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232almost identical14). In this study, SpO2 was decreased by ≥ 3% in both groups, indicating that the partic-ipants exhibited EIAH. A previous study reported that maximal performance capacity is impaired in highly trained cyclists working under 87% SaO2, but not under a milder desaturation level of 90%15). Arterial desaturation occurs in healthy, highly trained endurance athletes during heavy exercise, and the level of arterial desaturation is inversely related to maximal oxygen consumption (VO2max)10). Although drinking oxygenated water did not protect against EIAH in the present study, SpO2 was maintained at 90%. Given that exhaustion caused by EIAH shortens the exercise time and decreased SaO2 affects muscle fatigue16), drinking oxygenated water may protect athletes from performance decreases.Environments with low SpO2 include high-alti-tude situations, such as in-flight or alpine environ-ments. SpO2 decreases during exercise in a hypoxic environment and results in a lack of oxygen supply to tissues, leading to illness or fatigue17). Reports on mountaineering exercise show that SpO2 decreases and PR increases, particularly when physical condi-tions are not good. Understanding the hypoxic state of the body and managing and assessing its physical condition are effective for preventing acute mountain sickness14, 18). Reducing the decrease in SpO2 by drinking oxygenated water may prevent acute mountain sickness, prolong walking time, and increase walking speed. Similarly, in in-flight envi-ronments (altitude of 40,000 ft), the oxygen concen-tration is approximately 16%, and SpO2 is reported to decrease in individuals with respiratory dysfunc-tion19). Even in healthy individuals, SpO2 decreases significantly in in-flight environments20). Therefore, drinking oxygenated water before or during flight may suppress SpO2 reduction and help maintain physical condition.Oxygen is mainly absorbed into the body by inhalation. However, a previous study in pigs demon-strated that administering oxygenated water into the stomach increased SaO2. These results suggest that the administration of oxygenated water could be an alternative route of oxygen absorption21). In the present study, oxygenated water with an oxygen concentration of 110 ppm (110 mg O2/L) was used, but the speed of absorption into the body was unclear. In a previous study22), different concentra-tions of oxygenated water (40, 80, and 150 mg O2/L) were intraperitoneally injected in rabbits, and the authors demonstrated that different oxygen concentrations have different absorption speeds. Optimizing oxygen levels in water could potentially lead to better outcomes, and further research on this topic is warranted. Our data demonstrated that PR was higher in the OX group compared with the control group. We hypothesized that SpO2 decreases in hypoxic environments, as demonstrated in a previous study23). A previous study involving a walking exercise experiment reported that pulse oximeter measurements in the hand may provide false read-ings because of body movement, and this effect is more likely to be seen in the measurement of PR compared with that of SpO₂24). Alternatively, it is possible that our findings were caused by differ-ences in the physical abilities of the participants. Although participants were randomly assigned and their baseline demographic data were similar, some of the participants in the OX group may have had inferior physical abilities. However, we monitored exercise intensity using the Borg scale12, 25), and walking distances did not differ between groups. Although it is unclear why participants in the OX group had higher PR, it is important to be aware of this phenomenon to identify individuals who might experience harmful effects of increased PR. One strength of the current study is the experi-mental design involving a randomized placebo-con-trolled single-blinded trial, which would be expected to reduce the effects of bias. However, this study also involved several limitations. First, the sample size was small. Although a small sample size can result in type 2 error, it is unlikely that this error occurred in the current study because the differences between the groups were statisti-cally significant. Second, the participant selection method may have affected the results. We enrolled young adults aged 20-29 years. Although the changes in younger and older populations were not investigated, changes in SpO2 and PR may be more prominent in older participants. Third, the effects of oxygenated water under normal conditions remain unclear. In conclusion, the current study demonstrated that SpO2 was decreased and PR was increased under normobaric hypoxic condi-tions. Moreover, consumption of oxygenated water