The Effect of Two Commercially Available Beverages on Fluid Retention During Exercise in Heat

Occupational athletes such as wildland firefighters (WLFF) endure extreme conditions leading to high levels of fluid loss. While performing wildfire suppression, it is vital to maintain hydration in order to sustain exercise in high temperatures. PURPOSE: The purpose of this study was to compare two commercially available beverages (DD [60.9 mM Na+, 3.4% CHO] vs G [18.4 mM Na+, 5.9% CHO]) on hydration maintenance during submaximal exercise in the heat. METHODS: Ten aerobically fit males (22.5± 3.9 yrs, 82.2± 10.1 kg, 53.9± 5.9 ml•kg-1•min-1 VO2 Max) completed two 90-minute heat stress trials (39º C, 30% RH) working at 50% VO2 max followed by a 30-minute rest period in a heat chamber. Forty-five minutes into each trial, 150% of sweat rate was replaced with a randomly ordered beverage (G or DD). Subjects wore standard WLFF Nomex uniforms. Rating of perceived exertion (RPE) and heart rate (HR) were recorded every fifteen minutes. Blood samples were obtained pre- and post-exercise and post-trial to measure changes in hemoglobin (mg•dL-1) and plasma volume (%). Sweat rate was calculated by change in nude body weight. Data was analyzed using analysis of variance (ANOVA). RESULTS: There was a significant effect of time on HR following exercise (142.5 ± 6.3 at 15 min, vs 175.4 ± 4.7 b•min-1 at 90 min; p\u3c0.001). RPE was significantly lower in G than DD at 15 and 30 minutes following exercise (8.0 ± 0.6 and 6.7 ± 0.3 vs 10.4 ± 1.1 and 8.1 ± 0.8; for G and DD respectively; TrtXTime p=0.015). Additionally, hemoglobin was significantly affected by time (15. 8 ± 0.4 vs 16.3 ± 0.4 g•dL-1 post-trial; p\u3c0.01). Plasma volume, USG, and urine volume (mL) were not different (p=0.4, p=0.8, p=0.5 respectively) between trials. Sweat rate and percent dehydration from mid to post trial were not different (p=0.9, p=0.8 respectively) between beverages. CONCLUSION: Neither beverage (DD [60.9mmol/L Na+, 3.4% CHO] vs G [18.4mmol/L Na+, 5.9% CHO]) differed in fluid retention following exercise in the heat, however RPE was attenuated in G vs. DD. This study was supported by the US Forest Service