Acute Hypoxia Suppresses Exogenous Glucose Rate Of Appearance And Metabolic Clearance Rate During Steady-state Aerobic Exercise

Abstract

BACKGROUND: Previously, we observed that exogenous carbohydrate oxidation is reduced when lowlanders perform steady-state aerobic exercise after 5 h exposure to high altitude (HA). However, the underlying glucose kinetics that may contribute to the reduction in exogenous carbohydrate oxidation during steady-state aerobic exercise performed at HA have not been explored. PURPOSE: Determine glucose turnover responses to exogenous carbohydrate ingestion during metabolically-matched, steady-state aerobic exercise at HA and sea level (SL). METHODS: Using a randomized, crossover design, lowlanders (n = 8 males, mean ± SD, age: 23 ± 2 yr, body mass: 87 ± 10 kg, and VO2peak: SL 4.3 ± 0.2 L/min and HA 2.9 ± 0.2 L/min) consumed 145 g (1.8 g/min) of glucose while performing 80 min of metabolically-matched (SL: 1.66 ± 0.14 L/min V̇O2, 329 ± 28 kcal; HA: 1.59 ± 0.10 L/min V̇O2, 320 ± 19 kcal) treadmill exercise at SL (757 mmHg) and after 5 h of simulated HA (hypobaric hypoxia, 460 mmHg) exposure. Glucose rate of appearance (Ra), disappearance (Rd), and metabolic clearance rate (MCR), were determined during the last 40 min of exercise using primed, constant [6,6-2H2] glucose infusions and 13C glucose-labeled drinks. RESULTS: Exogenous glucose oxidation rate was lower (P < 0.05) at HA (0.35 ± 0.07 g/min) compared to SL (0.44 ± 0.05 g/min). Total glucose Ra was lower (P < 0.05) at HA (12.3 ± 1.5 mg/kg/min) compared to SL (13.8 ± 2.0 mg/kg/min). Exogenous glucose Ra was lower (P < 0.05) at HA (8.9 ± 1.3 mg/kg/min) compared to SL (10.9 ± 2.2 mg/kg/d), but there was no difference between endogenous glucose Ra at HA compared to SL. Glucose Rd and MCR were lower (P < 0.05) at HA (12.7 ± 1.7 mg/kg/min and 9.0 ± 1.8 mg/kg/min) compared to SL (14.3 ± 2.0 mg/kg/min and 12.1 ± 2.3 mg/kg/min). CONCLUSION: Ingesting carbohydrate during steady-state aerobic exercise performed 5 h after arriving at HA is associated with lower exogenous glucose Ra, glucose Rd, and MCR compared to SL. These data suggest that altered glucose kinetics, indicative of either a reduction in exogenous glucose absorption or release from the gut, may contribute to lower exogenous carbohydrate oxidation during exercise upon initial exposure to HA.