Acute Severe Hypoxia Impacts Whole-Body Fat Oxidation and Muscle Desaturation Kinetics During Exercise: Intensity...

The aim of this study was to investigate the effects of acute exposure to severe normobaric hypoxia versus normoxia on whole-body fat oxidation and muscle desaturation kinetics over a wide range of identical relative and absolute intensities.

Thirteen active men performed 1) two maximal incremental tests to determine maximal oxygen uptake ( ) in normobaric hypoxia (FiO 2 = 13.2%, 3740 m) and normoxia (FiO 2 = 20.7%, 375 m), and 2) two submaximal graded tests, after overnight fasting, to assess fat oxidation and muscle desaturation kinetics as a function of exercise intensity in each condition. Fat oxidation kinetics were measured using indirect calorimetry and characterized using a sinusoidal model. Vastus lateralis desaturation kinetics were measured using near-infrared spectroscopy and the tissue saturation index (TSI), and modeled through a double linear model.

Compared with the relative (% ) or absolute (% ) exercise intensity, absolute fat oxidation rates (g·min -1 ) were significantly higher in hypoxia than in normoxia, ranging from 55% to 85% ( P ≤ 0.023) and from 35% to 60% ( P ≤ 0.027), respectively. TSI was lower in hypoxia than in normoxia when compared at relative or absolute exercise intensity ( P ≤ 0.004), and muscle desaturation reached a maximal level only in hypoxia at high exercise intensity ( P ≤ 0.014).

Compared with normoxia, acute exposure to severe normobaric hypoxia enhanced fat oxidation pathways during moderate-to-high relative and low-to-moderate absolute exercise intensities while allowing greater and maximal muscle desaturation. These findings highlight the relevance of considering both relative and absolute intensities when comparing fat oxidation during exercise in hypoxia and normoxia and suggest that maximal muscle desaturation is not a limiting factor for fat oxidation during exercise.