Journal of applied physiology (Bethesda, Md. : 1985)Feb 16, 2008

How Training While Fasted Changes Metabolism During Exercise with Carbohydrate Intake

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Updated Mar 29, 2026

Abstract

Endurance training for 6 weeks resulted in a 7% increase in peak Vo(2) regardless of nutritional status during training.

Muscle adaptations, including succinate dehydrogenase activity, GLUT4, and hexokinase II content, were similarly enhanced in both fasted and carbohydrate-fed groups.
Fasted training led to a significant increase in fatty acid binding protein (FABPm) content.
Pre-exercise glycogen content was higher in the carbohydrate-fed group compared to the fasted group after training.
Fasted individuals experienced a reduction in exercise-induced glycogen breakdown compared to those who trained in a carbohydrate-fed state.
Intramyocellular triglyceride content and fat oxidation rates during exercise remained unchanged after the training period.

Simplified

Skeletal muscle gene response to exercise depends on nutritional status during and after exercise, but it is unknown whether muscle adaptations to endurance training are affected by nutritional status during training sessions. Therefore, this study investigated the effect of an endurance training program (6 wk, 3 day/wk, 1-2 h, 75% of peak Vo(2)) in moderately active males. They trained in the fasted (F; n = 10) or carbohydrate-fed state (CHO; n = 10) while receiving a standardized diet [65 percent of total energy intake (En) from carbohydrates, 20%En fat, 15%En protein]. Before and after the training period, substrate use during a 2-h exercise bout was determined. During these experimental sessions, all subjects were in a fed condition and received extra carbohydrates (1 g.kg body wt(-1) .h(-1)). Peak Vo(2) (+7%), succinate dehydrogenase activity, GLUT4, and hexokinase II content were similarly increased between F and CHO. Fatty acid binding protein (FABPm) content increased significantly in F (P = 0.007). Intramyocellular triglyceride content (IMCL) remained unchanged in both groups. After training, pre-exercise glycogen content was higher in CHO (545 +/- 19 mmol/kg dry wt; P = 0.02), but not in F (434 +/- 32 mmol/kg dry wt; P = 0.23). For a given initial glycogen content, F blunted exercise-induced glycogen breakdown when compared with CHO (P = 0.04). Neither IMCL breakdown (P = 0.23) nor fat oxidation rates during exercise were altered by training. Thus short-term training elicits similar adaptations in peak Vo(2) whether carried out in the fasted or carbohydrate-fed state. Although there was a decrease in exercise-induced glycogen breakdown and an increase in proteins involved in fat handling after fasting training, fat oxidation during exercise with carbohydrate intake was not changed.

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How Training While Fasted Changes Metabolism During Exercise with Carbohydrate Intake

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