Thermoneutral water immersion and hyperbaric oxygen do not alter cortisol regulation

Prior research has shown mixed results regarding how hyperbaric oxygen affects cortisol levels. Some studies suggest a rise in cortisol, while others find no significant change. This inconsistency may stem from uncontrolled variables in earlier experiments. It was already known that cortisol follows a natural daily rhythm, which can influence measurements. No prior work had resolved whether the changes observed in cortisol were due to the experimental conditions or the body's natural patterns. This gap motivated the current study to examine cortisol responses under tightly controlled conditions. Researchers aimed to eliminate confounding factors by using thermoneutral water immersion and hyperbaric oxygen in a consistent manner. The goal was to determine if these conditions alone could alter cortisol regulation.

The aim of this study was to assess how repeated exposure to thermoneutral water immersion and hyperbaric oxygen affects cortisol levels. Researchers wanted to isolate the effects of these conditions from other potential influences. They focused on both short- and long-term cortisol responses over five consecutive days. The study targeted Navy divers as a population accustomed to such environments. Participants were divided into three groups to compare different exposure types. Each group experienced a specific condition at a fixed pressure of 1.35 atm abs. The researchers measured cortisol before and after each session to track changes. This approach allowed them to evaluate whether the observed effects were consistent across all experimental conditions.

The study involved 32 Navy divers assigned to one of three experimental groups. Each group was exposed to a different condition for six hours over five consecutive days. One group breathed air while immersed in water. A second group breathed 100% oxygen in a dry hyperbaric chamber. The third group breathed 100% oxygen while immersed in water. All participants remained at rest during the sessions. Serum cortisol levels were measured one hour before and after each dive. Researchers calculated the change in cortisol concentration for each condition. They compared the results across all three groups to assess differences. The study controlled for variables like temperature and activity level to minimize external influences.

The change in cortisol levels after each dive was similar across all three groups. The air group showed an increase of 3.63 ± 5.56 µg/dL. The dry hyperbaric group had a rise of 4.91 ± 3.68 µg/dL. The immersed oxygen group experienced a 3.50 ± 3.48 µg/dL increase. These differences were not statistically significant (p > 0.05). No group showed a consistent trend in cortisol levels across the five days. Pre- and post-dive concentrations remained stable throughout the study. The results suggest that the observed changes were not due to the experimental conditions. Instead, they likely reflect the body's natural circadian rhythm of cortisol.

This study provides evidence that repeated exposure to thermoneutral water immersion and hyperbaric oxygen at 1.35 atm abs does not abnormally alter cortisol levels. The observed changes were not significantly different between the three experimental conditions. The lack of variation across days suggests no long-term effect on cortisol regulation. The authors propose that the changes seen are likely due to the body's natural daily rhythm. No prior work had resolved whether these conditions alone could disrupt cortisol. The study controlled for confounding factors to isolate the effects of immersion and oxygen. The findings suggest that these conditions, under ideal circumstances, do not disrupt cortisol regulation. The results highlight the importance of considering circadian patterns in future research.