The horse produces heat during exercise, and core body temperature can quickly increase. In an effort to dissipate the heat and maintain a normal core temperature, blood flow to the skin increases and the sweat glands are signaled to produce sweat. For the horse, sweating is the primary method of cooling. Sweating can result in significant fluid and electrolyte losses. The rate at which the horse is able to replace these losses is key to continued exercise performance and maintaining overall health.
Estimated sweat loss
Water represents approximately 65% of the horse’s total body weight, although this can vary with age, gender, health, and other factors. A common method to calculate sweat loss is based on this percentage of the total body weight loss during the exercise bout. During exercise, sweat losses can be significant enough to cause exhaustion, muscle soreness, fatigue, and dehydration. The following table provides some estimated examples of sweat losses at different exercise intensities:
| Racing and race training | Up to 10 liters (~2.5+ gallons) |
| Cross country phase of a 3-day event | Up to 20 liters (~5+ gallons) |
| 100-mile endurance ride | Up to 35 liters (~9+ gallons) |
Sweat is not just water.
The primary component in equine sweat is water; however, other nutrients are lost as well. Equine sweat also contains proteins and minerals. The major minerals lost in equine sweat are the ionic forms of sodium (Na+), chloride (Cl-), and potassium (K+), and to a lesser extent calcium (Ca2+), and magnesium (Mg2+). These minerals are collectively called electrolytes. By definition, an electrolyte is any of the ions (e.g., Na+, Ca2+) that, in biological fluid, regulate or affect most metabolic processes (e.g., the flow of nutrients and fluids into and waste products out of cells).
In a typical equine diet, protein lost in sweat should be easily replaced without additional supplementation. Electrolytes, however, can take much longer to rebound due to the concentrations lost in sweat and lack of stored electrolytes in the body. Supplementing electrolytes in the diet of an exercising horse is important in not only replacing lost electrolytes, but also in helping to rehydrate the horse. The following table shows the range of estimated electrolyte losses in equine sweat based on research with exercising horses. The ranges are mainly due to the different methods used to collect the sweat.
Estimated electrolyte concentrations in equine sweat
| Electrolytes | Sodium | Potassium | Chloride | Calcium | Magnesium |
| Range in I L of equine sweat | 2640 – 3800 mg | 1100 – 2300 mg | 4900 – 7100 mg | 123 – 124 mg | 52 – 56 mg |
| Approx. percent in equine sweat | ~29% | ~15.3% | ~54.1% | ~l.1% | ~0.5% |
You can lead a horse to water, but you can’t make them drink.
Maintaining electrolytes and fluid balance during exercise can be challenging because the horse’s thirst response may be decreased. Human sweat is hypotonic, such that the blood concentration of sodium increases with significant sweat loss. This stimulates a thirst response and is the reason humans can usually rehydrate by drinking lots of fluids. Equine sweat, on the other hand, is isotonic to hypertonic, depending on the amount of sweat lost. This means blood concentration of sodium may not increase. As a result, horses may not have a thirst response when sweating at very high rates.
Supplementing before and during exercise helps to replenish electrolytes as they are lost during exercise and helps to encourage drinking, thereby helping the horse to rehydrate. Horses do not store electrolytes or fluid in the body, except those carried in the digestive tract, mainly in the large intestine. (Whether the horse can mobilize water from the digestive tract for use during exercise is unclear.) Any excess electrolytes and fluid are absorbed and excreted via urine. Nevertheless, when fluid and electrolyte losses are anticipated, dietary alterations may be helpful. Increasing water intake prior to an event will make sure the horse is well hydrated. Providing electrolytes during training can help stimulate water intake and get the horse accustomed to drinking during an exercise event.
Maintaining adequate fluid and electrolyte balance during exercise has been shown to delay the onset of fatigue. During an exercise event that causes significant sweating, the horse should be provided with small amounts of electrolytes and water during breaks to help replenish stores as they are lost and prevent dehydration. Making sure the horse drinks is essential. Feeding electrolytes without adequate water intake can result in further dehydration. Conversely, providing water without electrolytes to a dehydrated horse can also create problems (e.g., decreased thirst response and increase the risk of electrolyte imbalances, neuromuscular disorders, etc.). Electrolytes should be provided for at least several days following an event. Horses experiencing significant losses (e.g., 100-mile endurance ride) may not be able to replenish lost electrolytes and fully rehydrate in one day.
Why does equine sweat foam?Equine sweat foams as a result of a protein present in the sweat called latherin. This protein is unique to equids and can also be found in equine saliva. Latherin helps to distribute sweat across the horse’s coat, thereby increasing heat loss and cooling during exercise. The concentration of latherin in equine sweat is highest in the first sweat being produced. As the exercise bout continues, the consistency of the horse’s sweat becomes more watery and the sweat no longer foams. Contrary to some beliefs, the presence of foamy sweat has nothing to do with the horse’s fitness level. |
Determining the need for an electrolyte
The necessity of an electrolyte supplement will depend on the amount the horse sweats. The rate of sweating and the amount of electrolytes lost (and therefore in need of replacing) depend on several factors, including the horse’s fitness leverl, the load being carried, intensity and duration of the exercise bout, and ambient temperature. (Note: Higher humidity can reduce the ability of sweat to evaporate, thus decreasing the ability of the horse to cool. Higher humidity does not necessarily cause the horse to sweat more.) A horse at light work (light trail ride) can likely replace lost electrolytes with a typical diet and access to salt. Horses competing in multiple events or a short term, high intensity event (e.g., multiple show jumping classes in a single day, Thoroughbred racehorse) may sweat at a moderate rate and require some electrolyte supplementation before, during and after the event. Endurance horses will likely have the highest sweat losses and require the highest amount of supplementation. These horses should receive electrolytes for days following an event. The form (e.g., granular, liquid, paste, etc.) in which the electrolyte is provided is not of great importance. Making sure the horse has access to water, however, is imperative. Again, the goal is to replace electrolyte losses in equine sweat and to help encourage rehydration. Many electrolyte supplements contain sugar (or flavoring agents) to help with palatability, but the goal of an electrolyte supplement is not to replace lost glucose (sugar). This will usually be accomplished with a typical diet.
An electrolyte supplement should mimic equine sweat, such that it provides the correct amount of electrolytes in the proper ratios. Human sweat and equine sweat are quite different in their concentration of nutrients; therefore, human electrolyte drinks and supplements will not adequately meet the horse’s needs. Maintaining the proper amount and balance of electrolytes is important to the horse’s overall health. If the ratio of electrolytes is not balanced, the horse could suffer further dehydration as well as tying up, synchronous diaphragmatic flutter (aka: thumps), colic, and death in extreme cases.
Summary
- Supplementing electrolytes in the diet of an exercising horse is important in not only replacing lost electrolytes, but also in helping to rehydrate the horse.
- An electrolyte supplement should be balanced to mimic equine
- If the ratio of electrolytes in relation to the others is not balanced, the horse could suffer neuromuscular disorders, as well as further dehydration.
- Amount of electrolyte supplementation required varies with duration and intensity of the exercise and the ambient
