Humidity Increases Carbohydrate Oxidation In Females During Walking Under Load Carriage In A Hot Environment

Female metabolic response to load carriage during environmental stress is an evolving field. Load carriage in the heat intensifies endogenous carbohydrate oxidation (CHOox) and reduces fat oxidation (Fatox) during exercise compared to thermoneutral conditions, particularly when core body temperature (Tc) is increased. However, it is unclear how humidity affects CHOox and Fatox in females during equivalent exercise and hot ambient temperatures. PURPOSE: To compare Tc, CHOox, Fatox, and oxygen uptake (V̇O2) in females between hot dry (HD; 40 °C, 20% RH) and hot humid (HH; 40 °C, 60% RH) environmental conditions during loaded walking at two different speeds. METHODS: Ten females (age 25 ± 5 y, body mass 65.8 ± 13.7 kg, V̇O2max 41.0 ± 2.8 ml·kg-1·min-1) walked for four min at a moderate (1.34 m·s-1) and fast (1.70 m·s-1) speed on a level treadmill while carrying a backpack loaded to 33% body mass (21.5 ± 4.6 kg) in both HD and HH environments. Gas exchange, CHOox, and Fatox were assessed via indirect calorimetry and averaged over the final two minutes of both walking speeds; Tc was recorded upon speed stage completion. Paired t-tests were used to compare between conditions. RESULTS: Compared to HD, HH conditions provoked higher Tc at both walking speeds (1.34 m·s-1: 38.3 °C ± 0.2 vs. 37.7 ± 0.2, p < 0.001; 1.70 m·s-1: 38.7 ± 0.2 vs. 37.9 ± 0.2, p < 0.001). CHOox was higher in HH compared to HD at 1.34 m·s-1 (0.70 ± 0.27 g·min-1 vs. 0.57 ± 0.19, p = 0.015) as well as at 1.70 m·s-1 (1.85 ± 0.55 vs. 1.60 ± 0.35, p = 0.039), respectively. Conversely, Fatox was higher in HD compared to HH at 1.34 m·s-1 (0.36 ± 0.08 g·min-1 vs. 0.32 ± 0.09, p = 0.038) and at 1.70 m·s-1 (0.25 ± 0.09 g·min-1 vs. 0.16 ± 0.11, p = 0.028), respectively. There were no differences in relative V̇O2 between conditions at either speed (1.34 m·s-1: p = 0.15, 1.70 m·s-1: p = 0.43). CONCLUSION: Higher humidity during heat stress increased carbohydrate oxidation in females during loaded walking at both moderate and fast walking speeds compared to a lower humidity hot environment. These metabolic changes were observed in tandem with higher core body temperature in the humid condition regardless of oxygen uptake indicating a thermal strain-induced metabolic shift in females. Funding provided through the Medical Technology Enterprise Consortium (#W81XWH-22-9-0014)