Dr. Femke Schaafstra, PhD Equine Nutrition, MSc, BSc
Electrolyte supplements are often thought to be needed only by high-level sport horses, especially those competing in endurance events. However, when horses sweat for a prolonged time electrolyte deficiency can occur. This might result in dehydration, impaired performance, fatigue, and in severe cases even ‘tying-up’. This article aims to improve the understanding of electrolytes and their use in horses.
Electrolytes are electrically charged minerals and are vital to many key functions in the body. They are needed for basal processes in the body, including conducting nervous impulses, contracting muscles, regulating hydration and pH levels (acid-base balance) of body fluids. The electrolytes of primary importance for the horse include Sodium (Na+), Potassium (K+), Chloride (Cl-), Magnesium (Mg2+), and Calcium (Ca2+) (Frape, 2010), with Na+, K+, and Cl- as the principal ions (Groenendyk et al., 1988). Sodium, Potassium, and Chloride are major players in the maintenance of the cellular and extracellular environment. Sodium is the principal cation (positively charged molecule) in the extracellular fluid and is involved in the maintenance of acid-base balance and osmotic regulation of body fluids (NRC, 2007). In the case of chronic Sodium depletion horses have decreased skin turgor, a tendency to lick objects, a slowed rate of eating or even stop eating, and a decreased water intake (Meyer, 1984). An acute Sodium deficiency might result in uncoordinated muscle contraction and chewing (Meyer, 1984). When a diet has a marginal or excessive sodium content, the horse appears to regulate the external balance by restricting or increasing urinary excretion (Tasker, 1967; Rose, 1990). In the diet, Sodium is normally accompanied by Chloride, which is the most common anion (negatively charged molecule) in the extracellular body fluid. Like Sodium, Chloride is involved in maintaining acid-base balance and osmotic regulation and is a component of gastric sections as hydrochloric acid and bile. When Sodium is sufficiently present in the diet, a deficiency in Chloride is unlikely to occur. However, when horses are supplemented with large amounts of Sodium bicarbonate metabolic alkalosis is induced and might result in decreased food intake, weight loss, muscle weakness, dehydration, constipation, decreased milk secretion, and depraved appetite (Tasker, 1980; Fettman et al., 1984; Coenen, 1991). Whereas Sodium and Chloride are found predominantly in extracellular fluids, Potassium is mainly retained within cells and, in particular, in muscle tissue (Meyer, 1987).
Table 1. Amount of electrolytes in constituents of equine diets (g/kg DM)1
In addition to maintaining acid-base balance and osmotic pressure, Potassium is the most quantitatively important cation involved in neuromuscular excitability (Kronfeld, 2001). A diet high in forages generally provides an adequate amount of electrolytes to the horse (Table 1) (Coenen, 2013) and the excessive intake of electrolytes is counteracted in the horse by renal and faecal excretion.
When horses are at rest, intake, absorption and faecal and urinary excretion are in a steady state. However, when horses start to exercise, the acid-base balance drops as soon as horses start to sweat (Coenen, 2005). During exercise, horses produce an enormous amount of metabolic heat and core body temperature can easily increase from 37°C at rest to 42°C (McKeefer, 2008). As for humans, the most effective way for horses to eliminate the excess heat produced is through evaporation of fluid from the body surface (sweating) (Figure 1) and the respiratory tract (Hodgson et al., 1994; Zeyner et al., 2013). When compared to human sweat, equine sweat is hypertonic with electrolyte concentrations higher than that of plasma. Equine sweat contains per liter approximately 3.1g Na, 1.8g K, and 6.3g Cl (Meyer, 1995; Flaminio & Rush, 1998; Waller & Lindinger, 2021). Potassium sweat concentration is 10 to 15 times greater than plasma Potassium concentration, whereas Chloride concentration in horse sweat is twice as high as plasma Chloride concentration. Sodium concentration in horse sweat is approximately equal to its concentration in plasma (Flaminio & Rush, 1998). Cutaneous losses, other than Sodium, Potassium, and Chloride can be neglected as a factor for the external balance (Meyer, 1995; Flaminio & Rush, 1998; McCutcheon & Geor, 1998). As sweat is rich in electrolytes with Na+, K+, and Cl- as the predominant ions, prolonged sweating and dehydration are associated with significant water and electrolyte depletion in exercised horses (Coenen, 2005; Waller & Lindinger, 2021). To balance these cutaneous losses internally, the horse can enforce electrolyte absorption from the gastrointestinal tract, reduce renal excretion and release electrolytes from eg. muscle tissue (Coenen, 2005). Intake of roughage could enforce the reservoir function of the gastrointestinal tract, as fermentation of fibers from roughage releases water and cations, which would then be available for absorption (Kronfeld, 2001).
