Introduction
Caffeine is an ingredient in a variety of beverages including coffee, tea, chocolate drinks, cola drinks. Caffeine is also included in some prescription medications (McKinley Health 2004). Caffeine has long been regarded as an ergogenic aid as it has been shown to increase muscle contractibility, aerobic endurance and enhancement of fat metabolism. (McKinley Health 2004). Caffeine acts as a mild stimulant to the central nervous system and the effects of caffeine can be modulated by the subject’s age and body mass (McKinley Health 2004). Caffeine has been shown to improve performance, including endurance, when taken prior to exercise (Ryu et al. 2001) as caffeine increases fatty acid metabolism. These fatty acids can be used as a fuel source, instead of glycogen. Many studies have found that caffeine enhances both the parasympathetic and sympathetic nervous systems (Nishijima et. al 2002). Further to this, Kruk et al. (2001) report that psychomotor performance (mental performance when being challenged physically) of subjects being is improved by taking caffeine.
There are conflicting reports as to whether or not caffeine has any significant effect on heart rate. Kruk et al. (2001) reported that, using subjects who are subjected to physical challenges, caffeine had no significant effect on heart rate, where as Nishijima et al. (2002) reported a difference in heart rate in groups ingesting caffeine. The form in which caffeine is ingested needs to be considered as pure caffeine and caffeine ingested as a coffee beverage can yield different results (Nishijima et al., 2002; Graham et. al 1998). Graham et al., (1998) suggest that other components in the coffee alter the effect of caffeine (Graham et al., 1998). Here we test the effect of moderate exercise on pulse rate and respiration rate after taking caffeine in coffee.
Methods:
Refer to School of Biological Sciences Human Biology laboratory manual (2013).
Refer to Appendix A for Null Hypothesis (Ho) and Alternative Hypothesis (HA).
Results:
Effect of caffeine on the change in heart rate: The heart rates of subjects enrolled in the study prior to and one hour after ingestion of caffeinated or decaffeinated coffee while performing an incremental exercise test and the changes in hear rates including the associated statistics such as mean, variance and standard deviation are summarized in Table 1 (see appendix). The mean changes in heart rates in caffeinated and noncoffeinated groups were 18.2 and 29.6, respectively (variance, 327.4; SD, 18.1 and variance, 87.8; SD, 9.4). The results, as analyzed by the change in heart (pulse) rate data using a two sample t-test assuming unequal variances, suggest no significant differences in the change in heart rate between the caffeine and noncaffeine subject groups (p=0.121). We also analyzed the resting (before exercise) heart (pulse) rate using a two-sample t-test assuming equal variances and found no significant differences between the caffeine and noncaffeine subject groups (p=0.458), suggesting no undue influence of the resting heart rates of the subjects in the two groups on the ultimate results of change in heart rates between caffeine and noncaffeine subject groups and randomization of subjects into the two groups as normal and expected.
The results remained the same when the data were analyzed with different parameters of statistics. Using a two sample t-test assuming equal variances, we found no significant differences change in heart rate between the caffeine and noncaffeine subject groups (p=0.11).
Effect of caffeine on the change in respiration rate: In the same study described above, we also measured the respiration rates of all the recruited subjects. The respiration rates of subjects enrolled into the study prior to and one hour after ingestion of caffeinated or decaffeinated coffee while performing an incremental exercise test and the changes in respiration rates including the associated statistics such as mean, variance and standard deviation are summarized in Table 2 (see appendix). The mean changes in respiration rates in caffeinated and noncoffeinated groups were 10.2 and 5.6, respectively (variance, 46.4; SD, 6.8 and variance, 28; SD, 5.3, respectively). The results, as analyzed by the change in respiration rate data using a two sample t-test assuming equal variances, suggest no significant difference in the change in respiration rate between the caffeine and noncaffeine subject groups (p=0.125). We also analyzed the resting (before exercise) respiration rate using a two-sample t-test assuming equal variances and found no significant differences between the caffeine and noncaffeine subject groups (p=0.716), suggesting no undue influence of the resting respiration rates of the subjects in the two groups on the ultimate results of change in respiration rates between caffeine and noncaffeine subject groups and randomization of subjects into the two groups as normal and expected.
Discussion:
The present study was undertaken to reexamine the effects of caffeine consumed in the form of coffee on heart rate and respiration rate while performing moderate exercise. Previous results on the effects of caffeine on heart rate were contradictory (Kruk, et al., 2001; Nishijima, et al., 2002). Kruck et al reported that caffeine had no significant effect on heart rate in individuals who are subjected to physical challenges, whereas the study by Nishijima et al reported a difference in heart rate in groups ingesting caffeine (Kruk, et al., 2001; Nishijima, et al., 2002). Our reexamination of the effects of caffeine in subjects included respiration rate changes in addition to the heart rate changes.
This was a double-blind randomized placebo-controlled study, albeit with a small sample size of 18 assigned equally into group A (caffeine) and group B (noncaffeine). Our results suggest that approximately 180 mg caffeine ingested in the form of coffee had no significant effect on both heart and respiration rates in subjects performing moderate exercise. While our results are consistent with those reported by Kruk et al (Kruk, et al., 2001) and contradict the findings of Nishijima et al (Nishijima, et al., 2002), caution must be exercised in the general applicability of the results given a small sample size employed in the present study. In addition, the form in which caffeine is ingested needs to be considered as pure caffeine and caffeine ingested as a coffee beverage can yield different results (Graham, Hibbert, & Sathasivam, 1998; Nishijima, et al., 2002). Graham et al. suggested that other components in the coffee can modulate and alter the effects of caffeine (Graham, et al., 1998).
While caffeine has been shown to increase muscle contractibility, aerobic endurance and enhancement of fat metabolism (McKinley Health 2004), the effects of caffeine can be modulated by the subject’s age and body mass (McKinley Health 2004). Since our study did not take into account the subject’s age and body mass in dosage used or data analysis, additional studies that are well controlled for all the factors mentioned above are required to establish the effects, if any, of caffeine on heart rates or respiration rates in humans.
References:
Graham, T. E., Hibbert, E., & Sathasivam, P. (1998). Metabolic and exercise endurance effects of coffee and caffeine ingestion. J Appl Physiol, 85(3), 883-889.
Kruk, B., Chmura, J., Krzeminski, K., Ziemba, A. W., Nazar, K., Pekkarinen, H., et al. (2001). Influence of caffeine, cold and exercise on multiple choice reaction time. Psychopharmacology (Berl), 157(2), 197-201.
McKinley Health 2004.
Nishijima, Y., Ikeda, T., Takamatsu, M., Kiso, Y., Shibata, H., Fushiki, T., et al. (2002). Influence of caffeine ingestion on autonomic nervous activity during endurance exercise in humans. Eur J Appl Physiol, 87(6), 475-480.
Apprendix:
Table 2. Respiration rates prior to and after exercise in caffeine test and control subjects
