The Role of Hydration Status in Diver Performance

Principal Investigator: David Hostler

Funding Agency: Navy Sea Systems

Period: 06/15 - 06/18

Abstract: This study will evaluate the effects of rehydration strategy on terrestrial performance and overnight recovery following immersion induced dehydration. Previous experimental approaches have assumed that water immersion led to dehydration by an immersion-induced diuresis that was thought to negatively affect exercise performance both in water and after regress. Data from our lab and others suggests that the immersion-induced diuresis does reduce the plasma volume when autotransfusion forces water from the intracellular and interstitial spaces into the intravascular space but does not lead to a true dehydration and will not compromise the diver while in water. While immersed, a new homeostatic set point is reached and the diver is effectively euhydrated as long as he remains in the water. Prehydration or rehydrating while immersed in water before this new set point is achieved would result in a plasma volume expansion that would cause further diuresis. After regressing from the water, however, there is a rapid and significant drop in plasma volume as the water leaves the vascular compartment and returns to the interstitial space and cells resulting in hypohydration. This drop in plasma volume after returning to land could compromise venous return and thus cardiac output, especially during exercise.

While hypohydration beyond a certain level impairs performance, overhydration is also undesirable. The doctrine of “drinking to stay ahead of thirst” has been associated with exercise-associated hyponatremia, which has occasionally proven fatal in soldiers and, in divers, carries the additional risk of pulmonary edema. The potential for pulmonary edema is complicated by the translocation of fluid to the thorax resulting in increased fluid and reduced volumes in the lung. Athletes and workers tend to voluntarily drink less fluid than is lost from sweating. Overhydration is almost always the result of bad training recommendations or misunderstanding fluid homeostasis by the individual. Body mass can be used to gauge sweat losses and fluid requirements in the short term but multiple studies have reported that soldiers operating in both cool and hot environments can drink according to individual desire (ad libitum) without experiencing excessive hypohydration or poor performance. Hydrating during immersion is analogous to attempting to hyperhydrate on land.

In aggregate, the previous studies performed for land-based activities demonstrate both knowing both when and how much to drink during activity is important. With regards to rehydration, the scenario of a diver transitioning from water-based to terrestrial activity requires careful examination. Increased water loss from both cold and warm environments adds additional complexity and should be examined before recommendations for rehydration can be made.