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1 School of Nursing, Faculty of Health Sciences, University of Western Ontario, London, Ontario N6A 5C1; and 2 Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada N1G 2W1
In this study
the effects of acute caffeine ingestion on exercise performance,
hormonal (epinephrine, norepinephrine, insulin), and metabolic (free
fatty acids, glycerol, glucose, lactate, expired gases) parameters
during short-term withdrawal from dietary caffeine were investigated.
Recreational athletes who were habitual caffeine users
(n = 6) (maximum oxygen uptake 54.5 ± 3.3 ml · kg
1 · min1
and daily caffeine intake 761.3 ± 11.8 mg/day) were tested under conditions of no withdrawal and 2-day and 4-day withdrawal from dietary
caffeine. There were seven trials in total with a minimum of 10 days
between trials. On the day of the exercise trial, subjects ingested
either dextrose placebo or 6 mg/kg caffeine in capsule form 1 h before
cycle ergometry to exhaustion at 80-85% of maximum oxygen uptake.
Test substances were assigned in a random, double-blind manner. A final
placebo control trial completed the experiment. There was no
significant difference in any measured parameters among days of
withdrawal after ingestion of placebo. At exhaustion in the 2- and
4-day withdrawal trials, there were significant increases in plasma
norepinephrine in response to caffeine ingestion. Caffeine-induced
increases in serum free fatty acids occurred after 4 days and only at
rest. Subjects responded to caffeine with increases in plasma
epinephrine (P < 0.05) at
exhaustion and prolonged exercise time in all caffeine trials compared
with placebo, regardless of withdrawal from caffeine. It is concluded that increased endurance is unrelated to hormonal or metabolic changes
and that it is not related to prior caffeine habituation in
recreational athletes.
norepinephrine; epinephrine; methylxanthines; performance; habituation
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