In human and equine athletes, loss of body fluid in sweat during prolonged exercise exceeds voluntary fluid replacement. This leads to a condition that has been termed involuntary dehydration. The magnitude of dehydration can be estimated by measuring body mass loss during the endurance event. This body mass loss usually persists for hours to days in the recovery period and water and electrolyte deficits are usually not fully replaced until one or more meals have been consumed following the exercise bout. Unfortunately, loss of body water and electrolytes can contribute to development of several medical or "metabolic" problems either during the exercise bout or even after successful completion of competition. Dehydration has long been considered the most important risk factor for development of medical problems in equine endurance athletes. However, recent evidence suggests that other factors, including a decrease in effective circulating volume and autonomic dysregulation leading to compromised tissue perfusion, may be equally if not more important factors.

Body fluid loss during endurance exercise

Figure 1 Humans, dogs, and horses all compete in endurance exercise events during which deficits in body water and electrolyte content may develop. However, mechanisms of thermoregulation and maintenance of body fluids and effective circulating volume vary between these species. Humans and horses utilize evaporative cooling via sweating while dogs thermoregulate via respiration (panting). Human sweat becomes hypotonic with endurance training while horse sweat remains nearly isotonic due to species differences in sweat glands. In contrast, dogs primarily lose water via evaporative cooling across the respiratory tract. Thus, the magnitude of electrolyte depletion that may develop during prolonged endurance competition also varies with species.