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AD LIBITUM WATER CONSUMPTION PREVENTS EXCESSIVE DEHYDRATION DURING SIMULATED WORK IN THE HEAT

Abstract

Annabel E. Leonard, Anne M. Mulholland, Jonathan E. Wingo, FACSM. The University of Alabama, Tuscaloosa, AL.

BACKGROUND: The National Institute for Occupational Safety and Health (NIOSH) recommends workers follow work-to-rest ratios and consume at least 237 mL of water every 15-20 minutes (≤1.4 L/h) to reduce heat injury risk. Ad libitum water intake of a smaller volume may also be effective if work-to-ratios are followed, but this has not been tested. Accordingly, this study tested the hypothesis that ad libitum room temperature water intake prevents excessive dehydration (body mass loss <2%) during simulated moderate intensity work in the heat when following a 45:15-min work:rest ratio. METHODS: Eight subjects [5 women; (mean±SD) age=25±5 y; body mass=74.8±11.6 kg; body fat=24.0±9.0%] completed one visit. Hydration status was measured upon arrival, with adequate hydration defined as urine specific gravity (USG) ≤1.020. Participants then completed 120 min of simulated moderate-intensity work in hot conditions [indoor wet bulb globe temperature=29.0±0.6 °C], utilizing the NIOSH-recommended 45:15-min work:rest ratio for the test conditions. Moderate work was achieved by 2.5 min of arm curls (4.5 kg at 20/min) and 20 min of treadmill walking (VO2=1.0-1.1 L·min-1), repeated once for a total of 45 min of work, and followed by 15 min of seated rest. Two 45:15 work:rest cycles were completed in succession. Subjects were provided an initial 500 mL of water at room temperature (21 °C) and instructed to drink at any time. Additional water was provided automatically if participants consumed all 500 mL prior to the end of the protocol. Whole body sweat loss was calculated from the difference in pre- and post-exercise nude body mass, adjusted for fluid consumed, respiratory water loss, metabolic mass loss, and urine output. RESULTS: Participants were adequately hydrated upon arrival (USG=1.008±0.005). Over 120 min of work and rest, participants drank 0.44±0.30 L of 21.4±4.4 °C water ad libitum, which did not fully replace sweat losses (-1.2±0.1 kg, P=0.001). This resulted in a body mass loss of 0.9%±0.4% (range: 0.3%-1.6%), which remained below the 2% threshold (P<0.001). CONCLUSION: Ad libitum consumption of room temperature water prevents body mass loss >2% during 120 min of simulated moderate intensity work in the heat when adhering to NIOSH work:rest guidelines. This study was funded by the Deep South Center for Occupational Health and Safety, a NIOSH Education and Research Center.

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