by: Nancy S. Loving, DVM
Principles that will help you know when and how to supplement horses with electrolytes.
One topic that horse owners repeatedly wonder about is that of electrolytes and specifically how and when they should be used. In general, horses participating in most equestrian sports don't need electrolyte supplementation. A horse that sweats during a short training ride or competition might lose some body water and salts, but these losses are usually replenished quickly upon eating hay, drinking, or visiting the salt block. However, there are other situations, involving horses that compete in distance riding pursuits (endurance, competitive trail, or the cross-country phases of eventing or combined driving), for example, when electrolyte supplementation is important to the horse's safekeeping and welfare. Electrolytes are salts--notably potassium, chloride, sodium, calcium, and magnesium--and are essential to proper body function.
Research conducted at endurance and competitive trail rides has demonstrated that many horses experience the greatest loss of fluids and electrolytes within the first 20 miles of exercise. Long-distance transport to an event adds to dehydration and electrolyte losses before the horse even begins to perform, so this, too, must be factored in. A horse that participates in protracted exercise, such as a distance trail horse, is working for many hours and, at times, for multiple consecutive days. Staying ahead of salt and fluid losses is instrumental in maintaining hydration, efficient muscle function, and the ability of the horse to perform to a safe standard and with enthusiasm for his work.
So, let's look at some principles that will help you decide when it is appropriate to provide electrolyte supplementation.
Energy into Locomotion
For muscles to obtain energy for locomotion, food substrates must be converted into energy fuel. In the horse, approximately 70-80% of this metabolic conversion to energy is liberated as heat. In the best situation, heat is dissipated from the inner recesses of the horse's body as sweat, which cools by evaporation of "water" from the skin. Even with moderate exertion during hot weather, a horse sweats. Think about what happens when you exert yourself even slightly on a hot day: Whether you are walking out to the pasture to retrieve your horse, are grooming your horse, or are riding at trot or canter, you sweat. This is a normal physiologic response of man or horse to muscular movement, particularly when the ambient conditions are hot and/or humid, or when the exertion is significant. The inherent problem in some equestrian pursuits is the persistence and duration of your horse's sweating process.
While a person might shed two liters of sweat in an hour, a horse has the potential to evaporate or drip away 15 liters in an hour. High humidity conditions make it hard for sweat to evaporate and cool the horse, so to compensate, the horse sweats more and for a longer time. In addition, there is a great difference between the salt content of horse sweat and human sweat--horse sweat tends to be a more concentrated salt solution. These salts are brought to the skin surface through the sweat pores along with body fluid. In contrast, human sweat has more water in it than salt. The longer a horse exercises, the hotter and more humid the climate, the less fit the horse, or the harder the effort--the more he sweats. And more sweat means more body salts (electrolytes) are lost along with evaporated fluid.
Mammals are composed largely of salt and water, with this body "water" present in a few predominant body compartments. The areas that contain most of this mixture are the extracellular fluid that surrounds the cells and includes the bloodstream; the intracellular fluid within the cells and tissues; and the intestinal compartment that provides fluid to the bloodstream (extracellular fluid) from intestinal circulation. Body water moves along a concentration gradient to maintain hydration of all fluid compartments when possible.
Electrolytes also follow an ionic (based on atomic charge) concentration gradient, diffusing across permeable cell membranes--salts at a higher concentration diffuse into an area with lesser concentration. Each fluid compartment contains electrolytes that can be lost in sweat. Electrolyte balance in the horse has a direct relationship to neuromuscular function and fatigue. Nerves and muscles interact through electrical impulses, which are controlled by the distribution of electrolytes within and outside each cell. In addition, electrolytes regulate the distribution of body water within the various fluid compartments. Peak performance relies on efficiency and synchrony of these functions; dehydration and electrolyte depletion causes a horse to fatigue.
So, in an exercising horse, how do sweat losses of electrolytes and changing proportions of salt cause imbalances between tissue compartments? Let's look at one example of a changing situation.
Normally, the extracellular fluid compartment contains a higher concentration of sodium, while the intracellular fluid compartment contains a higher concentration of potassium. Sweat losses reduce the concentration of these salts in their respective fluid compartments. Each salt possesses a specific negative or positive charge, and the system must remain in balanced electrical neutrality. Continued sweat loss of sodium ions (positive charge) from the extracellular fluid compartment stimulates potassium ions (positive charge) to diffuse from the intracellular fluid compartment into the extracellular fluid compartment. With continued sweating, potassium is subsequently lost from the circulation. This simple example demonstrates that as sodium is lost, potassium moves to fill the void, then it is lost, with a net deficit in whole-body sodium and potassium.
Each salt has many important roles: loss of sodium and potassium reduce neuromuscular irritability (abnormal responsiveness to stimuli), while losses of calcium and magnesium amplify this irritability. Sodium is necessary to drive the thirst reflex. Potassium dilates blood vessels to oxygenate working muscle tissues. The acid-base (pH) status of the horse is dependent on equilibration of electrolytes, particularly chloride, and the pH of the blood determines how much calcium is available for muscle contractility. This is an interactive system, dependent on relative salt concentrations.