Physical exercise can lead to high sweat rates and substantial water and electrolyte losses, especially in hot-humid conditions. Sweating during vigorous exercise is the principal route for electrolyte loss and is responsible for up to 65% of thermoregulation (Holbrook et al. 2005). Studies have shown that horses participating in intense exercise can sweat at rates of up to 15-20 L/h (Lindinger, 2008) and as much as 40 L of sweat has been recorded for endurance horses competing in hot and humid conditions. The amount of sweat an exercising horse will shed depends on a number of factors including intensity and duration of exercise, body weight, metabolic efficiency, temperature, humidity and heat acclimatization state (Swaka et al 2007).
In contrast to man, the horse produces hypertonic sweat in relation to plasma osmolality. The main ionic constituents in sweat are sodium (Na), potassium (K), chloride (Cl) as well as some calcium (Ca) and magnesium (Mg) (Munoz et al. 2010) all of which are vital for correct muscle cell functioning and nerve impulse transmission. Increased electrolyte loss during exercise, if uncompensated for, can result in electrolyte deficiencies leading to isotonic to hypotonic dehydration, hyponatremia, acid-base imbalances as well as cardiovascular and thermoregulatory instability (Sawka et al. 2007; Holbrook et al. 2005). This would undoubtedly have an adverse impact on the horsesâ€™ performance and health.
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In order to replace electrolyte losses sustained during an event, the oral administration of electrolyte solutions became a very important and common tool for restoring water volumes and electrolyte concentrations to normal values. Electrolytes are substances that break down in solution into electrically charged ions (Na+, K+, Cl-, Ca++, Mg++) which are contained within each body water compartment. The concentration and composition of these electrolytes play a crucial role in maintaining osmotic pressure as well as nerve and muscle activity (Bergero, 2004; Sawka and Montain, 2000). Na+ is the major extracellular cation and osmole that is functionally impermeable. It largely contributes to plasma tonicity and osmolality, including the maintenance of fluid balance between body compartments.
With the onset of exercise there is an increase in plasma Na+ forcing water to move from cells to plasma. With the increasing loss of Na+, the balance of ions on either side of cell membrane is disturbed altering water distribution and thus affecting both nerve and muscle function. Insufficient osmotic stimulus caused by the loss of Na+ leads to hypotonic hypohydration of plasma which results in the inadequate stimulation of the thirst mechanism since thirst is triggered by an increase in plasma Na+ concentration (Borer and Corley, 2006a). K+ is primarily an intracellular ion and is the most quantitatively important ion involved in neuromuscular excitability.
Around 70-75% of K+ is found in skeletal muscle. Intracellular water and K+ are required for the synthesis of muscle glycogen and the extracellular concentration of K+ is vital for the triggering of action potentials that initiate neuromuscular transmission. At the onset of exercise, K+ exits skeletal muscle when membranes depolarise raising plasma K+ concentration and increasing blood flow to working muscle through arteriolar vasodilation (Borer and Corley, 2006b). The amount of electrolytes lost through sweat largely depends on sweat electrolyte concentrations (Sawka et al. 2007).
K+ concentration in sweat is around 10 times higher than in plasma and thus, substantial amounts of body K+ are lost after prolonged sweating (Nyman et al. 1996). As the concentration of K+ in muscle and nerve cells decreases, muscles become weak due to reduced blood flow. Cramping and fatigue are also associated with cases of low plasma K+. Acid-base status is believed to affect the ratio of intracellular to extracellular K.
Acid-base balance is largely maintained by Cl- which is primarily an extracellular ion the concentration of which depends on total body water balance (the net difference between water intake and water loss) (Swaka et al. 2007) and is closely correlated to that of K+ plus Na+ (Jenkinson et al. 2006). The reabsorption of Cl- in the kidney is largely influenced by plasma Na+ concentration and acid-base balance. As Cl- is lost through sweat, bicarbonate is reabsorbed by the kidneys as to maintain ionic balance which could lead to alkalosis. The hypochloremic component of alkalosis is corrected when Cl- is replaced (Borer and Corley, 2006a).
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Ca++ is involved in excitation and contraction of cardiac muscle, nerve function and maintaining vascular tone (Borer and Corley, 2006a). Mg++ contributes largely to mediation of various metabolic and cellular functions, including those involving adenosine triphosphate (ATP) and the production of energy). Mg++ is also an essential coenzyme for the Na+-K+ ATPase pump (Borer and Corley, 2006b).
It is believed that replacing electrolyte losses with water alone leads to haemodilution (increased plasma volume) and decreased plasma osmolality (Hyyppa et al. 1996). 25 L of sweat equates to losses of around ~3,000 mmol Na+, ~3250 mmol Cl-, and ~750 mmol K+ (Schott, 2010) and thus it is vital that these losses are replaced. The most practical intervention commonly used by riders has been supplementation with oral electrolyte pastes. Some studies have shown electrolyte pastes increases voluntarily water intake, diminishes water shifts and losses, improves rehydration, attenuates body weight looses (Sampier et al. 2006; Schott et al. 2002) and moderates acid-base responses (Kronfeld, 2001).
Effective electrolyte supplements should provide a balanced mixture of NACl, KCl, Mg++ and Ca++ as readily dissolved salts (Lindinger, 2008). The main electrolytes present in the paste and the ratio of these electrolytes need to be taken into consideration. A Na+ to Cl- ratio of 1.4:1 is thought to be ideal. If the ratio is less than that, acidosis will develop after administration (Kronfeld, 2001). K+ content should be one third that of Na+. The osmolality of the solution should also be considered. Isotonic and hypotonic solutions are absorbed quickly while hypertonic solutions are theoretically more suitable due to the horse having hypertonic sweat. Leon et al. (1995) found that the small intestine of dehydrated horses did not absorb hypertonic solutions very well.
In order to enhance the intestinal absorption of electrolyte pastes dextrose is usually added as it serves as a direct source of glucose providing cellular energy and thus increasing intestinal absorption (Lindinger, 2008). Good quality forage in the diet usually meets the horses dietary requirements. However, horses in regular heavy work have an increased demand for Na+ K+, and Cl- . for horses in moderate exercise sweating 10 L per day would require 45g Na+, 80g K+, and 45g Cl- and these requirements are doubled if horses are in strenuous and/or prolonged exercise sweating 20L per day (Kronfeld, 2001).
There is a great deal of variability in articles reporting electrolyte losses and serum electrolyte concentrations. However, that is mainly due to the different test conditions which horses were under such as weather, terrain, speed, and distance. Generally, total electrolyte losses during moderate intensity exercise in hot/humid weather are twice as much as losses in cool conditions (McCutcheon and Geor, 1996) and serum electrolyte concentrations decreased (Schott et al. 1997). Therefore, all these factors need to be considered when deciding type and dose of electrolyte solutions.