CORRESPONDENCE: LETTER TO THE EDITOR
How Does Caffeine Increase Exercise Capacity But Decrease Myocardial Flow Reserve?
Tom M. McLellan, PhD*
* Operational Medicine Section, DRDC Toronto, 1133 Sheppard Avenue West, Toronto, Ontario M3M 3B9, Canada (Email: tom.Mclellan{at}drdc-rddc.gc.ca).
I read with interest a recent report by Namdar et al. (1) entitled, "Caffeine Decreases Exercise-Induced Myocardial Flow Reserve." The investigators highlighted some study limitations but failed to mention the potential order effect with the dosing of caffeine. All subjects in both groups completed the exercise testing without caffeine ingestion first and then repeated the testing 1 h later following a 200-mg dose of caffeine. Previous research by our group has shown that prior exercise alters the sensitivity of users of caffeine and makes them respond like caffeine-naive subjects during subsequent exercise tests (2). This effect of previous exercise was evident 5 h following the earlier exercise challenge, as we had also previously demonstrated that the ergogenic effect of a 5-mg/kg dose of caffeine lasted for 6 h for caffeine-naive subjects in contrast to the shorter 3-h benefit noted for caffeine users (3). The adenosine receptor sensitivity, therefore, may have been altered before the caffeine exercise test. A better design perhaps would have been to increase subject numbers and assign subjects to a placebo or caffeine group where subjects were matched according to fitness and caffeine habituation.
Namdar et al. (1) also make no reference to the exercise science literature that has reported a fairly consistent ergogenic effect of caffeine on exercise performance with no evidence of altered oxygen consumption or cardiac output (4), and that caffeine has been shown to demonstrate its ergogenic effect even more so during acute hypobaric exposure (5,6). Exactly how the findings from this current study can be viewed in terms of this other literature is puzzling.
Finally, care must also be taken in the generalization of findings with the ingestion of caffeine to the consumption of coffee. We know that other ingredients in coffee may interfere with the ergogenic effect of caffeine (7) and these other ingredients can also counter the negative effects of caffeine on glucose tolerance (8–10).
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1. Namdar M, Koepfli P, Grathwohl R, et al. Caffeine decreases exercise-induced myocardial flow reserve J Am Coll Cardiol 2006;47:405-410.[Abstract/Free Full Text]2. Bell DG, McLellan TM. Repeated exercise performance and caffeine ingestion Med Sci Sports Exerc 2003;35:1348-1354. 3. Bell DG, McLellan TM. Exercise performance 1, 3, and 6 hours after caffeine ingestion J Appl Physiol 2002;93:1227-1234.[Abstract/Free Full Text] 4. Graham TE. Caffeine and exerciseMetabolism, endurance and performance. Sports Med 2001;31:785-807.[CrossRef][Web of Science][Medline] 5. Berglund B, Hemmingsson P. Effects of caffeine ingestion on exercise performance at low and high altitudes in cross-country skiers Int J Sports Med 1982;3:234-236.[Web of Science][Medline] 6. Fulco CS, Rock PB, Trad LA, et al. Effect of caffeine on submaximal exercise performance at altitude Aviat Space Environ Med 1994;65:539-545.[Medline] 7. Graham TE, Hibbert E, Sathasivam P. Metabolic and exercise endurance effects of coffee and caffeine ingestion J Appl Physiol 1998;85:883-889.[Abstract/Free Full Text] 8. Shearer J, Farah A, de Paulis T, et al. Quinides of roasted coffee enhance insulin action in conscious rats J Nutr 2003;133:3529-3532.[Abstract/Free Full Text] 9. Graham TE, Sathasivam P, Rowland M, et al. Caffeine ingestion elevates plasma insulin response in humans during an oral glucose tolerance test Can J Physiol Pharmacol 2001;79:559-565.[CrossRef][Web of Science][Medline] 10. Thong FSL, Derave W, Kiens B, et al. Caffeine-induced impairment of insulin action but not insulin signalling in human skeletal muscle is reduced by exercise Diabetes 2002;51:583-590.[Abstract/Free Full Text]
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