The Effect of Hypoxia on Energy Expenditure and Substrate Utilization Following High-Intensity Interval Training

Many studies have investigated the effects of high intensity interval training (HIIT) on energy expenditure (EE) and substrate utilization in various populations. However, few studies assessed the specific interaction between hypoxia and exercise intensity on EE and substrate utilization during the post-exercise recovery period. In particular, matching both absolute and relative intensity in both hypoxia and normoxia situations within a single study has not been attempted. Therefore, the aim of this study was to examine the interaction of HIIT (intensity matched for HYPO condition) and hypoxia on EE and substrate utilization during recovery. Eleven recreationally active adults (five females and six males. Age 31.5 ± 10.2 yr) completed a ramp incremental test to determine maximal metabolic steady state in normoxia (MMSSN) and three HIIT sessions performed once in normoxia in the heavy intensity domain (NORM_H), the same absolute workload in hypoxia, equivalent to the severe domain (HYPO; FiO₂ ≈ 0.155), and matched relative intensity in normoxia in the severe domain (NORM_S). Resting metabolic rate (RMR), fat oxidation (FATox) and carbohydrate oxidation (CHOox) were measured before, 60 min post and 8 hr post exercise. Excess post-exercise oxygen consumption (EPOC) was measured during the first 10 min immediately following exercise. There was a main effect of condition on EPOC which was higher in NORM_S in comparison to both HYPO and NORM_H (P<0.05). By 40 min post, there was no difference between conditions on post-exercise resting metabolic rate (RMR), however FATox was higher in HYPO and NORM_S (P<0.05) compared to NORM_H. We concluded that hypoxia acts as additive stress during HIIT matched for absolute workrate, which leads to elevated FATox post-exercise. However, we observed a similar result in normoxia when matched for equivalent relative intensity. This effect may not be due to hypoxia per se but acts indirectly by altering relative exercise intensity.