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.
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.
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.