Figure 1. Considerable amounts of electrolytes and water can be lost in sweat and should be replaced.
Based on a dry matter intake for intensively exercising horses of 2.2kg/100kg BW/day (2.2% BW) a daily intake of approximately 1.3kg/100kg BW of a high-quality roughage is recommended to fulfill electrolyte requirements (Coenen, 2005). When the gastrointestinal tract is not able to compensate for electrolyte losses, the kidney may reduce water, Na+, and Cl- excretion (Robert et al., 2010). Furthermore, to maintain sweating losses of water and electrolytes, it is also likely that soft tissue, like skeletal muscle, are important sources of electrolytes and water. Although the gastrointestinal tract can serve as a temporary reservoir, and the horse is able to conserve water and ions during prolonged exercise via renal tubular conservation, these mechanisms seem to be insufficient to compensate for these losses (NRC, 2007; Robert et al., 2010). Significant electrolyte losses via perspiration and subsequent electrolyte imbalances may induce muscle fatigue and might even contribute to the development of exertional rhabdomyolysis (“tying up”) in some horses (Harris, 1989; Furman, 2015). Therefore, it is important to estimate the electrolyte losses of horses during training and competition by estimating the amount of sweat loss.
However, unstandardised methodological practices to determine sweat loss can produce inaccurate results. To develop a novel sweat scoring system Zeyner et al. (2013) studied the exercise-induced body weight losses in 35 Warmblood-type horses subjected to a low (LW) and a medium (MW) intensity exercise regimen under German conditions. Following each exercise, the horses were visually inspected and 5 distinct ‘sweat’ scores could be determined and associated with a defined sweat loss range (Table 2). Based on this technique, a horse performing light work (LW) at an ambient temperature of 20°C and a sweat score of 1 excretes approximately 4L of sweat corresponding to 40grams of electrolytes containing 12.4g Na+, 6.4g K+ and 22g Cl- (Meyer, 1995; Zeyner et al., 2013). The researchers concluded that this technique permits accurate estimation of exercise-induced sweat losses on an individual basis with higher accuracy than other available methods. However, this technique can be affected by several factors, including weather conditions (ambient temperature, relative humidity, precipitation), air movement, individual athletic fitness, transport, climatic adaptation, the character of the subsoil, and the temperament or degree of arousal of the horse (Ecker & Lindinger, 1995; Jansson et al., 1995; McCutcheon & Geor, 1998; Zeyner et al., 2013).
Table 2. Sweat scores with assigned sweat pattern (observed immediately after exercise without the saddle), their associated sweat and electrolyte loss ranges for practical use1
Not surprisingly, when the environmental temperature and humidity increase the thermal gradient between the skin and environment (skin-ambient vapor pressure) is reduced and cutaneous heat loss is impaired (Hodgson et al., 1994). The horse’s body can even gain heat from the environment when the ambient temperature exceeds skin temperature (greater than 35-36°C) (McCutcheon & Geor, 2008). In these circumstances, the cooling effect of sweat is limited as horses produce more sweat which just runs and drips off their body. In response to this extensive sweat loss, large amounts of Sodium and fluids are lost, which results in hypotonic dehydration (Hodgson et al., 1994). Dehydration of as little as 1.2% of body weight impairs thermoregulation and exercise performance (Hyyppä et al., 1996). Therefore, oral electrolyte supplementation in combination with water is recommended to restore the water and electrolyte balance.
