## 4 Discussion
This suggestion is in agreement with the report published by the European Union's SCF, which noted that \"if there are any interactions between caffeine and taurine, taurine might reduce the cardiovascular effects of caffeine\" [7]. SDNN, LF, and LF/HF are related to heart responsiveness and are thought to be mediated by interactions between sympathetic and parasympathetic nerve activities. Their reduction is reported to be associated with an increased risk of sudden arrhythmic death. In particular, SDNN values below 50 ms are classified as unhealthy, while healthy values are between 50 and 100 ms [41]. In our study, we found a reduction in these parameters for the caffeine and caffeine + taurine groups, in line with the study of [PERSON] et al. [42], which reported a decrease in SDNN and LF values after the consumption of 473 mL of energy drink, and with [PERSON] and [PERSON]'s study [43], which reported a decrease in LF and LF/HF parameters after the same amount of energy drink was consumed. However, several studies have reported that these parameters increased or did not change over time [24,44]. This may be because, in our study, the effects of caffeine and taurine were examined separately instead of using energy drinks. Furthermore, we evaluated the RMSSD, NN50, and HF parameters, which are related to cardiac parasympathetic function, and found that these parameters increased in the caffeine and caffeine + taurine groups. Our results agreed with studies reporting increased RMSSD and HF values after the consumption of varying amounts of energy drinks or caffeine capsules [23,44]. Furthermore, the increases in heart rate, RMSSD, NN50, and HF can be explained by the dominance of parasympathetic influence mediated via the release of acetylcholine by the vagus nerve. Since there are no studies investigating the acute effects of taurine on the cardiovascular system, we cannot make comparisons with our results in this regard. The oscillations of the heart have chaotic behavior. Therefore, nonlinear analysis of the cardiovascular system is gaining importance. It was reported that the cardiovascular system demonstrated increased complexity and chaotic behavior in several disease states [45]. There are very few studies in the literature investigating the effects of caffeine or energy drinks on the cardiovascular system using nonlinear analysis methods. In fact, no studies on the effects of taurine or a combination of taurine + caffeine have been conducted in this manner before our own study. We observed increases in the caffeine and caffeine + taurine groups' largest Lyapunov exponent, correlation dimension, approximate entropy, and Hurst exponent values at 60 min. These results are in line with studies that have reported increased Lyapunov exponent and correlation dimension values 60 min after participants consumed energy drinks or coffee [25] and increased approximate entropy values after 5 mg/kg caffeine intake when compared to the control group [26]. Since no studies have evaluated the Hurst exponent after caffeine or taurine consumption in the literature, there is no opportunity to compare the results we obtained. The nonlinear parameters largest Lyapunov exponent, correlation dimension, approximate entropy, and Hurst exponent indicate the complexity and chaotic dynamics of the system. The increased values of these parameters are related to pathological phenomena. Therefore, our results support the suggestion that caffeine and beverages containing caffeine and taurine have adverse effects on the cardiovascular system and may induce heart-related diseases. The central nervous system controls the heart from beat-to-beat through the vagus nerve and modulates heart rate. HRV is one of the major predictors utilized in a wide spectrum of cardiological and non-cardiological diseases. A high heart rate can cause tachycardia and also sudden cardiac death [22].