Body weight, fluid, electrolyte, and hormonal changes in horses competing in 50- and 100-mile endurance rides

To investigate effects of prolonged exercise on fluid and electrolyte losses in horses competing in 50- and 100-mile endurance competitions, with emphasis on recovery.

Changes in body weight (BW); PCV; serum osmolality; plasma total protein, lactate, aldosterone, and serum electrolyte concentrations; and exchangeable cation content were measured in 12 and 7 horses before and after and before, during, and after successful completion of 50- and 100-mile endurance rides, respectively.

BW was measured by use of a portable load bar scale, and blood samples were collected during the hour before ride start, at ride finish, and after approximately 2 and 18 hours (overnight) of recovery for horses competing in the 50-mile ride. For horses competing in the 100-mile ride, BW was measured and blood samples were collected at the start; after 50, 67, and 84 miles of the ride; at the finish; and after approximately 12 hours (overnight) of recovery.

BW decreased by (mean +/- SEM) 3.6 +/- 0.0% and 4.9 +/- 0.8% in horses that successfully completed rides of 50 and 100 miles, respectively. After the overnight recovery period, BW was 4.3 +/- 0.5% and 3.9 +/- 0.8% lower than preride values for horses performing the 50- and 100-mile rides, respectively. A decrease in plasma volume during the ride was reflected by an increase in plasma total protein concentration, but both measures returned to preride values after overnight recovery. Serum osmolality and serum electrolyte concentrations decreased and aldosterone concentration increased during prolonged exercise. Aldosterone concentration peaked after overnight recovery.

Despite apparent rapid return of plasma volume and ionic composition to near normal values, substantial depletion of body fluid and electrolyte stores persists after an overnight recovery period in horses that successfully complete 50 or 100 miles of endurance competition.