Re: “High Carbohydrate Ingestion in High Altitude” by Pesta et al.

Pesta et al. (2020) questioned the conclusion from our recent article (Margolis et al., 2019) that acute hypoxia blunts oxidation of exogenous carbohydrate ingested during exercise may conflict with general recommendations for increasing carbohydrate intake to support physical performance at high altitude (HA). We agree that carbohydrate is an important fuel source in lowlanders sojourning at HA, as there is an increased reliance on carbohydrate use for energy during metabolically matched steady-state exercise compared with sea level (SL) (Griffiths et al., 2019). Increased reliance on carbohydrate for fuel also appears to stimulate a natural desire for individuals to consume sweet carbohydrate-rich foods during initial HA exposure (Karl et al., 2018).

However, the studies cited by Pesta et al. (2020) questioning the validity of our conclusions (Margolis et al., 2019) involved resting subjects, whereas our observations were made in lowlanders performing metabolically matched steady-state aerobic exercise in hypoxia and normoxia. We found that the rate of exogenous glucose oxidation during acute hypoxic exposure was reduced compared with SL. Diminished exogenous glucose oxidation appeared to be the result of reduced uptake of glucose into peripheral tissue (Margolis et al., 2019).

Lower exogenous carbohydrate oxidation during exercise with acute (<8 hours) HA exposure compared with SL in our study is in agreement with previous findings from our laboratory (Young et al., 2018) and others (Peronnet et al., 2006; O'Hara et al., 2017, 2019). Reductions in exogenous glucose oxidation with acute HA exposure occurs whether exercise is matched for absolute (Peronnet et al., 2006; Young et al., 2018) or relative (O'Hara et al., 2017, 2019) exercise intensity. Corroborating findings across studies (Peronnet et al., 2006; O'Hara et al., 2017, 2019; Young et al., 2018; Margolis et al., 2019) using different exercise models strengthens our conclusion that exogenous glucose oxidation is lower during exercise at acute HA than at SL. We did not intend to imply that increasing carbohydrate intake has no benefits for lowlanders sojourning at HA, only that ingesting carbohydrate during aerobic exercise performed during initial hours of hypoxia might not produce the ergogenic benefit typically observed at SL. Recent work from our laboratory (Bradbury et al., 2020) confirms this speculation, showing no ergogenic effect of carbohydrate supplementation on 2-mile time trial performance during acute (<8 hours) HA exposure.

It is also important to note that recent investigations demonstrating a reduction in exogenous glucose oxidation during aerobic exercise are specific to acute (<8 hours) HA exposure. Earlier work from our laboratory (Young et al., 2018) showed that reductions in exogenous glucose oxidation during metabolically matched steady-state exercise were alleviated after 21 days of HA acclimatization. The time course of this HA-mediated adjustment in exogenous glucose oxidation is presently unknown and it is reasonable to speculate that it occurs much sooner than 21 days. As such, it is important to consider that recent results from our laboratory and others are only applicable after initial exposure to HA and the time point at which exogenous glucose oxidation is improved is not known.