Minneapolis, Apr 25: When military divers surface from an underwater mission, what they drink matters less than simply making rehydration a top priority before they continue with land-based operations. Researchers will present these findings at the 2026 American Physiology Summit in Minneapolis. The Summit is the flagship annual meeting of the American Physiological Society (APS).
Combat divers on a mission may spend up to four hours in the water followed by on-the-ground assignments. Dehydration can cause a host of physical and psychological symptoms and increases strain on the cardiovascular and renal systems. These factors can interfere with post-dive performance.
Becoming dehydrated while diving may seem counterintuitive, but it’s a true hazard due to changes in hydrostatic pressure, or the pressure exerted on the body. When immersed in water, hydrostatic pressure around the body increases. This makes the body think it’s overhydrated, triggering urine production to remove excess fluid. After getting out of the water, the hydrostatic pressure returns to normal, which causes dehydration.
Researchers analyzed the hydration status of 36 participants over the course of three separate four-hour dives, each followed by aerobic exercise tests. The divers received either no fluid replacement, replacement equal to half of the fluid they lost through urination or full fluid replacement. In each trial, the volunteers drank either plain water, a drink similar to a standard sports drink or a customized beverage with higher electrolyte levels than a sports drink, to rehydrate.
Total fluid replacement restored performance levels better than partial or no replacement, but the beverage composition did not make a significant difference. In other words, hydrating was more important than what the divers drank. These results could be “certainly translational to any type of exercise, training or athletic event,” said Hayden Hess, PhD, ATC, CSCS, corresponding researcher of the study.
Although replacing lost fluids, regardless of the type of beverage, is of utmost importance, there is one caveat: Divers and athletes “should avoid replacing fluids with drinks that they have not trained or practiced with, or are known to induce gastrointestinal distress,” Hess said. Sugary drinks or those with a high sodium content can often cause unpleasant gastrointestinal symptoms.
