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1 Exercise Physiology and Biomechanics Laboratory, Faculty of Kinesiology and Rehabilitation Sciences, K.U.Leuven, Belgium
2 Copenhagen Muscle Research Centre, Institute of Exercise and Sports Sciences, University of Copenhagen, Copenhagen, Denmark
3 Clinique Romande de Réadaptation, Sion, Switzerland
4 Gynaecology and Obstetrics Unit, Faculty of Medicine, K.U.Leuven, Belgium
The effects were compared of exercise in the fasted state and exercise with a high rate of carbohydrate intake on intramyocellular triglyceride (IMTG) and glycogen content of human muscle. Using a randomized crossover study design, nine young healthy volunteers participated in two experimental sessions with an interval of 3 weeks. In each session subjects performed 2 h of constant-load bicycle exercise (
75%
), followed by 4 h of controlled recovery. On one occasion they exercised after an overnight fast (F), and on the other (CHO) they received carbohydrates before (150 g) and during (1 g (kg bw)–1 h–1) exercise. In both conditions, subjects ingested 5 g carbohydrates per kg body weight during recovery. Fibre type-specific relative IMTG content was determined by Oil red O staining in needle biopsies from m. vastus lateralis before, immediately after and 4 h after exercise. During F but not during CHO, the exercise bout decreased IMTG content in type I fibres from 18 ± 2% to 6 ± 2% (P
= 0.007) area lipid staining. Conversely, during recovery, IMTG in type I fibres decreased from 15 ± 2% to 10 ± 2% in CHO, but did not change in F. Neither exercise nor recovery changed IMTG in type IIa fibres in any experimental condition. Exercise-induced net glycogen breakdown was similar in F and CHO. However, compared with CHO (11.0 ± 7.8 mmol kg–1 h–1), mean rate of postexercise muscle glycogen resynthesis was 3-fold greater in F (32.9 ± 2.7 mmol kg–1 h–1, P
= 0.01). Furthermore, oral glucose loading during recovery increased plasma insulin markedly more in F (+46.80 µU ml–1) than in CHO (+14.63 µU ml–1, P
= 0.02). We conclude that IMTG breakdown during prolonged submaximal exercise in the fasted state takes place predominantly in type I fibres and that this breakdown is prevented in the CHO-fed state. Furthermore, facilitated glucose-induced insulin secretion may contribute to enhanced muscle glycogen resynthesis following exercise in the fasted state.
(Received 14 January 2005;
accepted after revision 7 February 2005;
first published online 10 February 2005)
Corresponding author P. Hespel: Exercise Physiology and Biomechanics Laboratory, F.A.B.E.R. – K.U.Leuven, Tervuursevest 101, B-3001 Leuven (Heverlee), Belgium. Email: peter.hespel{at}faber.kuleuven.be
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