The muscular activity associated with training and competition results in a rapid increase in body temperature. The only way for horses to lose this excess body heat is via sweat production; in fact efficient muscle function is dependent on it.
Sweat also represents the major route of fluid and electrolyte loss in horses, taking with it significant amounts of water and electrolytes from the body.
 
Electrolyte composition of sweat
Chloride – 7gms/litre
Sodium – 4gms/litre
Potassium – 1.3gms/litre
Magnesium – .13gms/litre
Calcium – .25gms/litre
Phosphorus – .03gms/litre
 
Obviously these electrolytes must be replaced as quickly and efficiently as possible