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
During endurance exercise performed by human athletes and horses, body fluids are lost primarily in the form of sweat. In contrast, dogs competing in endurance events lose fluids in the form of respiratory water loss and via urine (Figure 1). Sweating is essential for effective thermoregulation in both human athletes and horses but in both species sweating rates are generally excessive. That is, if all sweat produced was fully evaporated, the cooling effect would far exceed the metabolic heat load generated. However, not all sweat produced evaporates as large amounts can drip from the body and the evaporative cooling potential of this "wasted" sweat is lost. This loss of sweat is exacerbated when evaporation is compromised by ambient conditions of high humidity, due to the already high atmospheric water vapor pressure that limits the gradient between be the skin surface and the surrounding air.
In horses, sweating rates during endurance exercise are directly related to core body temperature and have been demonstrated to remain fairly steady during treadmill exercise, despite lack of fluid replacement. With varying endurance work intensity (and increases in core body temperature) under moderate ambient conditions, total sweat losses during treadmill exercise bouts of 45 km have ranged from 3-7% of body mass. Similarly, field studies of endurance rides have also shown body mass losses ranging from 3-7% by the end of 80- to160-km rides. During actual competition horses are offered water and feed to promote fluid and fuel replacement at multiple times during the rides and in field studies in which body mass loss has been measured at multiple times during the rides, a consistent finding has been that the majority of body mass loss occurs during the first half of the competition and that body mass remains fairly steady from that point forward. Because body fluid in sweat continues throughout exercise, maintenance of body mass during the later stages of the ride can only be explained by water and feed intake at a rate matching ongoing fluid losses. Again, the net loss of body fluid that is sustained by endurance athletes can be characterized as involuntary dehydration because it occurs in the face of readily available rehydration fluids.
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.