Effect of a 20-day ski trek on fuel selection during prolonged exercise at low workload with ingestion of 13C-glucose.

Fuel selection was measured in five subjects (36.0 +/- 10.5 years old; 87.3 +/- 12.5 kg; mean +/- SD) during a 120-min tethered walking with ski poles (1.12 l O(2) min(-1)) with ingestion of (13)C-glucose (1.5 g kg(-1)), before and after a 20-day 415-km ski trek [physical activity level (PAL) approximately 3], using respiratory calorimetry, urea excretion, and (13)C/(12)C in expired CO(2) and in plasma glucose. Before the ski trek, protein oxidation contributed 9.7 +/- 1.6% to the energy yield (%En) while fat and carbohydrate (CHO) oxidation provided 73.5 +/- 5.5 and 16.7 +/- 6.5%En. Plasma glucose was the main source of CHO (52.9 +/- 9.5%En) with similar contributions from exogenous glucose (27.2 +/- 3.1%En), glucose from the liver (25.6 +/- 8.3%En) and muscle glycogen (20.9 +/- 4.0%En). Endogenous CHO contributed 46.6 +/- 3.9%En. Following the ski trek %En from protein, fat, CHO, exogenous glucose and endogenous CHO were not significantly modified (10.1 +/- 1.3, 15.8 +/- 6.7, 74.1 +/- 6.5, 28.7 +/- 3.0 and 45.5 +/- 7.5%En, respectively) but the %En from plasma glucose and glucose from the liver (41.1 +/- 3.6 and 12.4 +/- 4.0%En) were reduced, while that from muscle glycogen increased (33.0 +/- 4.5%En). These results show that in subjects in the fed state with glucose ingestion during exercise, CHO is the main substrate oxidized, with major contributions from both exogenous and endogenous CHO. Following a ~3-week period of prolonged low intensity exercise, the %En from protein, fat, CHO, exogenous glucose and endogenous CHO were not modified. However, the %En from glucose released from the liver was reduced (possibly due to an increased insulin sensitivity of the liver) while that from muscle glycogen was increased.