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

We evaluated the hypotheses that endurance training increases relative lipi d oxidation over a wide range of relative exercise intensities in fed and f asted states and that carbohydrate nutrition causes carbohydrate-derived fu els 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 a nd fasted states. Seven untrained (UT) men and seven category 2 and 3 US Cy cling Federation cyclists (T) exercised in the morning in random order, wit h 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 subjec ts 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(2p eak) 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) o ver dot o(2peak) when they fasted, and at 40% (V) over dot o(2peak) when fe d or fasted, but not at higher relative exercise intensities in either nutr itional state. Our results support the hypothesis that endurance training e nhances 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 a nd 75% (V) over dot o(2peak)). Because most athletes train and compete at e xercise 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.