Abstract
PurposePrevious research has reported inconsistent effects of hypoxia on substrate oxidation, which 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 on substrate oxidation at varying exercise intensities in normobaric hypoxia compared with normoxia.MethodsTwelve 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 min at 40%, 50% and 60% of altitude-specific dot{text{V}}O2max at 10–15% gradient with a 10 kg backpack. Indirect calorimetry was used to calculate carbohydrate and fat oxidation.ResultsThe 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% dot{text{V}}O2max (35.4 ± 12.4 vs. 50.1 ± 17.6%, p = 0.03), with a trend observed at 50% dot{text{V}}O2max (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% dot{text{V}}O2max (54.6 ± 17.8% vs. 55.1 ± 15.0%, p = 0.99).ConclusionsRelative 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.
Highlights
Disparate metabolic responses have been observed during exercise matched for relative intensities in hypoxia compared with normoxia (Young et al 1982; Braun et al 2000; Beidleman et al 2002; Lundby and Van Hall 2002; Friedmann et al 2004; Péronnet et al 2006; Katayama et al 2010; Morishima et al 2014; O’Hara et al 2017; Matu et al 2017)
It has been proposed that during exercise matched for relative intensities, the relative contribution of carbohydrate oxidation to energy expenditure is higher in hypoxia compared with normoxia when performed after breakfast consumption, but lower in hypoxia than normoxia when exercise was performed after breakfast omission (Griffiths et al 2019)
Relative exercise intensity was not significantly different between any trial at 40% (p = 0.39), or 60% V O2max (p = 0.18), a trend for an increased relative exercise intensity in hypoxia compared with normoxia after breakfast omission was observed at 50% V O2max (p = 0.06)
Summary
Disparate metabolic responses have been observed during exercise matched for relative intensities in hypoxia compared with normoxia (Young et al 1982; Braun et al 2000; Beidleman et al 2002; Lundby and Van Hall 2002; Friedmann et al 2004; Péronnet et al 2006; Katayama et al 2010; Morishima et al 2014; O’Hara et al 2017; Matu et al 2017). Increased expression of the transcription factor hypoxia inducible factor 1 alpha (HIF-1α) may upregulate the fatty acid-activated transcription factor peroxisome proliferator-activated receptor alpha (PPARα) as per the metabolic response to hypoxia (Aragones et al 2008) This response may be further stimulated by the fasted state (König et al 1999), subsequently inhibiting pyruvate dehydrogenase activity (Huang et al 2002) and enabling greater mobilisation and oxidation of fat stores (Spriet and Watt 2003)
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