## 4 Discussion
The post-test glucose values were more stable in the fasting participant, while the control participant showed greater fluctuation in their results. During fasting, glucose metabolism is adapted to maintain glycogen reserves [32]. Although glucose is not the primary fuel source in low-intensity ultraendurance tests, glycemic variability and hypoglycemia during the tests may be related to the demands of the test, glucose uptake, and inadequate nutritional strategies. Additionally, highly trained ultraendurance athletes who are keto-adapted have been found to have greater capacity for fat oxidation and glycogen preservation [33]. Therefore, in ultraendurance tests, diet and fasting induce a series of adaptations that are crucial for optimizing energy reserve economy. In this regard, the observed lactate values are generally low, except for specific efforts. Lactate has been observed as an energy substrate in low-intensity ultraendurance tests, which is reflected in the low values observe. Regarding muscle damage, higher creatine kinase (CK) levels were observed in the fasting participant. The literature shows that in half-marathon and marathon races, maximum CK values are obtained 24 h after completion. However, in 166 km races, maximum values are obtained after the event's completion [34]. In a narrative review, [PERSON] states that the ketogenic diet decreases CK levels after endurance exercise, such as 24 h after exhaustive exercise [35]. [PERSON] et al., in their review, determine that further research is needed in the field of dietary supplementation to attenuate muscle damage [36]. Therefore, we cannot determine whether the increase in CK in the fasting participant is simply due to a lower training status or could be related to the fasting situation. In the present investigation, we observed a decrease in lower limb strength at the end of each day. There was also a decrease in pre-test results for the last two days in the fasting participant. However, the control participant experienced a greater post-test decrease in hand strength, and for the pre-test, on the third and fourth days. Mountain ultraendurance tests have shown a decrease in leg strength, as the lower body is the most involved in this type of event [13], although this decrease is not universal. An increase in hand strength has been observed as a result of sympathetic activation [12], but this does not explain the decrease in hand strength for the control participant. The way the 10 kg weight was carried may have been a key factor, as the control participant relieved the weight of the backpack by using hand strength. Regarding leg strength, the significant decline in the fasting participant could be related to the lower supply of carbohydrate substrates to the muscle during the test, leading to the quicker onset of fatigue [22]. In addition to carbohydrates, the literature emphasizes the importance of considering fat-rich foods during the race [37]. Therefore, insufficient intake can negatively impact the performance and recovery of ultraendurance athletes in multi-stage events. The continuous effort over the four-day trial resulted in a significant weight decrease in the fasting participant compared to the control participant. The literature demonstrates that ultraendurance events substantially reduce the weight of athletes after the completion of the event [13]. This finding was consistent with the results obtained for the control participant. In contrast, we found that the fasting participant did not lose as much weight after the completion of the last two stages. Furthermore, on the last day before the trial, we observed that the control participant had lost 700 g, while the fasting participant had lost 2.7 kg compared to the first day. The literature concludes that exercise in a fasting state leads to increased adipose tissue lipolysis and peripheral fat oxidation [37].