need to ingest to prevent dehydration during exercise. 44 Thus, it appears that the use of body weight as a marker of hydration status may be more useful under steady-state (non-exercise, resting) conditions and less useful as a guide for monitoring plasma osmolality or volume during exercise. 46 Urine: Urine color, specific gravity (USG), and osmolality (UOsm) are routinely used to assess hydration status, with a variety of cut-off values used to define "dehydration."…”