Dietary Carbohydrates Influence the Performance Outcomes of Short-Term Heat Acclimation

Purpose Studies on heat acclimation (HA) involving high-intensity exercise report impairments in time trial (TT) performance in the heat immediately after HA. This study aimed to determine whether a high-carbohydrate (HC) diet could enhance 3.22-km run TT performance in the heat following exercise-intensive short-term HA. Methods Fourteen healthy military-age males were assigned to either an HC diet (70% kcal carbohydrate (CHO), n = 7) or lower CHO comparison (COMP, 35% kcal CHO, n = 7) to be consumed during and for 1 wk after a 6-d HA protocol. Results Baseline descriptive statistics, heat tolerance, and run performance were similar between groups (P > 0.05). Both groups acclimated, showing reduced heart rate, rectal and skin temperature, and increased sweat rate (each P ≤ 0.03) during heat stress testing 1 and 5 d after HA. The HC group demonstrated a faster TT (959 ± 103 s vs 1067 ± 172, P = 0.02, g = 0.71) than COMP 1 d post-HA. HC showed improved TT performance from baseline at 1 d (P = 0.01, g = 0.59) and 5 d post-HA (P = 0.04, g = 0.59). The HC group showed greater TT improvement at 1 d (-7.0 ± 4.9% vs -0.7 ± 4.7%, P = 0.03, g = 1.23) and 5 d post-HA (-8.4 ± 4.9% vs -2.8 ± 3.9%, P = 0.01, g = 1.18), with no group differences in HA outcomes or relative strain during TT. Conclusions Insufficient CHO intake during heavy daily exertion in unacclimated heat stress, such as in short-term HA, may affect post-HA performance outcomes unless addressed with additional CHO consumption.