Military divers often operate in cold water eliciting reductions in skin and core temperatures despite the use of thermal protection. Decreases in body temperatures triggers shivering and non-shivering thermogenesis, the latter of which is largely due to the activation of brown adipose tissue (BAT). Ten days of cool air exposure (15-16°C) and seven days of cold-water exposure (14°C) augments non-shivering thermogenesis. However, it is unknown if repeated exposure to cool head out water immersion (CWI) modifies non-shivering thermogenesis. This study tested the hypothesis that fourteen days of CWI for 4 h/d will increase BAT activation. Healthy adults were randomly assigned to either CWI (n=10, 5 women) or thermoneutral head out water immersion (35°C, TNW, n=9, 5 women). CWI and TNW exposures were 4 h in duration and were completed once daily for 14 d in a 16-d period. CWI consisted of the initial immersion in 28°C water and following a 0.5°C reduction in rectal temperature, the water temperature was increased to up to 30°C so that a mild hypothermic state was maintained during the 4 h. Rectal temperature, oxygen uptake (indirect calorimetry), and supraclavicular and trapezius skin temperatures (thermocouples) were measured preexposure and during the middle 3 h of immersion. This analysis was a part of a larger study. Therefore, during the 3 h period, all participants were exposed to hypoxia (FiO2: 0.14). The supraclavicular-trapezius skin temperature difference (Tsk diff) provided an index of BAT activation. Data collected on Day 1 and Day 14 are presented (mean ± SD). Peak decreases in rectal temperature from preexposure were greater in CWI (-0.7±0.3°C) vs. TNW (-0.2±0.4°C, p<0.001) and did not differ between Day 1 and Day 14 in either group (p=0.923). Average oxygen uptake was higher in CWI (0.54±0.09 L/min) vs. TNW (0.38±0.12 L/min, p<0.001) and did not differ between Day 1 and Day 14 in either group (p=0.443). Average Tsk diff did not differ between groups (CWI: 2.2±0.8°C, TNW: 1.6±0.1.1°C, p=0.090) and did not differ between Day 1 and Day 14 in either group (p=0.111). Peak decreases in rectal temperature were significantly correlated with average oxygen uptake (r=-0.552, p<0.001) but not average Tsk diff (r=-0.152, p=0.362). Fourteen days of CWI did not modify thermogenesis (oxygen uptake) nor BAT activation, as estimated from Tsk diff. However, a potential interaction between moderate hypoxia and BAT activation during CWI cannot be excluded. Future work is required to determine the effect of mild hypothermia, such as that experienced by divers operating in cold water even with thermal protection, on more direct markers of BAT activation. Funding: Offce of Naval Research (N00014-20-1-2593 & N00014-21-1-2276). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Breath hold divers have extraordinary tolerance to low levels of oxygen and high levels of carbon dioxide. Training induced improvements in apnea performance are speculated to be, at least partially, due to increases in oxygen carrying and pH buffering capacities, which may improve pulmonary gas exchange. However, it is unclear whether relatively brief (<4 h) daily exposures to moderate hypoxia and maximal apneas, such as may occur with diving, modifies pulmonary gas exchange efficiency (GEE) and function in people without breath hold diving experience. PURPOSE: Test the hypothesis that 14 days of daily 3 h of moderate hypoxia and 10 maximal apneas augments pulmonary GEE and function. METHODS: 17 healthy adults (27 ± 6 y; 11 women) completed 14 days of training within a 16-day period. Training days consisted of 10 maximal apneas and 3 h of exposure to normobaric hypoxia (FiO2 = 0.14). On Days 1, 7, 14 subjects completed pulmonary function testing, which included measurements of forced vital capacity (FVC), forced expiratory volume in one second (FEV1), FEV1/FVC, force expiratory flow at 25-75% (FEF25-75%) and peak expiratory flow (PEF), and a noninvasive assessment of pulmonary GEE in which the primary outcome variable was the alveolar-arterial O2 difference (O2 debt). Data are presented as mean ± SD. RESULTS: Apnea duration increased from Day 1 (69 ± 29 s) to Day 7 (83 ± 30 s, p < 0.01) and to Day 14 (90 ± 29 s, p < 0.01), but there was no change from Day 7 to Day 14 (p = 0.26). There were no changes in FVC (main effect: p = 0.14), FEV1 (main effect: p = 0.29), FEV1/FVC (main effect: p = 0.29), or FEF25-75% (main effect: p = 0.21). PEF increased from Day 1 (7.5 ± 1.9 L/min) to Day 7 (8.0 ± 1.9 L/min, p = 0.03) and Day 14 (8.4 ± 1.5 L/min, p = 0.01), but was not changed from Day 7 to Day 14 (p = 0.13). O2 debt did not differ (main effect: p = 0.25) between Days 1 (-0.7 ± 6.1 mmHg), 7 (2.3 ± 7.0 mmHg) and 14 (0.2 ± 6.5 mmHg). CONCLUSIONS: PEF increased with as little as 7 days of moderate hypoxia and maximal apnea training. However, no other changes in pulmonary function or gas exchange efficiency were observed. Further research is required to better understand how relatively brief daily exposures to moderate hypoxia and maximal apneas improves breath-hold capabilities FUNDING: Office of Naval Research (N00014-20-1-2593)