Dehydration: Physiology, Assessment, and Performance Effects

Abstract

This article provides a comprehensive review of dehydration assessment and presents a unique evaluation of the dehydration and performance literature. The importance of osmolality and volume are emphasized when discussing the physiology, assessment, and performance effects of dehydration. The underappreciated physiologic distinction between a loss of hypo-osmotic body water (intracellular dehydration) and an iso-osmotic loss of body water (extracellular dehydration) is presented and argued as the single most essential aspect of dehydration assessment. The importance of diagnostic and biological variation analyses to dehydration assessment methods is reviewed and their use in gauging the true potential of any dehydration assessment method highlighted. The necessity for establishing proper baselines is discussed, as is the magnitude of dehydration required to elicit reliable and detectable osmotic or volume-mediated compensatory physiologic responses. The discussion of physiologic responses further helps inform and explain our analysis of the literature suggesting a ≥ 2% dehydration threshold for impaired endurance exercise performance mediated by volume loss. In contrast, no clear threshold or plausible mechanism(s) support the marginal, but potentially important, impairment in strength, and power observed with dehydration. Similarly, the potential for dehydration to impair cognition appears small and related primarily to distraction or discomfort. The impact of dehydration on any particular sport skill or task is therefore likely dependent upon the makeup of the task itself (e.g., endurance, strength, cognitive, and motor skill).

National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active

“…Many laboratory studies have demonstrated a consistent association between hypohydration and decreased exercise performance. 180,181 However, when the same variables were monitored in some uncontrolled field studies, the data seemed to conflict.…”

Evaluation of ergonomic risk factors, acute traumatic injuries, and occupational exposures at an airline catering facility.

“…Performance can be affected by dehydration from sweat losses and fluid restriction or unavailability [Cheuvront and Kenefick 2014]. Work factors, such as physical exertion, hot environments, and cold environments can contribute to sweat losses.…”

“…Employees reported that they were permitted to drink water only on breaks, drinking water was not available in most work areas, the drinking fountain in the storeroom had been replaced with an eye wash, the water fountains located in the receiving area and the cafeteria did not meet the needs of employees in departments that did not have a drinking fountain, and open bottles of water were prohibited in work areas. Thirst is not a good indicator of dehydration and may be satisfied well before total body water is fully restored even when dehydration is substantial [Cheuvront and Kenefick 2014]. Employee reports of not having enough water to drink suggest that some employees were thirsty because of dehydration; however, additional employees might have been dehydrated without feeling thirsty.…”

“…These and other symptoms may be distracting and affect performance. Additionally, tasks that have an aerobic or endurance component, such as day-long manual labor, appear to be significantly impaired by dehydration [Cheuvront and Kenefick 2014]. Thus, the lack of access to drinking water in work areas led to involuntary fluid restriction, which could increase the risk of dehydration that could affect employees' work performance.…”

Why do team-sport athletes drink fluid in excess when exercising in cool conditions?

“…Over-drinking appears to be reported when athletes consume fluid in the absence of body fluid deficits to magnitudes which would stimulate the physiologically driven responses for compensatory water acquisition (≥2% change in plasma osmolality (POsm) and / or ≥10% change in blood volume) (Cheuvront et al 2013;Cheuvront and Kenefick 2014) and thus in excess of homeostatic needs.…”