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FLUID DELIVERY SCHEDULE AND COMPOSITION: FLUID BALANCE, PHYSIOLOGIC STRAIN, AND SUBSTRATE USE IN THE HEAT

Abstract

A.M. Rosales, W.S. Hailes, A.N. Marks, P.S. Dodds, B.C. Ruby, FACSM

University of Montana, Missoula, MT

Hydration position stands propose exogenous volume intake considerations but remain ambiguous regarding frequency parameters. PURPOSE: To determine the effects of micro-dosing or bolus-dosing plain water or a carbohydrate-electrolyte solution (MW, BW, MCE, BCE, respectively) on fluid retention, heat stress, and carbohydrate (CHO) oxidation during exercise in the heat. METHODS: In a repeated measures cross-over design, males (n=12, 77.6±11.3 kg, VO2 peak 55.4±6.6 ml.kg-1.min-1) completed four 2-hour trials (treadmill, 1.3 m.s-1, 5% grade) in a heat chamber (33°C, 30% RH) outfitted with a 15 kg pack and standardized USFS uniform. Fluid delivery during the experimental trials was based on 100% of calculated loss from a pre-experiment familiarization trial under identical conditions. Micro-dosed fluids were provided at 22 doses.h-1 (46±11 ml.dose-1), while bolus-dosed fluids were provided as a single dose.h-1 (1005±245 ml.dose-1). CE trials delivered 62±15 g CHO.hr-1 and 878±214 mg Na+.hr-1. Nude body weight (BW) and urine volume were recorded pre, during, and post exercise. Heart rate and core temperature were recorded to determine physiologic strain index (PSI), while steady state expired air samples were collected to determine VO2 and CHO oxidation rates. Sweat rate was calculated from BW, urine output, fluid intake, and estimated respiratory water loss. A two-way repeated measures ANOVA was used to determine differences. Statistical significance was established at p<0.05. RESULTS: Total BW loss (n=11, -0.56±0.25 kg, p>0.05), cumulative urine output (n=10, 729±427 ml, p>0.05), and sweat rate (0.79±0.20 L.h-1, p>0.05) were similar across trials. PSI at hour 1 was significantly lower than hour 2 (3.6±0.7, 4.5 ±0.9, respectively, p<0.05), however there were no differences across trials. CHO oxidation was significantly higher in the CE trials when compared to the W trials (1.5±0.3, 0.8±0.2, g.min-1, respectively, p<0.05), but was not different between dosing styles of identical composition. CONCLUSION: These data demonstrate that physiological strain, sweat rate, fluid retention, and CHO oxidation during continuous work in the heat are unaffected by varied fluid delivery schedules of equal volume.

Supported by the United States Forest Service (USFS), National Technology and Development Program.

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