Sodium, Potassium, and Chloride in Exercising Horse Diets

Most forages for horses are high in potassium and have a sufficient amount of chloride. Therefore, when provided with forage at the recommended rate of at least 1-2% of body weight, maintenance potassium requirements can be met by most idle equines. Sodium content of grass and alfalfa (lucerne) is usually low, so there is greater risk of a deficiency of sodium than of chloride or potassium based on dietary intake.

The apparent digestibilities of sodium, potassium, and chloride are approximately 90, 80, and 90%, respectively, indicating the efficient utilization of these macrominerals by the horse. Higher intakes of sodium and chloride also increase calcium and phosphorus absorption and retention.

Provision of sodium and chloride requirements for nonexercised horses is often met by access to a salt block. Voluntary salt consumption normally increases in hot and humid weather and may increase if horses are on pasture during periods of plant growth. A small number of horses, when restricted to box stalls and allowed free access to salt, consume excess quantities of salt as a result of boredom.

In exercising horses, muscular activity results in substantial energy expenditure. However, the process of conversion of metabolic energy to mechanical work is only 20 to 25% efficient with the result that 75 to 80% of the chemical energy is converted to heat within skeletal muscle cells.

As in human athletes, the principal mechanism of heat loss in the horse during exercise is evaporation, primarily in the form of sweating. A heavily exercised horse can lose as much as 10 to 15 liters of sweat per hour. Therefore, prolonged or strenuous exercise results in significant fluid losses and, in addition, extensive losses of ions, predominantly sodium, potassium, and chloride.

Several factors affect the extent of sweat fluid and ion losses associated with exercise. Sweating rate, which can be used to estimate the rate of fluid and ion losses, is largely determined by the duration and intensity of exercise undertaken. Other factors such as terrain, footing, the weight of the rider and tack, training status, and ambient temperature can contribute to the horse’s work output and the need to increase dissipation of heat via sweating.

Total sweat fluid losses in hot, dry conditions may be double the amount lost in cool, dry conditions. Heavy sweat production can strip the horse of ions at a higher rate than what is provided by unsupplemented diets.

For horses in training and competition, it is important to replace these minerals promptly, as ongoing heavy sweat losses without adequate replacement can result in electrolyte imbalances or depletion that contributes to poor exercise performance. A commercial electrolyte supplement such as Restore SR (in Australia, look for Restore) can be provided to meet these additional needs; composition of such supplements should be carefully evaluated to ensure the specific needs of a particular horse or group of horses are met.