Author(s): HASSANEIN, Y., IBRAHIM, D., MURIAS, J., TOWNSEND, N., Institution: HAMAD BIN KHALIFA UNIVERSITY, Country: QATAR, Abstract-ID: 2231

INTRODUCTION:
The maximum fat oxidation rate (MFO) has been suggested to provide useful information regarding endurance training status in athletes, but also as an index of metabolic health in untrained populations. Training in hypoxia is a popular method used by athletes for the purpose of performance enhancement and is emerging as a therapeutic intervention to improve cardiometabolic health. It is known that the addition of hypoxia induces a shift towards carbohydrate utilization, however the few studies that examined fat oxidation in hypoxia reported equivocal results. This may be due to differences in the methods used to normalize relative intensity. The aim of this study was to evaluate MFO matched intensity relative to the respiratory compensation point (RCP) in both normoxia (NORM) and hypoxia (HYPO).
METHODS:
Seventeen recreationally active adults (5F/12M; age: 36.2±7.4 yr; weight: 76.6±13.1 kg) performed a ramp and a step test, in NORM (FiO2≈21%) and HYPO (FiO2≈13.5%), on separate days. The gas exchange threshold (GET), RCP and VO2peak were determined from the ramp tests. The step test involved 6 constant load stages (4-8 min) matched for intensity relative to the RCP in each condition. Indirect calorimetry was used to estimate MFO rate during the step test. Each participant’s diet, fasting hours, and testing time were consistent between conditions. Paired t-tests and a 2-way ANOVA were used to examine differences between NORM and HYPO. Pearson correlation was used to assess the relationship between MFO and GET.
RESULTS:
Fat oxidation rate was decreased in HYPO across all stages (main effect of condition: P<0.001; η_p^2=0.64). Subsequently a 25% decrease in MFO in HYPO (0.26 ±0.08g.min-1) compared to NORM (0.35±0.08g.min-1; P<0.001; d=1.24) was observed. %VO2peak where MFO occurred was similar in HYPO (39±21%) and NORM (39±21%; p=0.97), however the correlation between MFO and GET (as %VO2peak) was not significant in either NORM (r = 0.3; P>0.05) or HYPO (r = 0.07; P>0.05).
CONCLUSION:
MFO was markedly decreased in hypoxia during step incremental cycling where each workload was normalized to RCP. Furthermore, the decrease in fat oxidation was apparent at all workloads and was greater than expected from the lower absolute exercise intensity. This suggests the reduced fat oxidation was not fully accounted for by a decrease in absolute workrate alone, but there was an independent effect of reduced oxygen availability. Hypoxia did not alter the relative intensity, as a percentage of VO2peak where the MFO occurred.