Dose response effects of a caffeine-containing energy drink on muscle performance: a repeated measures design

Journal Title (Medline/Pubmed accepted abbreviation): J. Int. Soc. Sports Nutr.
Year: 2012
Volume: 9
First Page: 21
doi: 10.1186/1550-2783-9-21
Summary of Background and Research Design

Background: Caffeine is a popular stimulant that is present in many energy drinks and pre-workout supplements.  However, the dose of caffeine that is optimal for resistance training has not yet been established.

Hypothesis: Caffeine, administered at 3 mg/kg body weight, will be more effective at increasing power than 1 mg/kg caffeine or no caffeine at all.

Subjects: Active men (9) and women (3) who were “light” caffeine consumers (< 60 mg per day, or 1 cup of coffee)

Experimental design: randomized, double-blinded, placebo-controlled, crossover trial

Treatments : Fure® Proenergetics® energy drink with:
No caffeine (placebo)
1 mg caffeine per kg body weight (69 ± 10 mg, about 1 cup of coffee or 2 sodas)
3 mg caffeine per kg body weight (207 ± 30 mg)

Protocol : The participants were first weighed and evaluated for body composition.  One repetition maxima (1RM) was determined for the half squat and bench press.  The participants then came to the laboratory 3 more times, one for each treatment, separated by at least 48 hrs.  They were encouraged to consume a light meal within the 2 hours preceding their appointment at the laboratory.  Upon arrival, they consumed the test beverage and then rested for 60 min.  Respiratory gases were monitored, from which resting energy expenditure was calculated.  Blood pressure and heart rate were also assessed.  After a standardized warm up, power-load tests were performed.  Concentric bench press and half squat performance were evaluated by increasing load from 10% to 100% of their previously determined VO2max.  Participants started with their arms or legs at 90° and then elevated the bar as fast as possible.  Velocity, acceleration, and power were recorded.  The day after the trial, the participants reported any side effects including difficulty sleeping or gastrointestinal discomfort.

Summary of research findings
  • Caffeine increased blood pressure and heart rate in a dose dependent manner.
  • Resting energy expenditure and respiratory exchange ratio were similar between trials.
  • Power output of the half-squat was similar between the placebo and 1 mg/kg caffeine trials.  Power output was significantly greater with 3 mg/kg caffeine.
  • Similarly, power output of the bench press was significantly higher with the 3 mg/kg dose of caffeine, while the power output was similar for 0 and 1 mg/kg caffeine.
  • The 3 mg/kg dose of caffeine led to an increased frequency of negative side effects, with 17% of the participants experiencing gut/abdominal discomfort with the high dose only.  Additionally, 17% of the participants felt tachycardia (increased heart rate) and heart palpitations with only the high dose of caffeine.  While there were some feelings of insomnia with the placebo and 1 mg/kg caffeine, 25% of the participants had difficulty sleeping after the high dose.

Key practice applications

In healthy athletes that normally consume about 60 mg of caffeine per day or less, 1 mg/kg of caffeine was not sufficient to improve power endurance, but 3 mg/kg improved performance.  However, this dose led to side effects including self-reported abnormal heart rhythms and sleep difficulties. 
The effects of caffeine can vary greatly between different individuals, resulting in either positive or negative outcomes.  It is easy to build a tolerance to caffeine, so frequent caffeine consumers may require more caffeine to elicit performance benefits.  As always, try supplements in practice before using them in competition to prevent disappointing outcomes due to unexpected negative side effects.
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