Carbohydrate supplementation maintains physical performance during short-term energy deficit despite reductions in exogenous glucose oxidation

Exogenous glucose oxidation is reduced 55% during aerobic exercise after 3 days of complete starvation. Whether energy deficits more commonly experienced by athletes and military personnel similarly affect exogenous glucose oxidation and what impact this has on physical performance remains undetermined. This randomized, longitudinal parallel study aimed to assess the effects of varying magnitudes of energy deficit (DEF) on exogenous glucose oxidation and physical performance compared with energy balance (BAL). Participants consumed a 4-day BAL diet, followed by a 6-day 20% (<i>n</i> = 10), 40% (<i>n</i> = 10), or 60% (<i>n</i> = 10) DEF diet. At the end of each energy phase, participants performed 90-min of steady-state cycle ergometry (56 ± 3% V̇o_2peak) while consuming a glucose drink (80 g), followed by a time to exhaustion (TTE) performance test. Substrate oxidation (g/min) was determined by indirect calorimetry and ¹³C-glucose. Muscle glycogen (mmol/kg dry wt) and transcript accumulation were assessed in rested fasted muscle collected before exercise in each phase. Muscle glycogen was lower (<i>P</i> = 0.002) during DEF (365 ± 179) than BAL (456 ± 125), regardless of group. Transcriptional regulation of glucose uptake (<i>GLUT4</i> and <i>IRS2</i>) and glycogenolysis (<i>HKII</i> and <i>PKM</i>) were lower (<i>P</i> < 0.05) during DEF than BAL, independent of group. Regardless of group, exogenous glucose oxidation was 10% lower (<i>P</i> < 0.001) during DEF (0.38 ± 0.08) than BAL (0.42 ± 0.08). There was no evidence of a difference in TTE between BAL and DEF or between groups. In conclusion, despite modest reduction in exogenous glucose oxidative capacity during energy deficit, physical performance was similar compared with balance.<b>NEW & NOTEWORTHY</b> Short-term (6-day) energy deficit reduced exogenous glucose oxidation during exercise. Though less exogenous glucose was used for fuel, young healthy individuals appear to have a metabolic resilience to short-term periods of low energy availability, with no observed differences in the ability to take up and oxidize exogenous glucose between minimal (20%), moderate (40%), and severe (60%) energy deficits. Similar metabolic responses to carbohydrate supplementation independent of deficit severity likely contributed to sustainment of physical performance.