South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
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.
Mechanism of involuntary dehydration: In both horses and human athletes, insufficient thirst,
due to concurrent loss of water and electrolytes in sweat, appears to be the most important
mechanism causing involuntary dehydration. However, two species differences appear to make
the magnitude of involuntary dehydration greater in equine endurance athletes. First, equine sweat
remains isotonic during prolonged exercise while human sweat becomes hypotonic, in
comparison to plasma tonicity. Thus, horses lose comparatively greater amounts of electrolytes in
each litre of sweat produced. As a consequence, plasma osmolality rises more slowly in horses
than human endurance athletes and because a rise in plasma tonicity is the most potent stimulus
for thirst, inhibition of thirst likely plays a greater role in horses than in human athletes. Second,
transcellular fluid reserves in the lumen of the equine gastrointestinal tract are substantially
greater (12-15% of body mass) in comparison to those in human athletes (1-2% of body mass).
Thus, the equine athlete has a larger “fluid reserve” that can be called upon to replace sweat fluid
losses during endurance exercise. As a result of these species differences, horses appear to be able
to tolerate a relatively greater total loss of body fluid reserves during prolonged endurance
competition.
Exercise hyperthermia: In human athletes, exercise hyperthermia leading to excessive sweating
and eventual volume depletion was commonly assumed to be the most important mechanism
responsible for development of collapse and altered mentation during and after endurance
competition. This mechanism gained initial support as a consequence of a 1969 publication
authored by Wyndham and Strydom and entitled