Journal of applied physiology (Bethesda, Md. : 1985)Apr 1, 1993

How carbohydrate loading affects energy use and breakdown during long exercise

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Updated Mar 29, 2026

Abstract

Total muscle glycogen utilization was greater in carbohydrate-loaded subjects (134 +/- 11 mmol/kg wet weight) compared to non-carbohydrate-loaded subjects (95 +/- 12 mmol/kg wet weight).

Carbohydrate oxidation rates during exercise were similar in both carbohydrate-loaded and non-carbohydrate-loaded conditions.
Blood glucose oxidation was also comparable between the two groups.
Liver glucose appearance varied, being higher in the non-carbohydrate-loaded group.
Muscle glycogen content was significantly greater in carbohydrate-loaded subjects at the start of exercise.
Muscle glycogen breakdown rates remained high in carbohydrate-loaded subjects but decreased in non-carbohydrate-loaded subjects after 60 minutes.

Simplified

This study compared liver glucose turnover, blood glucose oxidation, and muscle glycogen utilization in 15 male endurance-trained cyclists who rode for 180 min at 70% of maximal O2 consumption in either a carbohydrate-(CHO) loaded (CL) or a non-CHO-loaded (NL) state. Total CHO oxidation during exercise was similar in the CL and NL subjects (492 +/- 77 vs. 448 +/- 43 g, respectively), as were blood glucose oxidation (103 +/- 19 vs. 99 +/- 7 g, respectively) and liver glucose appearance (110 +/- 15 vs. 127 +/- 16 g, respectively). However, total muscle glycogen utilization was greater in CL than NL subjects (134 +/- 11 vs. 95 +/- 12 mmol/kg wet wt; P < 0.05), the former of which had higher muscle glycogen content at the start (194 +/- 4 vs. 124 +/- 7 mmol/kg wet wt; P < 0.05) and throughout the trial. Whereas high rates of muscle glycogen breakdown were maintained throughout the trial in CL subjects, rates of muscle glycogenolysis in NL subjects decreased to 26 mmol.kg wet wt-1.h-1 after 60 min of exercise (P < 0.05) when their muscle glycogen content had declined to 70 mmol/kg wet wt. Comparable rates of blood glucose and overall CHO oxidation in CL and NL subjects, despite a slowing of muscle glycogenolysis in the NL group, could be explained by an accelerated breakdown of glycogen in the nonworking muscles to redistribute CHO (lactate) to the working muscles for oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

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How carbohydrate loading affects energy use and breakdown during long exercise

Abstract

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