Author(s): GARCIA, J., ALLENDER, P.1, BROCHERIE, F.2, MALATESTA, D.1, Institution: FRENCH INSTITUTE OF SPORT (INSEP), Country: FRANCE, Abstract-ID: 1285

INTRODUCTION:
The heterogeneity of the effects of acute hypoxia on substrate oxidation during exercise matched for relative intensity compared with normoxia (N) has been previously reported [1]. While fat oxidation kinetics over a wide range of intensities in N has already been well described [2, 3], the homologous kinetics in normobaric hypoxia (NH) and its comparison at identical relative intensities with N are unknown. Therefore, this study aimed to investigate the effect of acute NH vs. N on whole-body fat oxidation kinetics at identical relative intensities during a submaximal graded cycling test.
METHODS:
Thirteen active men (24.9±3.0 yrs) performed i) two maximal incremental cycling tests to determine maximal oxygen uptake (VO2max) in either NH [hypoxic chamber; inspired fraction of oxygen (FiO2) = 12.7%, ~4000 m] or N [FiO2 = 20.7%, ~375 m], and ii) two submaximal graded cycling tests to assess whole-body fat oxidation in each condition, using indirect calorimetry after an overnight fast and 20-min acclimation to NH. A sinusoidal model was used to characterize, with three independent parameters (dilatation, symmetry and translation), the whole-body fat oxidation kinetics and to determine Fatmax [i.e., the intensity eliciting the maximal fat oxidation (MFO)] in both conditions [2].
RESULTS:
VO2max was significantly lower (50.4±5.3 vs. 61.5±7.5 ml/kg/min, p<0.001) and Fatmax was significantly higher (63.9±6.7 vs. 57.7±4.3 %VO2max, p<0.001) in NH vs. N. MFO (0.66±0.22 vs. 0.49±0.12 g/min, p=0.005) and whole-body fat oxidation rates for exercise intensities ranging from 50 to 85% VO2max (p<0.019) were significantly higher in NH vs. N. The mean kinetics in NH was characterized by a significantly greater dilatation (widening of the curve, p=0.01) with no significant difference for symmetry (p=0.17) and translation (p=0.79) vs. N.
CONCLUSION:
Whole-body fat oxidation kinetics differed between normobaric hypoxia and normoxia when compared at the same relative exercise intensities, with greater dilatation, MFO, Fatmax and fat oxidation rates during moderate- and high-intensity exercises in normobaric hypoxia. Exercise intensity may thus play a central role in the effect of hypoxia on fat oxidation, confirming previous findings involving carbohydrate intake [4]. For the same relative moderate- to high-intensity exercise, the greater reliance on fat oxidation in normobaric hypoxia vs. normoxia seems consistent with the increased oxidation of plasma non-esterified fatty acids, representing a more important part of total fat oxidation during moderate- to high-intensity exercise. The present findings support that acute exposure to normobaric hypoxia vs. normoxia after overnight fasting enhances the metabolic pathways of fat oxidation during moderate- to high-intensity exercise.

REFERENCES:
[1] Griffiths, J Int Soc Sports Nutr, 2019
[2] Cheneviere, Med Sci Sports Exerc, 2009
[3] Jeukendrup and Wallis, Int J Sports Med, 2005
[4] O’Hara et al., Physiol Rep, 2017