Introduction 28 38 28 41 45 19 36 28 41 45 28 41 45 42 44 47 9 10 48 11 9 5 9 13 17 24 25 33 13 2 2 Materials and methods Subjects 1 Table 1 n Characteristics   Age (years) 23 ± 3 Height (m) 1.83 ± 0.04 Body mass (kg) 72.5 ± 9.6 −2 21.7 ± 1.9 Body fat percentage (%) 9.8 ± 2.4 Fat free mass (kg) 65.3 ± 7.9 −1 5.1 ± 0.3 120 min −1 3.9 ± 1.1 −1 9.0 ± 2.6 1 5.2 ± 0.2 2 −1 −1 59.8 ± 5.6 W max 398 ± 41 BMI Plasma glucose 120 min Pretesting 2max W max 31 2 23 Diet and activity before testing −1 Experimental trials 1 W max 13 2 2 −1 −1 1 13 13 40 Fig. 1 13 2 2 W max  Protocol a m vastus lateralis 3 13 3 −1 2 2 −1 2 2 −1 −1 13 −1 −1 13 t t −1 −1 t t W max 2 2 t t t Beverages −1 −1 2 2 w w w w Tracer infusion 13 2 2 −1 −1 13 −1 −1 13 13 −1 −1 −1 Blood- and breath-sample analysis g 16 13 12 13 27 2 2 Calculations 26 1 \documentclass[12pt]{minimal}
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\begin{document}$${\text{Fat}}\,{\text{oxidation}}\,{\text{rate}}\quad \quad = \quad 1.695\,\,V{\text{O}}_{{\text{2}}} - 1.701\,\,V{\text{CO}}_{{\text{2}}} $$\end{document} 2 \documentclass[12pt]{minimal}
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\begin{document}$${\text{Carbohydrate}}\,{\text{oxidation}}\,{\text{rate}}\quad = \quad 4.585\,V{\text{CO}}_{{\text{2}}} - 3.22\,V{\text{O}}_{{\text{2}}} $$\end{document} 2 2 −1 −1 3 \documentclass[12pt]{minimal}
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\begin{document}$$ {\text{TTR}}\quad = \quad {\left( {^{{13}} {\text{C}} \mathord{\left/ {\vphantom {{\text{C}} {^{{{\text{12}}}} {\text{C}}}}} \right. \kern-\nulldelimiterspace} {^{{{\text{12}}}} {\text{C}}}} \right)}_{{{\text{sa}}}} - {\left( {^{{13}} {\text{C}} \mathord{\left/ {\vphantom {{\text{C}} {^{{{\text{12}}}} {\text{C}}}}} \right. \kern-\nulldelimiterspace} {^{{{\text{12}}}} {\text{C}}}} \right)}_{{bk}} $$\end{document} R a R d 32 49 R d R a 4 \documentclass[12pt]{minimal}
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\begin{document}$$ R_{a} = \frac{{F - V{\left[ {{{\left( {C_{2} + C_{1} } \right)}} \mathord{\left/ {\vphantom {{{\left( {C_{2} + C_{1} } \right)}} 2}} \right. \kern-\nulldelimiterspace} 2} \right]}\,\;{\left[ {{{\left( {E_{2} + E_{1} } \right)}} \mathord{\left/ {\vphantom {{{\left( {E_{2} + E_{1} } \right)}} {{\left( {t_{2} - t_{1} } \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {t_{2} - t_{1} } \right)}}} \right]}}} {{{{\left( {E_{2} + E_{1} } \right)}} \mathord{\left/ {\vphantom {{{\left( {E_{2} + E_{1} } \right)}} 2}} \right. \kern-\nulldelimiterspace} 2}} $$\end{document} 5 \documentclass[12pt]{minimal}
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\begin{document}$$ R_{d} = R_{a} - V{\left( {\frac{{C_{2} - C_{1} }} {{t_{2} - t_{1} }}} \right)} $$\end{document} F −1 −1 V −1 C 1 C 2 −1 t 1 t 2 E 2 E 1 13 2 13 2 −1 6 \documentclass[12pt]{minimal}
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\begin{document}$${\text{Pr}}^{{{\text{13}}}} {\text{CO}}_{{\text{2}}}  = {{\left( {{\text{TTR}}_{{{\text{CO}}_{{\text{2}}} }} V{\text{CO}}_{{\text{2}}} } \right)}} \mathord{\left/ {\vphantom {{{\left( {{\text{TTR}}_{{{\text{CO}}_{{\text{2}}} }} V{\text{CO}}_{{\text{2}}} } \right)}} {{\left( {k{\text{Ar}}} \right)}}}} \right. \kern-\nulldelimiterspace} {{\left( {k{\text{Ar}}} \right)}}$$\end{document} CO2 13 12 2 −1 k 2 −1 13 2 30 7 \documentclass[12pt]{minimal}
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\begin{document}$$Ar = {\left( {{{\text{TTR}}_{{{\text{CO}}_{{\text{2}}} }} V{\text{CO}}_{{\text{2}}} } \mathord{\left/ {\vphantom {{{\text{TTR}}_{{{\text{CO}}_{{\text{2}}} }} V{\text{CO}}_{{\text{2}}} } {k2F}}} \right. \kern-\nulldelimiterspace} {k2F}} \right)}$$\end{document} F 13 −1 R ox −1 8 \documentclass[12pt]{minimal}
