## 4 Discussion
These physiological mechanisms provide crucial insights into why individuals with a higher RHR exhibit increased HOMA-IR levels. In particular, the negative correlation between cholesterol levels and the RHR is intriguing and may significantly contribute to the current literature. According to the research findings, total cholesterol (TC), high-density lipoprotein (HDL), and low-density lipoprotein (LDL) levels did not show a significant relationship with the RHR. However, in the group with an RHR of 80 bpm or higher, 15.5% had triglyceride (TG) levels of200 mg/dL or higher, which was statistically significant. This suggests that individuals with lower cholesterol levels tend to have a lower RHR. Particularly, higher TG levels may increase sympathetic nervous system activation, leading to a higher RHR. This indicates a complex interaction between lipid metabolism and autonomic nervous system activity, necessitating further research into these interactions. Other previous studies have also mentioned that demographic characteristics (e.g., age, gender) and lifestyle factors (e.g., smoking status, alcohol consumption) are associated with the RHR, consistent with our findings. For example, older adults generally have a higher RHR, and smokers and drinkers have a higher RHR compared to non-smokers and non-drinkers [25]. Our study also found that these factors support the association with the RHR. Another study contends that overactivation of the sympathetic nervous system is associated with stimulation of the renin-angiotensin-aldosterone system (RAAS), which can lead to insulin resistance and eventually diabetes [31]. In this study, activation of the sympathetic nervous system in relation to an elevated RHR is hypothesized to inhibit insulin in pancreatic beta cells and activate the RAAS to increase and cause insulin resistance [19], but further investigations into the physiological mechanisms are needed. This also raises other questions about the mechanisms driving the association between the RHR and HOMA-IR. The RHR is commonly used as a marker of cardiovascular fitness in epidemiologic studies. A low RHR is often observed in athletes and people engaging in endurance exercise and is attributed to an increased one-time cardiac output. The independent role of cardiovascular fitness has been noted in other studies, and the findings of the present study imply that this effect may be attributed in part to the effect of cardiovascular fitness on insulin sensitivity [32]. Such a diversity of opinion needs to be considered, and further studies are needed to explore how the relationship between cardiovascular fitness and insulin sensitivity may influence the correlation between the RHR and HOMA-IR. Although this study suggests that the RHR may be an important predictor of insulin resistance, a more comprehensive approach that considers the role of cardiovascular fitness is required. Therefore, further studies including variables related to cardiovascular fitness may contribute to the accurate prediction and management of insulin resistance. There are some limitations to interpreting the results of this study. First, as this is a cross-sectional study using the KNHANES data, no causal inferences can be made. There is also the possibility of under- or over-measurement as the survey progresses. Second, not all influences on HOMA-IR could be considered outside of the variables included in the analysis. Third, the cardiovascular fitness levels were directly measured in assessing the impact of the RHR on insulin resistance. Nevertheless, this study is significant in that it analyzed the association between the RHR and HOMA-IR using the KNHANES data, which are representative and reliable. In conclusion, the findings of this study showed that an increase in the RHR significantly increased the odds of having a HOMA-IR \(\geq\) 2.5. To improve fitness in relation to the RHR, lifestyle modifications should be made with encouragement to engage in regular exercise.