Exogenous glucose oxidation during endurance exercise in hypoxia

Effects of Standing after a Meal on Glucose Metabolism and Energy Expenditure

In the past decade, university students have become more sedentary. A sedentary lifestyle is associated with an increased risk of obesity and cardiovascular disease. Methods that decrease sedentary lifestyles, such as the use of standing desks to increase physical activity, have been extensively examined. However, the effects of postprandial standing and sitting on energy metabolism have not yet been compared. Therefore, the present study investigated the effects of standing after a meal on energy expenditure and glucose metabolism. Ten males participated in the present study. The experiment was initiated with 300 g of rice ingested as a carbohydrate load. The subjects maintained a standing or sitting position for 120 min after the meal. Energy expenditure was calculated from VO2 and VCO2 using the indirect calorimetry method. Glucose metabolism was assessed by measuring blood glucose levels and the exogenous glucose metabolic rate. Energy expenditure through standing after eating was approximately 0.16 ± 0.08 kcal/min higher than that through sitting. Blood glucose dynamics did not significantly differ between the standing and sitting positions. Furthermore, no significant differences were observed in the dynamics of the exogenous glucose metabolic rate between the standing and sitting positions. Standing for 2 h after a meal increased energy expenditure by 10.7 ± 4.6% without affecting glucose metabolism.

Exogenous glucose oxidation during endurance exercise under low energy availability

The present study was conducted to determine the effect of endurance exercise under low energy availability (EA) on exogenous glucose oxidation during endurance exercise. Ten active males (21.4 ± 0.6 years, 170.4 ± 1.4 cm, 62.4 ± 1.5 kg, 21.5 ± 0.4 kg/m²) completed two trials, consisting of two consecutive days (days 1 and 2) of endurance training under low EA (19.9 ± 0.2 kcal/kg fat free mass [FFM]/day, LEA trial) or normal EA (46.4 ± 0.1 kcal/kg FFM/day, NEA trial). The order of these two trials was randomized with at least a 1-week interval between trials. As an endurance training, participants performed 60 min of treadmill running at 70% of maximal oxygen uptake (V˙O2max) during two consecutive days (on days 1 and 2). On day 1, the endurance training was performed with consumed individually manipulated meals. During the endurance exercise on day 2, exogenous glucose oxidation was evaluated using ¹³C-labeled glucose, and respiratory gas samples were collected. In addition, blood glucose and lactate concentrations were measured immediately after exercise on day 2. Body composition, blood parameters, and resting respiratory gas variables were evaluated under overnight fasting on days 1 and 2. Body weight was significantly reduced in the LEA trial on day2 (day1: 61.8 ± 1.4 kg, day 2: 61.3 ± 1.4 kg, P < 0.001). There were no significant differences between trials in ¹³C excretion (P = 0.33) and area under the curve during the 60 min of exercise (LEA trial: 40.4 ± 3.1 mmol•60min, NEA trial: 40.4 ± 3.1 mmol•60min, P = 0.99). However, the respiratory exchange ratio (RER, LEA trial: 0.88 ± 0.01, NEA trial: 0.90 ± 0.01) and carbohydrate oxidation (LEA trial: 120.1 ± 8.8 g, NEA trial: 136.8 ± 8.6 g) during endurance exercise showed significantly lower values in the LEA trial than in the NEA trial (P = 0.01 for RER and carbohydrate oxidation). Serum insulin and total ketone body concentrations were significantly changed after a day of endurance training under low EA (P = 0.04 for insulin, P < 0.01 for total ketone). In conclusion, low EA during endurance exercise reduced systemic carbohydrate oxidation; however, exogenous glucose oxidation (evaluated by ¹³C excretion) remained unchanged during exercise under low EA.

Exogenous glucose oxidation during endurance exercise in hypoxia

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_iO₂) = 20.9%, 10.6 ± 0.3 km/h], (b) exercise at the same relative exercise intensity with NOR in hypoxia (HYPR, F_iO₂ = 14.5%, 9.4 ± 0.3 km/h), and (c) exercise at the same absolute exercise intensity with NOR in hypoxia (HYPA, F_iO₂ = 14.5%, 10.6 ± 0.3 km/h). The subjects consumed ¹¹³C‐labeled glucose immediately before exercise, and expired gas samples were collected during exercise to determine ¹³C‐excretion (calculated by ¹³CO₂/¹²CO₂).

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 (¹³C‐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 (¹³C‐excretion) during exercise was attenuated compared with the same exercise in normoxia.