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/O₂ consumption (O₂)] 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 O₂ (O_{2 peak}) for 2 h, 60% O_{2 peak} for 1.5 h, and 80% O_{2 peak} 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% O_{2 peak}. 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% O_{2 peak} in T and at 40 and 59% O_{2 peak} in UT subjects. Training decreased (P < 0.05) mean RER values compared with UT subjects at 22% O_{2 peak} when they fasted, and at 40% O_{2 peak} 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% O_{2 peak}). However, a training effect on RER was not apparent at high relative exercise intensities (59 and 75% O_{2 peak}). Because most athletes train and compete at exercise intensities >40% maximal O₂, they will not oxidize a greater proportion of lipids compared with untrained subjects, regardless of nutritional state.

crossover concept; lipid oxidation; carbohydrate oxidation; indirect calorimetry

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