Exogenous glucose oxidation during endurance exercise in hypoxia

Abstract

Purpose Endurance exercise in hypoxia promotes carbohydrate (CHO) metabolism. However, detailed CHO metabolism remains unclear. The purpose of this study was to evaluate the effects of endurance exercise in moderate hypoxia on exogenous glucose oxidation at the same energy expenditure or relative exercise intensity. Methods Nine active healthy males completed three trials on different days, consisting of 30 min of running at each exercise intensity: (a) exercise at 65% of normoxic maximal oxygen uptake in normoxia [NOR, fraction of inspired oxygen (F i O 2 ) = 20.9%, 10.6 ± 0.3 km/h], (b) exercise at the same relative exercise intensity with NOR in hypoxia (HYPR, F i O 2 = 14.5%, 9.4 ± 0.3 km/h), and (c) exercise at the same absolute exercise intensity with NOR in hypoxia (HYPA, F i O 2 = 14.5%, 10.6 ± 0.3 km/h). The subjects consumed 113 C‐labeled glucose immediately before exercise, and expired gas samples were collected during exercise to determine 13 C‐excretion (calculated by 13 CO 2 / 12 CO 2 ). Results The exercise‐induced increase in blood lactate was significantly augmented in the HYPA than in the NOR and HYPR ( p = .001). HYPA involved a significantly higher respiratory exchange ratio (RER) during exercise compared with the other two trials ( p < .0001). In contrast, exogenous glucose oxidation ( 13 C‐excretion) during exercise was significantly lower in the HYPA than in the NOR ( p = .03). No significant differences were observed in blood lactate elevation, RER, or exogenous glucose oxidation between NOR and HYPR. Conclusion Endurance exercise in moderate hypoxia caused a greater exercise‐induced blood lactate elevation and RER compared with the running exercise at same absolute exercise intensity in normoxia. However, exogenous glucose oxidation ( 13 C‐excretion) during exercise was attenuated compared with the same exercise in normoxia.

Isotope tracer assessment of exogenous glucose oxidation during aerobic exercise in hypoxia

“…Our data challenge common recommendations to increase the carbohydrate intake during exercise at high altitude (>2,500 m) to fuel exercise metabolism and augment endurance capability (Koehle, Cheng, & Sporer, 2014). As such, we read with interest the recent report by Sumi et al (Sumi, Hayashi, Yatsutani, & Goto, 2020), as their findings, on the surface, appear to confirm our previous results (Margolis et al, 2019;O'Hara et al, 2017O'Hara et al, , 2019Young et al, 2018). Their randomized crossover study aimed to investigate the effects of acute hypoxia on exogenous glucose oxidation in nine unacclimatized lowlanders performing 30-min of absolute or relative intensity-matched aerobic exercise.…”

“…While these data appear confirmatory, there are methodological limitations in the work by Sumi et al (2020) that should be acknowledged. Most noteworthy, the only glucose provided by Sumi et al (2020) was the 0.5 g of oral 13 C-glucose isotope tracer before exercise. Surprisingly, no additional glucose (i.e., tracee) was provided.…”

“…As such, the tracer/tracee ( 13 CO 2 / 12 CO 2 ) ratios measured by Sumi et al (2020) reflects the oxidation of a quantitatively trivial amount of exogenous glucose. The article referenced by Sumi et al (2020) justifying their approach for measuring 13 C-excertion after only ingesting the tracer noted that the fasting 13 C-glucose breath test was a proposed clinical screening tool designed to reflect the efficiency of hepatic energy utilization (Tanaka et al, 2013). Furthermore, Sumi et al (2020) do not appear to calculate exogenous glucose oxidation (Peronnet, Rheaume, Lavoie, Hillaire-Marcel, & Massicotte, 1998).…”

“…The negligible amount of glucose provided in tracer form (i.e., our experimental placebo) expectedly yielded 0 g of oxidized exogenous glucose (Young et al, 2018). As such, the amount of exogenous glucose oxidized by Sumi et al (2020) should be essentially 0 g. If the authors had used a plasma precursor method they could have distinguished plasma glucose oxidation from total carbohydrate oxidation, with the balance between the two representing glycogen oxidation, but as stated in the manuscript, plasma 13 C-glucose enrichments were not measured.…”

“…Taking the delay between 13 CO 2 production in tissues and at the mouth into account is necessary to ensure the accurate assessment of exogenous glucose oxidation (Peronnet et al, 1998). The 30-min exercise bout used by Sumi et al (2020) was likely…”