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Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph N1G 2W1, and Department of Medicine, McMaster University, Hamilton, Ontario, Canada L8N 3Z5
This study
examined the effects of caffeine (Caf) ingestion on muscle
glycogen use and the regulation of muscle glycogen phosphorylase (Phos)
activity during intense aerobic exercise. In two separate trials, 12 untrained males ingested either placebo (Pl) or Caf (9 mg/kg body wt) 1 h before cycling at 80% maximum
O2 consumption (
O2 max) for
15 min. Muscle biopsies were obtained from the vastus lateralis at
0, 3, and 15 min of exercise. In this study, glycogen "sparing"
was defined as a 10% or greater reduction in muscle glycogen use
during exercise after Caf ingestion compared with Pl. Muscle glycogen
use decreased by 28% (Pl 255 ± 38 vs. Caf 184 ± 24 mmol/kg dry
muscle) after Caf in six subjects [glycogen sparers (Sp)]
but was unaffected by Caf in six other subjects [nonsparers
(NSp), Pl 210 ± 35 vs. Caf 214 ± 37 mmol/kg dry muscle]. In both groups, Caf significantly increased resting free fatty acid
concentration, significantly increased epinephrine
concentration by twofold during exercise, and increased the Phos
a mole fraction at 3 min of exercise
compared with Pl, although not significantly. Caf improved the energy
status of the muscle during exercise in the Sp group: muscle
phosphocreatine (PCr) degradation was significantly reduced (Pl 47.9 ± 3.6 vs. Caf 40.4 ± 6.7 mmol/kg dry muscle at 3 min) and the
accumulations of free ADP and free AMP (Pl 6.8 ± 1.3 vs. Caf 3.1 ± 1.4 µmol/kg dry muscle at 3 min; Pl 8.7 ± 0.8 vs. Caf 4.7 ± 1.1 µmol/kg dry muscle at 15 min) were significantly reduced.
Caf had no effect on these measurements in the NSp group. It is
concluded that the Caf-induced decrease in flux through Phos
(glycogen-sparing effect) is mediated via an improved energy status of
the muscle in the early stages of intense aerobic exercise. This may be
related to an increased availability of fat and/or ability of
mitochondria to oxidize fat during exercise preceded by Caf ingestion.
It is presently unknown why the glycogen-sparing effect of Caf does not
occur in all untrained individuals during intense aerobic exercise.
glycogen phosphorylase; cellular energy status; fat metabolism; variability
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