## 4 Discussion  
This study was performed to evaluate the effects of caffeine and taurine components on the cardiovascular systems of young adults and to analyze HRV both linearly and nonlinearly. As a principal finding, compared with taurine and the combination of caffeine + taurine, caffeine was found to have the most negative effects on the cardiovascular system. It was also found that caffeine had adverse effects on the linear and nonlinear parameters of HRV. However, the presence of taurine may have repressed the adverse effects of caffeine. Caffeine and taurine are accepted as the major ingredients of energy drinks that cause improvements in neurocognitive performance and positive ergogenic effects [10]. It was reported that high doses of caffeine are characterized by multiple cardiac comorbidities, including atrial fibrillation, palpitations, and tremors [7]. In addition, a transient increase in blood pressure and heart rate occurs due to the release of adrenaline after the consumption of caffeine [33]. Furthermore, low doses of taurine demonstrated cardioprotective actions such as decreasing platelet aggregation, reducing blood pressure, and having antiarrhythic properties [7]. However, it was suggested that for higher doses of taurine, these beneficial effects were overridden [8]. Consistent with the effects of caffeine, [PERSON] et al. [34] reported a significant increase in both systolic blood pressure and diastolic blood pressure 60 min after the intake of a 250 mg caffeine capsule, while [PERSON] et al. [27] reported an increase in blood pressure values 60 min after the ingestion of 240 mg caffeine. These were in line with our results. This increase in blood pressure may be because of sympathetic over-activation, antagonism of adenosine receptors, and increased norepinephrine release [35]. Furthermore, [PERSON] et al. [36] reported a significant increase in the heart rate related to caffeine intake after 60 min. We also found a significant elevation in heart rate values in the caffeine group 60 min after ingestion. On the other hand, in our study, we did not observe any changes in systolic blood pressure, diastolic blood pressure, or heart rate in the taurine group. Contrary to our results, it has been reported in the literature that the ingestion of taurine doses below the 10 g/day safe limit [37] reduces blood pressure and heart rate [7]. Since peak plasma concentrations of taurine occur at 60 min to 150 min after ingestion [38], the reason why no change was observed may be the duration of our study. On the other hand, for the caffeine + taurine group, the heart rate values increased in line with the study of [PERSON] et al. [11], which reported a large increase in heart rate one hour after  
Figure 4: Time course of changes in the approximate entropy of experimental groups. \(\Delta\): statistically significant versus the pre value of the caffeine group (\(p\) = 0.035). the ingestion of a combination of caffeine + taurine at rest. Increases in the systolic blood pressure and diastolic blood pressure values of the caffeine + taurine group were also found in our study; the increase was smaller in the caffeine + taurine group than in the caffeine group. This is because taurine may have a reducing effect on blood pressure and heart rate variability due to inducing vasodilation and its hypotonic effect. Caffe consumption has been reported to increase vasoconstriction and the release of norepinephrine, which causes a high heart rate and blood pressure [39]. However, taurine has the opposite effect of caffeine by suppressing norepinephrine, increasing norepinephrine turnover, and reducing angiotensin II-mediated vasoconstriction [40]. In this case, the increased release of norepinephrine due to the presence of caffeine may have shown a taurine suppressive effect, therefore reducing the negative effects caused by caffeine, such as increases in heart rate and blood pressure [7]. Therefore, taurine may have repressed the effect of caffeine.