A High-carbohydrate Diet Improves Running Endurance In The Heat Following Short-term Acclimation

PURPOSE: Intense exercise during heat acclimation (HA) may impair endurance performance in the heat immediately after HA, affecting available athletic potential despite HA adaptations. This study sought to determine if a high-carbohydrate (HC) diet can rescue 3.22 km run time trial (TT) performance in the heat following exercise-intensive short-term HA. METHODS: Fourteen healthy males completed a baseline heat stress test (HST) consisting of 45 min walk at 50% V̇O2max followed by a 3.22 km run TT in the heat (37 °C, 40 % relative humidity). They were assigned to a eucaloric, isonitrogenous (1.4 g·kg-1·d-1) diet consisting of either 70 %kcal carbohydrate (CHO) (HC, n = 7) or lower CHO comparison (COMP, 35 %kcal carbohydrate, n = 7) based on metabolic rate and activity level. Participants consumed their diet throughout six days of HA involving 20-30 min high-intensity interval running with 60 min isothermal exercise to sustain rectal temperature (Tre) ≥ 38.5 °C. RESULTS: There were no group differences in baseline nutritional intake, HST responses, or run time (p > 0.05). During HA and post-testing the HC group reported a significantly higher mean CHO intake (HC: 5.9 ± 0.6 g·kg-1·d-1 vs. COMP: 3.2 ± 0.4, p < 0.01) with higher fat intake in COMP (1.81 ± 0.2 g·kg-1·d-1 vs. HC: 0.53 ± 0.1, p < 0.01). There were no group differences in caloric or protein consumption. The HST one day following HA showed evidence of HA for both groups as evidenced by reduced heart rate, post-exercise Tre, and skin temp (each p < 0.01), with increased sweat rate (HC: p = 0.03, COMP: p < 0.01). However, only the HC group improved absolute and relative 3.22 km TT performance from baseline (HC: - 69 ± 51 sec / - 7.0 ± 4.9 % vs. COMP: - 5 ± 47 sec / - 0.7 ± 4.7 %, p = 0.03, g = 1.23), despite no group differences in relative strain during the TT. CONCLUSIONS: Nutritional intake, notably CHO, can significantly influence endurance performance outcomes following exercise-intensive HA and should be considered in future analyses.