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Carbohydrate Availability and Muscle Energy Metabolism during Intermittent Running


Medicine & Science in Sports & Exercise: January 2008 - Volume 40 - Issue 1 - p 96-103
doi: 10.1249/mss.0b013e3181586b2c
BASIC SCIENCES: Original Investigations

Purpose: To examine the influence of ingesting a carbohydrate-electrolyte (CHO-E) solution on muscle glycogen use and intermittent running capacity after consumption of a carbohydrate (CHO)-rich diet.

Methods: Six male volunteers (mean ± SD: age 22.7 ± 3.4 yr; body mass (BM) 75.0 ± 4.3 kg; V˙O2max 60.2 ± 1.6 mL·kg−1·min−1) performed two trials separated by 14 d in a randomized, crossover design. Subjects consumed either a 6.4% CHO-E solution or a placebo (PLA) in a double-blind fashion immediately before each trial (8 mL·kg−1 BM) and at 15-min intervals (3 mL·kg−1 BM) during intermittent high-intensity running to fatigue performed after CHO loading for 2 d. Muscle biopsy samples were obtained before exercise, after 90 min of exercise, and at fatigue.

Results: Subjects ran longer in the CHO-E trial (158.0 ± 28.4 min) compared with the PLA trial (131.0 ± 19.7 min; P < 0.05). There were no differences in muscle glycogen use for the first 90 min of exercise (~2 mmol of glucosyl units per kilogram of dry matter (DM) per minute). However, there was a trend for a greater use in the PLA trial after 90 min (4.2 ± 2.8 mmol·kg−1 DM·min−1) compared with the CHO-E trial (2.5 ± 0.7 mmol·kg−1 DM·min−1; P = 0.10). Plasma glucose concentrations were higher at fatigue in the CHO-E than in the PLA trial (P < 0.001).

Conclusions: These results suggest that CHO-E ingestion improves endurance capacity during intermittent high-intensity running in subjects with high preexercise muscle glycogen concentrations. The greater endurance capacity cannot be explained solely by differences in muscle glycogen, and it may actually be a consequence of the higher plasma glucose concentration towards the end of exercise that provided a sustained source of CHO for muscle metabolism and for the central nervous system.

1Institute of Food, Nutrition and Human Health, Massey University, Auckland, NEW ZEALAND; 2School of Sport and Exercise Sciences, Loughborough University, Loughborough, UNITED KINGDOM; 3Sunderland Royal Hospital, Sunderland, UNITED KINGDOM; and 4Centre for Integrated Systems Biology and Medicine, School of Biomedical Sciences, Nottingham University, Nottingham, UNITED KINGDOM

Address for correspondence: Andrew Foskett, Ph.D., Institute of Food, Nutrition and Human Health, Massey University, Private Bag 102 904, North Shore Mail Centre, Auckland, New Zealand; E-mail:

Submitted for publication May 2007.

Accepted for publication August 2007.

© 2008 American College of Sports Medicine