Effects on Energy Metabolism

Acute administration of caffeine produces a 5-25% increase in the basal metabolic rate. Inactive subjects exhibit a greater increase in resting metabolic rate than do exercise-trained subjects. It is concluded that endurance training seems to result in a reduced thermogenic response to a caffeine challenge.

These modifications of energy metabolism were associated with significant increases in serum free fatty acids, glycerol, and lactate concentrations, whereas inconsistent findings were reported for blood glucose levels. Acute administration of caffeine was shown to decrease insulin sensitivity and to impair glucose tolerance, possibly as a result of elevated plasma epinephrine. However, it is not understood why a large and long-term epidemiolo-gical study associated significant lower risks for type 2 diabetes in both men and women with total caffeine intake. The lipolytic effect is generally explained by the inhibition of phosphodiesterase, the release of catecholamine, or adenosine receptor antagonism. The increased availability of free fatty acids and their oxidation may have a glycogen-sparing effect. However, increasingly more results do not support the hypothesis that caffeine improves endurance performance by stimulating lipolysis, and some of the ergogenic effects in endurance exercise performance may occur directly at the skeletal muscle and CNS levels. In addition, this effect may be suppressed by the simultaneous ingestion of a high-carbohydrate meal, which is a common practice prior to competition.

Despite the controversy among scientists concerning the ergogenic potential of caffeine on sport performance, it is accepted that caffeine will not improve performance during short-term, high-intensity work, whereas an increase in both work output and endurance in long-term exercise is expected. Most studies also show that the duration and the magnitude of the ergogenic effect of caffeine are greater in nonusers than in users.

Based on the assumption that caffeine may enhance athletic performance, the International

Olympic Committee defined an upper concentration limit of 12 mg/ml in urine samples, above which an athlete was disqualified. However, in the World Anti-Doping Agency Executive Committee Meeting (September 2003), it was observed that the stimulant effect of caffeine is obtained at levels lower than 12. As a consequence, caffeine was removed from the 2004 list of prohibited substances because athletes must be allowed to behave like other people in society and may thus be allowed to drink coffee.

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