Eur J Appl Physiol (1991) 62:424-429

Applied Physiology European Journal of

arid Occupational Physiology © Springer-Verlag 1991

The effects of caffeine on graded exercise performance in caffeine naive versus habituated subjects S. L. Dodd 1, E. Brooks 1, S. K. Powers 3, and R. Tulley 2 1 Department of Kinesiology and 2 Pennington Biomedical Research Center, Louisiana State University, Baton Rouge, Louisiana, USA 3 Center for Exercise and Sports Sciences, University of Florida, Gainesville, Florida, USA Accepted January 14, 1991 Summary. The physiological effects of caffeine on subjects habituated to caffeine is relatively unstudied compared to those of caffeine naive subjects during graded exercise. Thus, the purpose of this investigation was to determine the effects of caffeine on maximal oxygen consumption (~'rO2max) and the anaerobic threshold in these two populations. Seventeen moderately trained males were classified according to caffeine usage: (1) caffeine consumption 25 mg. day-1 or less (CN) (n = 8) or (2) caffeine consumption above 300 mg. day-1 (CH) (n = 9). The subjects were tested post-absorptive on the same cycle ergometer on three occasions with 7 days separating the tests. One hour before each test the subject ingested either a gelatin capsule (C); 3 mg.kg -1 body weight of caffeine (C3); or 5 mg-kg -1 body weight of caffeine (C5). The subject then performed an incremental l/O2m,x test beginning at 50W and the work rate was increased 30 W every 2 rain until the subject could not maintain the power output. Serial venous blood samples were drawn over 30 s at the end of each stage. The CN group significantly increased resting heart rate (fc) and expired ventilation volume (IRE) after C3 and C5 and VO2 after C5. No significant differences were found for exercise IkE, IkO2, respiratory excharge ratio, fc or time to exhaustion. There were no significant differences (P < 0.05) in the lactate threshold or the ventilatory threshold between treatment in either group. The CH subjects showed a significant increase (P

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lactate values were not different between groups nor at any point during the test. In addition, the thla for the C N group (170 W) was not statistically different than in the C H g r o u p (140 W). There were no significant differences in the thvent between treatments in either group (Fig. 2). The method of analysis used was to identify the point at which there was an increase in the ventilatory equivalent for 02 ( ; E / ~ r O 2 ) without an increase in the ventilatory equivalent for CO2 ([2E/J2CO2). As can be seen, the thwnt is identical across treatments in both groups. The C H subjects show a significant increase ( P < 0.05) in F F A during the C3 and C5 treatments, but no change in C, between the administration of caffeine and the beginning of exercise (Fig. 3). There are no differences in FFA between treatments from the first stage of exercise until the end. The greatest effect of caffeine on FFA is seen in the C N subjects during C3 and C5 treatments. F F A were significantly increased ( P < 0.05) at all times during C3 and C5 treatments when compared to the control values.

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Discussion The primary finding of this study was that caffeine ingestion had no significant effect on the th~a o r ~rO2max during graded exercise in either the CN or CH group• However, caffeine did have some differential physiological effects on CN subjects at rest when compared to CH subjects. First, caffeine elicited an increased fc at rest in the CN subjects when compared with the CH group• This finding would be supported by the notion of a developed tolerance to caffeine as purported by Robertson et al. (1981) and supported by the data of Robertson et al. (1978). The finding that lie at rest was elevated in the CN group but not in the CH group during caffeine administration suggests that the central ventilatory stimulatory effects of caffeine (Trippenbach et al. 1980) were greater in CN subjects as compared with CH subjects. Indeed, this finding is supported by the data of Robertson et al. (1978). Resting VO2 and RER were not different after C3 caffeine administration in either group. However, in the CN group, the C5 treatment caused a significantly increased resting metabolic rate. This increase is consistent with the findings of LeBlanc et al. (1985), who found a 15% increase in resting metabolic rate with similar doses of caffeine.

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Fig. 3. Blood free fatty acid (FFA) concentration for the CH and CN groups at each work rate during three treatments. * Significantly different from control values at P300 mg.day-1. CN, caffeine consumption