Respiratory gas-exchange ratios during graded exercise in fed and fasted trained and untrained men

We evaluated the hypotheses that endurance training increases relative lipid oxidation over a wide range of relative exercise intensities in fed and fasted states and that carbohydrate nutrition causes carbohydrate-derived fuels to predominate as energy sources during exercise. Pulmonary respiratory gas-exchange ratios [(RER) = CO2 production/O-2 consumption (V o(2))] were determined during four relative, graded exercise intensities in both fed and fasted states. Seven untrained (UT) men and seven category 2 and 3 US Cycling Federation cyclists (T) exercised in the morning in random order, with target power outputs of 20 and 40% peak (V) over dot o(2) ((V) over dot o(2peak)) for 2 h, 60% (V) over dot o(2peak) for 1.5 h and 80% (V) over dot o(2peak) for a minimum of 30 min after either a 12-h overnight fast or 3 h after a standardized breakfast. Actual metabolic responses were 22 +/- 0.33, 40 +/- 0.31, 59 +/- 0.32, and 75 +/- 0.39% (V) over dot o(2peak) T subjects showed significantly (P < 0.05) decreased RER compared with UT subjects at absolute workloads when fed and fasted. Fasting significantly decreased RER values compared with the fed state at 22, 40, and 59% (V) over dot o(2peak) in T and at 40 and 59% (V) over dot o(2peak) in UT subjects. Training decreased (P < 0.05) mean RER values compared with UT subjects at 22% (V) over dot o(2peak) when they fasted, and at 40% (V) over dot o(2peak) when fed or fasted, but not at higher relative exercise intensities in either nutritional state. Our results support the hypothesis that endurance training enhances lipid oxidation in men after a 12-h overnight fast at low relative exercise intensities (22 and 40% (V) over dot o(2peak)) However, a training effect on RER was not apparent at high relative exercise intensities (59 and 75% (V) over dot o(2peak)). Because most athletes train and compete at exercise intensities >40% maximal (V) over dot o(2), they will not oxidize a greater proportion of lipids compared with untrained subjects, regardless of nutritional state.