Substrate oxidation and the influence of breakfast in normobaric hypoxia and normoxia

Purpose: Reported substrate oxidation responses in hypoxia are divergent, and may be due to differences in methodological design, such as pre-exercise nutritional status and exercise intensity. This study investigated the effect of breakfast consumption versus omission on substrate oxidation at varying exercise intensities in normobaric hypoxia compared with normoxia. Methods: Twelve participants rested and exercised once after breakfast consumption and once after omission in normobaric hypoxia (4300 m: FiO2 ~11.7%) and normoxia. Exercise consisted of walking for 20-minutes at 40%, 50% and 60% of altitude-specific V̇O2max at 10-15% gradient with a 10 kg backpack. Indirect calorimetry was used to calculate carbohydrate and fat oxidation. Results: The relative contribution of carbohydrate oxidation to energy expenditure was significantly reduced in hypoxia compared with normoxia during exercise after breakfast omission at 40% (22.4 ± 17.5% vs. 38.5±15.5%, p = 0.03) and 60% V̇O2max (35.4±12.4 vs. 50.1±17.6%, p = 0.03), with a trend observed at 50% V̇O2max (23.6±17.9% vs. 38.1± 17.0%, p = 0.07). The relative contribution of carbohydrate oxidation to energy expenditure was not significantly different in hypoxia compared with normoxia during exercise after breakfast consumption at 40% (42.4±15.7% vs. 48.5±13.3%, p = 0.99), 50% (43.1±11.7% vs. 47.1±14.0%, p = 0.99) and 60% V̇O2max (54.6±17.8% vs. 55.1±15.0%, p = 0.99). Conclusions: Relative carbohydrate oxidation was significantly reduced in hypoxia compared with normoxia during exercise after breakfast omission but not during exercise after breakfast consumption. This response remained consistent with increasing exercise intensities. These findings may explain some of the disparity in the literature