Heat Stress Drifts Cardiometabolic Parameters During Prolonged Load Carriage Exercise

Prolonged load carriage in a temperate environment leads to a reduction in carbohydrate oxidation (CHOOx) and increase in fat oxidation (FatOx) over time, though it is unclear how heat stress affects CHOOx and FatOx under such conditions. Studies examining cardiometabolic effects of heat stress are typically <45-60 min and do not include load carriage. PURPOSE: Compare cardiorespiratory and metabolic responses to 90-min of load carriage in hot (HOT: 37 °C / 20% relative humidity [RH]) and temperate (TEMP: 20 °C / 40% RH) environments. METHODS: Eight males (age 27 ± 4 y, body mass 77.8 ± 13.5 kg, V̇O2peak 49.4 ± 3.2 ml.kg-1.min-1, body fat 11.6 ± 3.2 %) completed two bouts of load carriage treadmill exercise in HOT and TEMP conditions in a randomized crossover fashion. Subjects performed 90-min of continuous loaded (30% body mass; 23.4 ± 3.7 kg) walking at 1.56 m·s-1 with incline (6.3 ± 0.9 %) set to elicit first ventilatory threshold. CHOOx, FatOx, and workload (%V̇O2peak) were determined using indirect calorimetry. Heart rate (HR) was monitored throughout exercise. All variables were analyzed using separate two-way (time [8 timepoints: Minutes 10, 25, 35, 40, 50, 65, 80, 90] x condition [TEMP, HOT]) repeated measures ANOVAs with a priori paired t-tests. RESULTS: There was an overall effect of time on CHOOx (p = 0.018) and FatOx (p < 0.001). Between first and last timepoints, CHOOx was sustained in HOT (Min 10: 2.79 ± 0.53 to Min 90: 2.57 ± 0.77 g·min-1, p = 0.36) but CHOOx reduced in TEMP (2.63 ± 0.52 to 2.10 ± 0.50 g·min-1, p = 0.009), though there were no between-condition differences. Both conditions increased FatOx from min 10 to 90 (HOT: 0.12 ± 0.14 to 0.30 ± 0.21 g·min-1, p = 0.036; TEMP: 0.15 ± 0.09 to 0.41 ± 0.21, p = 0.005). HR was higher in HOT than TEMP (p < 0.01) at every timepoint (p < 0.05) except min 10 (p = 0.07). HR drift from min 10 to 90 was greater in HOT than TEMP ((+23 ± 10 vs. +10 ± 11 bpm; p = 0.017). Independent of condition, workload (%V̇O2peak) increased from min 10 to 90 (HOT: 60.7 ± 4.4 to 64.9 ± 3.3, p < 0.01; TEMP: 58.6 ± 5.1 to 61.8 ± 4.7 %V̇O2peak, p = 0.01). CONCLUSION: There was a higher HR and HR drift in HOT compared with TEMP load carriage. Both FatOx and oxygen consumption increased over time in HOT and TEMP. CHOOx was only reduced over time in TEMP conditions while CHOOx was sustained in HOT, which may affect nutritional needs and recommendations.