Author(s): PARKER, C., JOHNSON, M., HUNTER, K., SHARPE, G., WILLIAMS, N., Institution: NOTTINGHAM TRENT UNIVERSITY, Country: UNITED KINGDOM, Abstract-ID: 1604

INTRODUCTION:
It is increasingly common for endurance-based athletes to perform exercise in hypoxic environments. Concerningly, several studies have observed greater elevations in markers of gastrointestinal (GI) injury, endotoxemia, and GI discomfort when exercising in hypoxia compared to normoxia (1, 2). However, it must be noted that for each of these studies, participants exercised at the same absolute intensity during both normoxia and hypoxia trials. This means relative intensity may have been higher in hypoxia and thus, be the main contributor to the observed exacerbated GI injury and discomfort. Therefore, the aim of this study was to investigate the differences in GI injury (intestinal fatty acid binding protein (I-FABP)), endotoxemia (lipopolysaccharide binding protein (LBP)) and GI discomfort between normobaric hypoxia and normoxia when running at the same environment-specific relative intensity.
METHODS:
In a counterbalance design, 12 participants completed two experimental trials, one in normoxia (NORM) (FIO2=20.9%) and one in normobaric hypoxia (HYP) (FIO2=14.0%). Each experimental trial involved a 60-minute treadmill run at 85% of the respiratory compensation point (RCP) determined in both 14.0% FIO2 and 20.9% FIO2. Blood samples were obtained pre, immediately-post (post0) and 60-min post exercise (post60). The occurrence and severity of GI discomfort were assessed at post0 using a questionnaire that rated the presence of 14 symptoms.
RESULTS:
Treadmill speed at 85% of the RCP was lower in HYP (9.8 ± 0.6 km/h) than Norm (11.2 ± 0.7 km/h) (p < 0.001). Despite this, pre-post0 ∆ was greater in HYP than NORM for I-FABP (780 ± 350 vs 388 ± 317 pg/mL) (p = 0.008) and LBP (2659 ± 2569 vs 607 ± 1730 ng/mL) (p = 0.031). GI discomfort severity (10.9 ± 8.6 vs 6.1 ± 4.4) (p = 0.016) and incidence (2.77 ± 2.71 vs 1.54 ± 1.20) (p = 0.026) during exercise were greater in HYP than NORM. Specifically, symptoms associated with the lower GI tract were more severe in HYP compared to NORM (4.08 ± 4.79 vs 2.08 ± 2.14) (p = 0.039).
CONCLUSION:
We have shown for the first time that 60 min of running at the same, environment-specific relative intensity in acute normobaric hypoxia (FIO2=14.0%) significantly elevates markers of GI injury, endotoxemia and discomfort to a greater degree than when performed in normoxia (FIO2=20.9%). This suggests that exacerbations of GI injury, endotoxemia and discomfort are due to the concurrent physiological strain induced by exercise and hypoxia rather than exercise intensity alone.

References
1) Hill, Garrett W., et al. "Prolonged treadmill running in normobaric hypoxia causes gastrointestinal barrier permeability and elevates circulating levels of pro-and anti-inflammatory cytokines." Applied Physiology, Nutrition, and Metabolism 45.4 (2020): 376-386.
2) McKenna, Zachary J., et al. "Exercise in hypobaric hypoxia increases markers of intestinal injury and symptoms of gastrointestinal distress." Experimental Physiology 107.4 (2022): 326-336.