Substrate Oxidation During Prolonged Upper and Lower-Body Exercise with Glucose Ingestion

Abstract

Substrate oxidation during 120-min upper and lower-body exercise (UBE and LBE: arm cranking vs cycling) was compared in recreational cyclists (n = 6, 67 ± 6 kg, VO2max = 45 ± 5 and 69 ± 7 mL·kg−1·min−1 for UBE and LBE respectively) and kayakers (n = 6, 75 ± 8 kg, VO2max = 51 ± 9 and 55 ± 4 mL·kg−1·min−1 for UBE and LBE respectively) at ∼50% of their maximal power output on each ergometer (cyclists: 69 ± 13 and 182 ± 22 W vs kayakers: 89 ± 10 and 159 ± 6 W, UBE and LBE respectively), with water (22 mL·kg−1) or 13C-labelled glucose ingestion (2 g·kg−1), using indirect respiratory calorimetry corrected for urea excretion in urine and sweat combined with tracer techniques. Protein oxidation was not different between cyclists and kayakers but its contribution to the energy yield was higher in UBE than in LBE in both cyclists (7.4 ± 0.2 vs 3.8 ± 0.1 %, in UBE and LBE, respectively) and kayakers (6.0 ± 0.2 vs 4.8 ± 0.1 %, in UBE and LBE, respectively) because of the lower energy expenditure during UBE than LBE. When only water was ingested, total carbohydrate (CHO) oxidation was higher during LBE than UBE in both cyclists and kayakers, also because of the higher workload sustained. The contribution of CHO oxidation to the energy yield was similar during UBE and LBE in kayakers (66.6 ± 6.6 and 68.0 ± 6.4 %, respectively) but was higher in cyclists during UBE than LBE (70.6 ± 6.7 vs 65.2 ± 5.7 %). Glucose ingestion increased total CHO oxidation during UBE and LBE especially in cyclists during UBE (cyclists: 79.6 ± 6.4 and 73.0 ± 2.1 %; kayakers: 78.4 ± 1.1 and 76.8 ± 2.1 %, in UBE and LBE, respectively). This was due to a large oxidation rate of exogenous glucose which was significantly higher in kayakers than cyclists and during LBE than UBE (0.64 ± 0.16 vs 0.71 ± 0.15 g/min in UBE and LBE, respectively, in kayakers, vs 0.51 ± 0.15 and 0.62 ± 0.14 g/min in cyclists). The contribution of exogenous glucose oxidation was also slightly higher in kayakers than in cyclists especially during LBE (kayakers: 25.7 ± 6.1 and 21.2 ± 4.6 %; cyclists: 24.3 ± 6.7 and 18.6 ± 4.0 %, in UBE and LBE respectively). Exogenous glucose ingestion significantly spared endogenous CHO: the contribution of endogenous glucose oxidation was significantly reduced to 52-55, vs 65-71 % with water ingestion. These results show that substrate selection and exogenous glucose oxidation are different during prolonged exercise at ∼50 % of the maximal workload during arm cranking and cycling, and favours CHO oxidation during arm cranking. This phenomenon is more pronounced in subjects who regularly perform LBE than in those accustomed to UBE. Supported by NSERC