The ingestion of carbohydrate-electrolyte (CHO-E) solutions has been shown to improve capacity during high-intensity intermittent running (LIST) (Nicholas et al. 1995). Nicholas et al. (1999) reported that this was due to a sparing of muscle glycogen. Initial muscle glycogen concentrations in that study were moderate (approx 350 mmol kg^-1 dry matter), therefore the purpose of the present study was to examine the effect of CHO ingestion on muscle glycogen utilisation and intermittent running capacity on subjects with high pre-exercise muscle glycogen concentrations.

Six men (mean ± (S.E.M.); age 23.0 years ( ± 0.4); body mass 75.0 kg ( ± 1.2); Î{special}J{special}_max 60.0 ml kg^-1 min^-1 ( ± 0.7)) volunteered, with informed consent, to participate in this study. Subjects performed two trials separated by 14 days in a randomised crossover design, consuming either a 6.4 % hypotonic CHO-E solution (HYP) or a placebo (PLA) in a double-blind fashion prior to the trials (8ml kg^-1BM) and at 15 min intervals (3ml kg^-1BM) until fatigue. Following a glycogen depleting trial and 48 h high CHO diet (10g kg^-1BM day^-1) subjects ran LIST to fatigue. Muscle biopsy samples were obtained pre-LIST, 90 min and fatigue.

All subjects ran longer in HYP (158.0 min ± 11.6) compared to PLA (131.0 min ± 8.0; P = 0.03; Wilcoxon). There were no differences in muscle glycogen concentrations pre-exercise (HYP 533 ± 31 mmol kg^-1 DM v PLA 512 ± 41 mmol kg^-1 DM) or at 90 min (HYP 344 ± 37 mmol kg^-1 DM v PLA 359 ± 24 mmol kg^-1 DM; paired Student’s t test). There was a trend for muscle glycogen utilisation to be greater in PLA post 90 min (4.2 ± 1.2 mmol kg^-1 DM min^-1) compared to HYP (2.5 ± 0.3 mmol kg^-1 DM min^-1; P = 0.1) although this failed to reach statistical significance. Plasma glucose concentrations were higher at fatigue in HYP (P < 0.001; ANOVA) although hypoglycaemia did not occur in PLA.

These data suggest that the ingestion of a 6.4 % hypotonic CHO-E solution will improve endurance capacity during intermittent exercise (LIST) in subjects with elevated pre-exercise muscle glycogen concentrations. This appears to occur through higher plasma glucose concentrations which may result in elevated rates of exogenous CHO oxidation late in exercise as indicated by the tendency towards a lower rate of muscle glycogen utilisation.