Miles J. Ortiz, Beverley K. Buchanan, Abaigeal G. Doody, Katherine G. Baur, Daniel A. Baur. Virginia Military Institute, Lexington, VA.

BACKGROUND: Soldiers are frequently required to carry heavy loads in challenging environments that substantially increase energy expenditures. However, few studies have investigated the influence of load carriage on substrate utilization nor how responses are impacted by hypoxia. METHODS: Healthy male subjects (n=10) performed 3 exercise tests on a treadmill consisting of the following conditions: 1) unloaded normoxic (UL: FiO2=20.93%), 2) loaded (~30 kg) normoxic (LN), and 3) loaded hypoxic simulating 3,650 m (LH: FiO2=~13%). Exercise consisted of 3 x 10 min walking (separated by 5 min rest) with stages matched with the UL condition for relative intensity (40.8 ± 3.2 %VO2max), absolute VO2 (1.7 L/min), and walking velocity (1.45±0.15 m/s). Breath-by-breath analysis via an automated metabolic system and blood sampling were used to determine metabolic responses including carbohydrate (CHO) oxidation, the relative contribution of CHO to energy expenditure (%CHO), blood glucose, and blood lactate. Data were analyzed via two-way repeated measures ANOVA with post hoc one-way ANOVA tests to identify differences in the case of significant interactions (α=0.05). RESULTS:CHO oxidation was reduced with LH (0.9 ± 0.2 g/min) versus UL (1.3 ± 0.3 g/min) and LN (1.2 ± 0.2 g/min) at the same relative intensity (p < 0.05). However, LH increased CHO oxidation at the same absolute intensity (1.7 ± 0.3 g/min) and velocity (3.2 ± 0.8 g/min) versus UL (absolute: 1.3 ± 0.3 g/min; velocity: 1.2 ± 0.3 g/min; p < 0.001) and LN (absolute: 1.4 ± 0.3 g/min; velocity: 2.2 ± 0.6 g/min; p < 0.01). CHO oxidation was also increased with LN versus UL at the same velocity (p = 0.001). With LH versus the other conditions, %CHO was increased at rest (LH: 80.5 ± 14.6%; UL: 49.9 ± 14.1%; LN: 56.2 ± 15.5%; p < 0.05) and when matched for absolute intensity (LH: 78.9 ± 13.3%; UL: 59.6 ± 12.8%; LN: 62.2 ± 10.6%; p < 0.05) and velocity (LH: 96.8 ± 11.3%; UL: 53.7 ± 13.3%; LN: 71.0 ± 12.6%; p < 0.001). Additionally, %CHO was increased with LN versus UL at the same velocity (p = 0.025). Blood glucose was increased with LH (+~10-15 mg/dL) at the same relative intensity and velocity (p < 0.05). Finally, lactate was increased with LH during exercise (+~ 0.35-5.8 mmol/L; p < 0.05) and with LN versus UL (2.4 ± 1.7 vs. 0.9 ± 0.5 mmol/L) at the same velocity (p = 0.05). CONCLUSIONS: CHO utilization is substantially increased with heavy load carriage when maintaining a consistent walking velocity, and this effect is exacerbated by hypoxia. These findings support the importance of CHO intake for soldier’s that bear heavy loads and suggest that alterations in marching pace may be warranted for maintaining CHO availability and limiting fatigue. GRANT OR FUNDING INFORMATION: This study was funded by the Jackson Hope New Directions in Research grant.

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