Running in the heat similarly reduces lipid oxidation and peak oxygen consumption in trained runners and inactive individuals

This study compared oxygen consumption and substrate oxidation while exercising in hot and temperate conditions in individuals with different physical activity status (i.e., inactive individuals vs. trained runners). 10 inactive individuals (IA: 26 ± 6 y; 79.1 ± 14.1 kg; 40.7 ± 5.1 ml·kg^-1·min^-1) and 10 trained runners (TR: 25 ± 6 y; 69.5 ± 9.1 kg; 63.1 ± 5.1 ml·kg^-1·min^-1) completed two incremental exercise tests (4 min stages) until exhaustion in temperate (TEMP: 18.7 ± 0.1 °C; 43.2 ± 4.1% relative humidity) and hot (HOT: 34.4 ± 0.2 °C and 42.6 ± 1.6% relative humidity) conditions. Expired gas and blood lactate concentrations were measured at the end of each stage. Peak oxygen consumption similarly decreased in HOT compared to TEMP for IA and TR (-13.2 ± 4.5% vs. -15.2 ± 7%; P=0.571; ES=0.25). In HOT compared to TEMP, lipid oxidation, from 30 to 70% of V̇O_2peak, was reduced for both groups (IA: P=0.023, ES=0.43; TR: P<0.001, ES=0.72) while carbohydrate oxidation was increased for TR ( P=0.011; ES=0.45) but not for IA ( P=0.268; ES=0.21). Core temperature was different between conditions for TR (higher in HOT, P=0.017; ES=0.66) but not for IA (P=0.901; ES=0.25). Despite reduced physiological capacities in IA, both populations demonstrated reductions in lipid utilisation and peak oxygen consumption in hot compared to temperate conditions. However, the increased carbohydrate oxidation in HOT for TR were not observed in IA, potentially explained by lower thermal strain.