Hydration considerations for combat divers

Diving induces diuresis which can result in marked dehydration following the dive. In combat diving scenarios requiring prolonged dive exposures followed by terrestrial operations, this dehydration can negatively impact performance. In a series of studies, we examined several rehydration strategies for restoring performance following prolonged exposure. Our data provide valuable insights into countermeasures that can be issued depending on the dive profile and operational conditions. PURPOSE: To determine the optimal rehydration strategy following prolonged water immersion to restore post-immersion performance across a range of performance assessments. To do so, we tested the hypothesis that full fluid replacement would restore post-immersion performance compared to partial or no fluid replacement following prolonged water immersion. METHODS: Thirty-six subjects completed baseline performance testing (i.e., non-immersed control) and up to three experimental trials (90 total observations) consisting of a four-hour water immersion (10°C [n=9], 18°C [n=18], 20°C [n=9]) followed by an aerobic performance test (time trial [n=9], time to exhaustion at 85%VO 2 max [n=9], or an intermittent exercise performance test [n=18]). Following immersion, subjects received no fluid replacement (33 observations), partial fluid replacement (1L or 50% replacement [15 observations]) or full fluid replacement (42 observations). To better understand the role of fluid composition, in a subset of full fluid replacement trials, subjects received either plain water, a 6.5% CHO drink, or a 6.5% CHO+50 mmol/L NaCl drink. Performance was quantified as a percentage change from baseline (i.e., non-immersed control). Immersion-induced diuresis was quantified by the total urine output during each immersion and percentage change in body mass provided an index of post-immersion hydration status. Data were analyzed using one-way linear mixed models and data are presented as mean±SD. RESULTS: Total urine output across the four-hour immersion was higher in 10°C (1.7±0.5 L) compared to 20°C (1.4±0.4 L; p=0.04), but 18°C was not different from either 10 or 20°C (1.5±0.4 L; p>0.23). However, this did not result in different post-immersion hydration status as a function of water temperature (10°C: -2.1±0.4%; 18°C: -2.2±0.6%; 20°C: -2.4±0.9%; p=0.46) and all conditions resulted in >2% body mass loss (i.e., dehydration) after immersion. Compared to baseline, full fluid replacement restored performance (-3.7±9.7%) compared to both partial (-15.8±13.9%; p< 0.01) and no fluid replacement (-16.8±9.8%; p< 0.01). Performance was not modified by fluid composition (water: -1.7±5.6%; CHO: -2.4±7.7%; CHO+NaCl: -5.7±13%; p=0.30). CONCLUSION: Across the series of studies, four hours of water immersion results in dehydration (i.e., >2% body mass loss) and reduced performance across of range of parameters (e.g., duration, intensity, continuous or intermittent). Full fluid replacement appears to be the best strategy for restoring performance compared to either partial or no fluid replacement. Additionally, our data suggests that fluid replacement, rather than fluid composition, is more important. Thus, when considering operation logistics, combat divers should focus on replacement >50% volume loss during the dive. Supported by Naval Sea Systems Command N00024-15-C-4105 and N00024-20-C-4307 This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.