The effects of environmental hypoxia on substrate utilisation during exercise: a meta-analysis

Abstract

Background: A better understanding of hypoxia-induced changes in substrate utilisation can facilitate the development of nutritional strategies for mountaineers, military personnel and athletes during exposure to altitude. However, reported metabolic responses are currently divergent. As such, this systematic review and meta-analysis aims to determine the changes in substrate utilisation during exercise in hypoxia compared with normoxia and identify study characteristics responsible for the heterogeneity in findings.

Methods: A total of six databases (PubMed, the Cochrane Library, MEDLINE, SPORTDiscus, PsychINFO, and CINAHL via EBSCOhost) were searched for published original studies, conference proceedings, abstracts, dissertations and theses. Studies were included if they evaluated respiratory exchange ratio (RER) and/or carbohydrate or fat oxidation during steady state exercise matched for relative intensities in normoxia and hypoxia (normobaric or hypobaric). A random-effects meta-analysis was performed on outcome variables. Meta-regression analysis was performed to investigate potential sources of heterogeneity.

Results: In total, 18 studies were included in the meta-analysis. There was no significant change in RER during exercise matched for relative exercise intensities in hypoxia, compared with normoxia (mean difference: 0.01, 95% CI: -0.02 to 0.05; n = 31, p = 0.45). Meta-regression analysis suggests that consumption of a pre-exercise meal (p < 0.01) and a higher exercise intensity (p = 0.04) when exposed to hypoxia may increase carbohydrate oxidation compared with normoxia.

Conclusions: Exposure to hypoxia did not induce a consistent change in the relative contribution of carbohydrate or fat to the total energy yield during exercise matched for relative intensities, compared with normoxia. The direction of these responses appears to be mediated by the consumption of a pre-exercise meal and exercise intensity.

Keywords: Altitude; Carbohydrate; Exercise; Fat; Oxidation; Substrate; Systematic review.

Fig. 1

Flow chart of study selection

Fig. 2

Forest plot of mean differences (means ± 95% CI) for studies investigating the effects of hypoxia on RER during exercise matched for relative intensities. The size of the circle represents the relative weight of the trial. CIs are represented by a horizontal line through their representative circles. The diamond quantifies the overall mean difference (means ± 95% CI). A, B, C and D refer to the different trial arms of each study. Details of which are provided in Table 1

Fig. 3

Forest plot of mean differences (means ± 95% CI) for studies investigating the effects of hypoxia on absolute carbohydrate oxidation during exercise matched for relative intensities. The size of the circle represents the relative weight of the trial. CIs are represented by a horizontal line through their representative circles. The diamond quantifies the overall mean difference (means ± 95% CI). A and B refer to the different trial arms of each study. Details of which are provided in Table 2

Fig. 4

Forest plot of mean differences (means ± 95% CI) for studies investigating the effects of hypoxia on absolute fat oxidation during exercise matched for relative intensities. The size of the circle represents the relative weight of the trial. CIs are represented by a horizontal line through their representative circles. The diamond quantifies the overall mean difference (means ± 95% CI). A and B refer to the different trial arms of each study. Details of which are provided in Table 2

Fig. 5

Assessment of risk of bias (Cochrane’s collaboration tool)