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

TLDR

The study tested whether endurance training increases relative lipid oxidation across exercise intensities in fed and fasted states and whether carbohydrate intake shifts fuel use toward carbohydrates during exercise. RER was measured during graded exercise at 20, 40, 60, and 80 % VO₂peak in 7 trained and 7 untrained men under fed and fasted conditions. Training increased lipid oxidation at low intensities in the fasted state but had no effect at higher intensities, and fasting lowered RER; thus athletes exercising above 40 % VO₂max will not oxidize more lipids regardless of nutrition.

Abstract

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