Hypohydration impairs both dry and evaporative heat loss (or, if the air is warmer than the skin, dehydration aggravates dry heat gain). Hypohydration delays sweating onset and skin vasodilitation. It also reduces sweating sensitivity. Hypohydration may be associated with either reduced or unchanged sweating rates at a given metabolic rate in the heat. The physiological mechanisms mediating the reduced dry and evaporative heat loss from hypohydration include both the separate and combined effects of plasma hyperosmolality and reduced blood volume.

Hypohydration and Fatigue
A common complaint of hypohydrated persons is skeletal muscle fatigue; however, little research had been conducted to address whether hypohydration reduces skeletal muscle performance (in absence of heat stress). Recent research at our laboratory demonstrated that, in temperate conditions, hypohydration (4 percent body weight loss) reduced single-leg knee endurance time by 18 percent compared with euhydration. The mechanism(s) responsible for this are unclear, as hypohydration does not seem to markedly alter skeletal muscle glycogen utilization. To study possible mechanism(s), subjects are repeating these exercise experiments inside of a nuclear magnetic resonance (NMR) magnet and 3'P spectra are being collected. It is hypothesized that hypohydration might accelerate depletion of adenosine triphosphate (ATP) and PCr or impair the ability to buffer hydrogen and Pj ions produced during exercise.

Hyperhydration
Hyperhydration, or greater than normal body water, has been suggested to improve, above euhydration levels, thermoregulation and exercise-heat performance. The concept that hyperhydration might be beneficial for exercise performance arose from the adverse consequences of hypohydration. It was theorized that body water expansion might reduce the cardiovascular and thermal strain of exercise by expanding blood volume and reducing blood tonicity, thereby improving exercise performance. Studies that have directly expanded blood volume (e.g., infusion) have usually reported decreased cardiovascular strain during exercise, but have reported disparate results on heat dissipation and exercise-heat performance. Studies that have attenuated plasma hyperosmolality during exerciseheat stress generally report improved heat dissipation, but have not addressed exercise performance.

Ten studies have been published that evaluated hyperhydration effects on thermoregulation in the heat. Briefly, 6 of 10 studies observed smaller core temperature increases during exercise with hyperhydration. Together, these studies support the notion that hyperhydration can provide a thermoregulatory benefit; however, most of these studies suffer from serious experimental design flaws. Examination of their data indicates that "control" conditions generally do not represent euhydration, and that there may have been order problems so that subjects were more heat acclimated during hyperhydration trials. Recent studies at our laboratory have controlled for these confounding factors, and observed no thermal advantage with either water hyperhydration or glycerol hyperhydration during exercise-heat stress.

1. Adolph EF, et al. Physiology of man in the desert. New York: Interscience, 1947.

2. Committee on Military Nutrition. In: Marriott BM, ed. Nutritional needs in hot environments. Institute of Medicine, Washington DC: National Academy Press, 1993.