Effect of Hypoxia on Maximal Fat Oxidation Rate During Incremental Cycling : Relationship to the Gas Exchange Threshold

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 to exercise 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 in normoxia (NORM) and hypoxia (HYPO) during exercise at intensities matched relative to the respiratory compensation point (RCP).

Methods

Seventeen recreationally active adults (5 females/12 males; age: 36.2 ± 7.4 year; body mass: 76.6 ± 13.1 kg) performed a ramp and a step incremental test, in NORM (The fraction of inspired oxygen; FiO₂ ≈ 21%) and HYPO (FiO₂ ≈ 13.5%), on separate days. The gas exchange threshold (GET), RCP, and the maximum volume of oxygen consumption (V̇O_2max) were determined from the ramp tests. The step test involved 6 constant load stages (4-8 minutes) 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_p² = 0.64). A 25% decrease in MFO in HYPO (0.25 ± 0.08g.min^-1) compared to NORM (0.35 ± 0.08g.min-1; P < 0.001; d =1.24) was observed. The percent of V̇O_2max at which MFO occurred was significantly different in HYPO (34% ± 5) compared to NORM (39% ± 7; p = 0.008, d = 0.95). In contrast, MFO happened at a similar V̇O₂ as a percent of GET and RCP (P > 0.05), however the correlation between MFO and GET (as %V̇O_2max) was not significant in either NORM (R₂ = 0.05; P > 0.05) or HYPO (R₂ = 0.04; P > 0.05).

Conclusion

MFO was markedly decreased in hypoxia during step incremental cycling in HYPO compared to NORM. Furthermore, the decrease in fat oxidation was apparent in HYPO compared to NORM at all workloads. This suggests the reduced fat oxidation was not fully accounted for by a decrease in absolute work rate alone which occurred due to normalization of relative intensity in HYPO, but an independent effect of reduced oxygen availability may affect regulation of substrate utilization. Hypoxia did not alter the relative intensity as a V̇O2 percent of GET and RCP where the MFO occurred.