Horses in work sweat to keep themselves cool. But in that process, they lose essential electrolytes that are necessary for body function, including healthy hydration. 

Regain® is a daily electrolyte replacement for horses in light to intense work, or in hot dry conditions. It provides the electrolytes that horses most often lose in their daily work and supports better overall health. 

Sodium and Chloride: the main components of sweat and help support hydration
Potassium: One of main factors in maintaining good hydration, and together with sodium help to maintain a good fluid balance in the body
Calcium and Magnesium: Important for muscle contraction and relaxation, and together support healthy muscle function.