Acute Hypoxia Decreases Maximum Fat Oxidation Rate During Step Incremental Exercise Normalized to Respiratory Compensation Point.

Whether fat oxidation (FATox) is altered during exercise in hypoxia remains equivocal due to differences in experimental protocols. Furthermore, to date no investigation has reported the effect of hypoxia on maximal fat oxidation rate (MFO). Therefore, the aim of this study was to assess substrate utilization in normoxia and hypoxia and determine MFO. Seventeen active adults (12 M/5F) performed ramp and step incremental testing in normoxia (FiO2 = 0.209; NORM) and normobaric hypoxia (FiO2 = 0.135; HYPO). Respiratory compensation point (RCP) determined from ramp testing was used to normalize relative intensity across 6 constant workrate steps in the moderate and heavy domain. Indirect calorimetry was used to measure cardiorespiratory responses and estimate substrate utilization and MFO. Linear mixed modeling was used to compare measurements in NORM and HYPO, where intensity was expressed as a function of absolute or relative workrate. Cardiorespiratory responses to exercise were similar in NORM and HYPO when the workrate was expressed as a function of relative intensity. FATox was decreased across all stages in HYPO (p < 0.001), which was associated with a 22% decrease in MFO (HYPO: 0.26 ± 0.07 g·min-1, NORM: 0.34 ± 0.07 g·min-1; p < 0.001, d = 1.16). MFO occurred at a similar percentage of V ˙ $\dot{\mathrm{V}}$ O2max in both NORM (38 ± 8%) and HYPO (38 ± 8%; p = 0.89, d = 0.04). MFO was decreased in HYPO regardless of whether the workrate was expressed as a function of relative or absolute intensity. This suggests that hypoxia may exert a direct effect on regulation of fuel selection during exercise, independent of the reduced absolute workrate when normalizing relative intensity to RCP.