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\begin{document}$$ R_{{{\text{ox}}}} \,{\text{palmitate}} = R_{{\text{d}}} \,{\text{palmitate}}{\left( {{\Pr ^{{13}} {\text{CO}}_{{\text{2}}} } \mathord{\left/ {\vphantom {{\Pr ^{{13}} {\text{CO}}_{{\text{2}}} } {F16}}} \right. \kern-\nulldelimiterspace} {F16}} \right)} $$\end{document} R d −1 F −1 37 13 2 2 18 R d 9 \documentclass[12pt]{minimal}
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\begin{document}$$ R_{{{\text{ox}}}} \,{\text{plasma}}\,{\text{glucose}} = R_{{{\text{ox}}}} \,{\text{plasma}}\,{\text{glucose}}{\left( {{\text{during}}\,{\text{exercise}}} \right)} $$\end{document} Muscle-sample analyses 29 21 22 39 41 43 44 Statistics P F t P Results Plasma metabolite and hormone concentrations 2 2 P P 2 P 2 P 2 2 3 P 3 P Fig. 2 W max n Crosses P  Fig. 3 W max n Crosses P  Plasma tracer kinetics 2 32 R a R d −1 −1 R a R d R ox −1 −1 R a R a R d R ox 2 R a R d, R a R d R ox 2 R a R d P 2 R a R d R ox P 2 44 Table 2 Tracer kinetics and respiratory measures during exercise   First hour Second hour Third hour Exercise average CON CHO CON CHO CON CHO CON CHO Glucose R a −1 −1 21.1 ± 2.7 a 28.8 ± 4.8 a 38.1 ± 7.5 a 30.4 ± 4.7  a R d −1 −1 21.9 ± 3.1 a 30.3 ± 7.9 a 39.5 ± 7.2 a 31.7 ± 4.7  a Palmitate R a −1 −1 2.49 ± 0.80 a 3.55 ± 0.78 a 6.43 ± 0.89 a 4.36 ± 0.71 a R d −1 −1 2.48 ± 0.79 a 3.50 ± 0.79 a 6.36 ± 0.88 a 4.32 ± 0.70 a R ox −1 −1 2.28 ± 0.73 a 3.49 ± 0.72 a 6.35 ± 0.73 a 4.26 ± 0.59 a R a 91.5 ± 5.9 90.9 ± 8.6 98.7 ± 5.83 95.6 ± 8.8 99.4 ± 7.8 99.8 ± 7.0 97.2 ± 6.2 96.0 ± 6.8  Ac recovery 81.7 ± 5.7 – 86.5 ± 4.7 – 86.2 ± 4.1 – 85.8 ± 3.9 – Respiratory measures         2 2,666 ± 91 2,690 ± 90 2,713 ± 88 2,709 ± 98 2,773 ± 93 2,772 ± 109 2,717 ± 89 2,724 ± 97 2 2,361 ± 81 2,412 ± 76 2,336 ± 76 a 2,327 ± 88 a 2,342 ± 79 a  RER 0.89 ± 0.01 0.90 ± 0.01 0.86 ± 0.01 a 0.84 ± 0.01 a 0.86 ± 0.01 a n W max R a R d R ox R a ox a Ac recovery 13 2 a P Substrate source utilization at rest −1 −1 −1 −1 −1 Substrate source utilization during exercise W max 2max −1 5 −1 P −1 P 4 4 5 P 4 P 4 −1 P 4 −1 P 4 Fig. 4 a b c d −1 W max n Crosses P  Fig. 5 −1 W max Crosses P  5 P 5 5 P 5 −1 P 4 5 4 5 5 Skeletal muscle lipid content 6 P P 2 −2 Fig. 6 n W max Asterisk P Crosses P  Skeletal muscle glycogen content P P P 6 P Discussion R a R d R a R d R ox IMTG use during prolonged exercise 46 1 37 11 20 41 44 5 6 IMTG utilization during prolonged exercise with carbohydrate supplementation 28 41 45 42 44 47 9 10 R a R d R ox 4 5 6 R a 2 4 2 6 2 44 4 6 48 48 48 11 2 2 3 48 Effects of carbohydrate supplementation on muscle glycogen use 8 14 6 6 5 9 13 17 24 25 34 35 34 35 15 4 12 5 9 13 17 24 25 Glycogen sparing occurs early in exercise with carbohydrate supplementation 5 7 5 4 R a R d 2 2 34 Conclusions This study provides compelling evidence that carbohydrate supplementation spares muscle glycogen utilization in a time-dependent manner during prolonged exercise in endurance-trained cyclists. Carbohydrate supplementation augments plasma glucose uptake and oxidation rate, while reducing muscle glycogen use during the early stages of exercise, thereby sparing type I and II muscle fiber glycogen stores. Carbohydrate supplementation during exercise lowers plasma FFA rate of appearance, uptake, and oxidation, but does not modulate the use of IMTG as a substrate source during cycling exercise.