## 4 Discussion
The results of our study showed that serum 25-(OH) D (60.22%, \(p<0.01\)), and intact PTH (23.86%, \(p<0.05\)) were increased significantly in HWE between pre- and post-tests, whereas OPN (\(-20.45\%\), \(p<0.05\)) was decreased significantly in HWE between pre- and post-tests. Furthermore, OC (\(-7.89\%\), \(p<0.05\)) and calcium (\(-2.99\%\), \(p<0.01\)) were decreased significantly in CON between pre- and post-tests, whereas CTX (30.94%, \(p<0.05\)) was increased significantly in CON between pre- and post-tests. However, no significant interaction effects were found on other markers including intact PTH, OPG, RANKL, and OPN. Vitamin D plays a critical role in promoting the intestinal absorption of phosphate and calcium [57], and participating in regular physical activity is a well-known stimulator of serum 25-(OH) D [58]. In accordance with our results, [PERSON] et al. [55] reported that the serum 25-(OH) D increased significantly following 12 weeks of resistance training in young women, but the level of PTH was not significantly changed. It is an interesting result in the present study where the HWE group increased both vitamin D and PTH level after four-months of exercise intervention. Generally, serum calcium level is mediated by vitamin D and PTH [59] and the changes in these levels may increase or decrease the mineralization of bone. It is assumed that an increase in PTH level may help to maintain serum calcium level. In particular, our participants were healthy and had no bone-related disease, thus it is possible that PTH is increased for calcium homeostasis. [PERSON] and [PERSON] [56] reported that resistance circuit training for 12 weeks significantly increased PTH in healthy and sedentary young women, which agrees with the present result. Authors assumed that this increase in PTH after resistance training may promote anabolic activities in bone metabolism. However, it is difficult to explain the direct relations or physiological mechanism between the variables in the present study and conflicting results have been observed in previous studies. [PERSON] et al. [60] reported that Nordic walking training for six weeks significantly decreased serum 25-(OH) D in post-menopausal women and [PERSON] et al. [54] reported that a recruit training program for four-months significantly decreased serum 25-(OH) D in men, whereas these values remained at baseline levels in women. Vitamin D is an important factor for bone health, and its deficiency leads to osteoporosis and CVD in adults [61; 62]. Moreover, physically inactive individuals have two times lower vitamin D levels than active adults [63]. Although it is difficult to determine the direct relations between HWE and increased serum 25-(OH) D, several studies support our results that increase in physical activity is an effective manner to maintain optimal vitamin D level in later life [64; 65]. In this study, the serum 25-(OH) D increased from an inadequate level (12-20 ng\(\cdot\)mL\({}^{-1}\)) to a sufficient level (\(\geq\)20 ng\(\cdot\)mL\({}^{-1}\)) in HWE. Bone turnover markers provide information on important biochemical indicators such as bone resorption and bone formation in managing osteoporosis [66]. A significant association between bone turnover markers and the risk of fractures has been identified for females and males [67]. [PERSON] et al. [68] reported that soccer training for 15 weekssignificantly increased plasma OC (37 \(\pm\) 15%) and CTX (42 \(\pm\) 18%) in middle-aged premenopausal women. A significant increase in OC has also been observed in untrained young women following sixteen weeks of exercise intervention [69]. Additionally, exercise training caused augmentation in resting plasma OC and CTX in elderly sedentary men with no resistance training [70]. However, CTX has been shown to increase shortly or immediately after impact exercise [26]. Thus, impact exercise may have a high potential to improve bone health in the general population.