As already discussed, equine sweat is isotonic or slightly hypertonic relative to plasma, containing high concentrations of Na+, K+, and Cl- and electrolyte supplements should therefore contain these ions. Additionally, small amounts of Ca2+ and Mg2+ are also lost, and because these are important for cellular function, they should also be replaced (Meyer, 1995). Normally, Potassium intake greatly exceeds requirements due to the high K+ concentrations in most types of roughages (Table 1) and there will be no need for Potassium supplementation if the horse has a normal food intake post-exercise (Jansson et al., 1995; Nyman et al., 1996). However, the Sodium content of the horse’s diet is often low and Sodium deficits should be replenished in exercising horses. It is, therefore, important that they have free access to common salt (NaCl) to prevent Sodium depletion. However, providing horses with a salt block would not be of any benefit. According to Jansson et al. (1996), voluntary salt intake of exercising horses was not increased when they were offered salt blocks. This might be since a salt block is a concentrated salt solution, which may cause taste aversion and/or a temporary inhibition of the Sodium appetite (Jansson et al., 1996). So, additional NaCl intake could therefore be improved in the form of a salty, high palatable feed or adding table salt to your horse’s daily ration on exercise days (Coenen et al., 1995). As Sodium is absorbed from the intestinal lumen by diffusion through waterfilled channels, cotransport with organic solutes (eg. glucose and amino acids), cotransport with Cl- and the countertransport of Na+ in exchange for H+, Sodium uptake could be increased by the addition of glucose and glycine to the electrolyte solution (Wright & Loo, 2006). However, the addition of glucose in an isotonic electrolyte solution might fail to adequately replenish electrolyte losses and impair the rehydration process. To be isotonic, fluids containing glucose have fewer electrolytes, and subsequently decreased amounts of electrolytes could be absorbed (Sosa León et al., 1995; Monreal et al., 1999). Hence, rapid and efficient restoration of fluid loss and electrolyte imbalances in exercise-like dehydrated horses could be achieved by providing the horse with an isotonic electrolyte solution similar to equine sweat (Monreal et al., 1999).
Trainers/riders apply different methods to avoid dehydration and decrease horse performance capacity. In general, procedures to stimulate water intake through electrolyte consumption to compensate for fluid and electrolyte losses are used. Supplementation during or after exercise as well as preloading is practiced, but there is still no specific procedure recommended. During endurance training or rides, fluid losses appear to be balanced by water intake and as renal absorption seems insufficient to compensate for Na+ and Cl-, supplementation of these ions could be of benefit (Robert et al., 2010). Carlson (1985) suggested an easy and cheap recipe for an electrolyte solution very similar to 1L of equine sweat containing one level tablespoon (17grams) of Sodium chloride (common salt) and one level tablespoon of Lite Salt (Sodium and Potassium chloride) dissolved in 4L of water. However, horses might refuse the saline solution due to taste aversion and should be trained to drink during competition. In the case of post-exercise replacement, providing concentrated electrolytes may have a detrimental effect as water absorption may be impaired during recovery (Chapman, 2011). Because when horses ingest concentrated electrolyte solutions (slurries and pastes) a net flux of water from the extracellular fluid compartment into the upper gastrointestinal tract is caused. So, water is moving in the wrong direction and induces further dehydration of the horse. In contrast, the provision of water alone results in a dilution of the extracellular fluid and results in renal excretion of water with additional electrolytes (Lindinger & Ecker, 2012). Thus, providing only water would not be as successful in rehydrating the horse as the administration of a glucose-electrolyte solution (Hyyppä et al., 1996). As Sodium and fluid loss after exercise result in hypotonic dehydration, an insufficient osmotic stimulus may lead to a lack of desire to drink (Flaminio & Rush, 1998). Therefore, providing the horse salt in their feed and free excess to water would be recommended (Jansson et al., 1996). Preloading a large volume of electrolytes is another strategy that could be applied. Waller & Lindinger (2021) studied the effect of supplementation of water (1L) or electrolytes dissolved in 1L or 3L water before exercise on horses' performance, plasma, and sweating response. They concluded that provision of a hypotonic electrolyte supplement 1h before exercise provides a source of water and ions that potentially support muscle and whole-body ion balance during periods of prolonged sweat ion losses (Waller & Lindinger, 2021).
SUMMARY
Electrolytes play an important role in hydration and cellular function in horses. Horses at rest are perfectly able to fulfill their electrolyte requirements by a high-quality forage-rich diet. However, during prolonged or high-intensity exercise horses lose large quantities of electrolytes and fluid by sweating. To determine the amount of electrolytes lost, sweat rate could be scored with help of a novel sweat scoring technique. Replenish of electrolytes and rehydration of the exercising horse is important and oral isotonic electrolyte supplementation similar to equine sweat is recommended. Providing salt topped on feed and free access to water or iso- or hypotonic solutions before ingestion of dry feeds at least 1h prior to training or competition benefits muscle function and exercise performance during periods of prolonged sweating.
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