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Sweating during vigorous exercise is the principal route for electrolyte loss and is responsible for up to 65 of thermoregulation Holbrook et al. 2005.
Electrolytes are substances that break down in solution into electrically charged particles called ions (Na+, K+, Cl-, Ca++, Mg++) and have a crucial role in maintaining osmotic pressure as well as nerve and muscle activity (Bergero, 2004).
The amount of electrolytes lost through sweat depends on a) total sweat losses and b) sweat electrolyte concentrations (Sawka et al. 2007). Potassium concentration in sweat is around 10 times higher than in plasma and thus, substantial amounts of body potassium are lost after prolonged sweating (Nyman et al. 1996).
Studies have shown that horses participating in heavy prolonged exercise such as endurance horses can sweat at rates of up to 15-20 L/h (Lindinger, 2008)
In contrast to man, the horse produces hypertonic sweat in relation to plasma osmolality. The major electrolytes lost in sweat are sodium (Na), potassium (K), chloride (Cl) as well as some calcium (Ca) and magnesium (Mg) (Munoz et al. 2010). Increased electrolyte loss during exercise, if uncompensated for, can result in electrolyte deficiencies which could lead to dehydration and hyponatremia as well as cardiovascular and thermoregulatory instability (Sawka et al. 2007; Holbrook et al. 2005). This would undoubtedly have an adverse impact on the horses performance and health. 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. Some studies have shown that it also increases voluntarily water intake (Sampier et al. 2006; Schott et al. 2002) as well as moderating acidbase responses (Kronfeld, 2001)
It is believed that replacing electrolyte losses with water alone leads to haemodilution (increased plasma volume) accompanied by decreased plasma osmolality (Hyyppa et al. 1996)
Insufficient osmotic stimtdus caused by the loss of sodium leads to hypotonic hypohydration of plasma which results in the inadequate stimulation of the thirst mechanism since thirst is only triggered by an increase in plasma sodium concentration. This is why the unwillingness to drink is common in exercising horses.
Conclusion and Clinical Relevance-Oral administration of electrolyte pastes to dehydrated horses increases voluntary WI and improves rehydration during the rehydration period. Rehydration is more rapid and complete when NaCl is a component of the electrolyte paste.( American Journal of Veterinary Research January 2002, Vol. 63, No. 1, Pages 19-27 Effect of oral administration of electrolyte pastes on rehydration of horses. Harold C. Schott II, DVM, PhD Shannon M. Axiak, DVM Kristina A. Woody, DVM Susan W. Eberhart, BS)
Total dosages and administration rates vary among competitors and individual horses and with environmental conditions. When competing in hot, humid conditions, horses commonly receive 28-57 g electrolyte mixture at every veterinary checkpoint, with additional supplementation on the course during longer loops (Holbrook et al. 2005).
Several studies have shown that the replacement of water and electrolyte deficits induced by exercise can be hastened when rehydration solutions containing electrolytes, compared to plain water, are provided (Nyman et al. 1996; Butudom et al. 2002). Attenuation of body fluid and bwt losses induced by prolonged exercise can be achieved by supplementation with oral electrolyte pastes (Dusterdieck et al. 1999) or by providing salt water as the rehydration solution (Butudom et al. 2002). The more practical intervention used by most endurance riders has been supplementation with oral electrolyte pastes. However, to date, there is no evidence that use of oral electrolyte pastes enhances performance in competing endurance horses. (Sampier et al. 2006)
Sampier et al (2006) found that supplementing horses with high doses of NaCl and KCI increased water intake but did not improve performance. However, this study had a few limitations, including a small sample number (n=8) as well as the absence of a control group.
Electrolyte supplementation of endurance horses has been primarily aimed at preventing fatigue and clinical manifestations of altered neuromuscular excitability. It may also increase water intake, diminish water shifts and losses, and moderate acidbase responses. Precise relationships of changes in plasma electrolyte concentrations during strenuous and prolonged exercise and the improvement or deterioration of performance have not been well determined in exercising horses (BODY FLUIDS AND EXERCISE:
David S. Kronfeld.
Horses may also become mildly to severely dehydrated during exercise. In contrast to human studies, the benefits of enteral fluids administered to horses before exercise are not convincing. Study differences in the intensity and duration of exercise, the environment in which the exercise took place, and the volume of
fluids administered have produced variable results. While enteral fluids usually have to be administered by nasogastric intubation prior to exercise, after exercise most horses will voluntarily consume electrolyte solutions. In one study, dehydration resolved faster following exercise in horses offered free choice electrolyte solutions than in horses offered water alone. Similarly, concentrated electrolyte pastes administered before and during exercise, combined with free access to water, have been shown to restore fluid, electrolyte and acid base balance in dehydrated horses more rapidly than water alone.( Enteral fluid therapy in large animals)
If high amounts of sodium are lost during sweating, plasma sodium concentration will be unchanged or decreased. The thirst mechanism may, therefore, not be adequately stimulated since thirst is mainly triggered by an increase in plasma sodium concentration and/or a drop in plasma volume (Anderson 1978). An unwillingness to drink despite signs of dehydration is a well known problem in horses during prolonged exercise (Nyman et al. 1996)
Electrolytes and water are intimately relatedand the maintenance of osmotic balance is important.
When electrolyte deficiencies occur the horse will stop drinking to conserve this balance to the point of clinical
Dehydration (Backhouse, 2000).
The performance horse is frequently subjected to periods of heat stress, usually as a result of the metabolic heat production of exercise, and also in situations of prolonged transport or exposure to ambient heat loads when in hot, humid conditions. When heat stress periods are short the thermoregulatory abilities of the horse easily cope despite high rates of heat storage. However, when body heat storage is prolonged, such as can occur during prolonged exercise or transport, dehydration ensues due to sweat losses of water and electrolytes. Dehydration further contributes to increased stress on many physiological systems and impairs cognitive and physical performance. If continued, heat stress can develop into heat strain, which severely comprises health and wellbeing and may be life threatening. Evaporation of sweat is the primary means for thermoregulation, but at best the thermoregulatory efficiency of sweating is ~30%, resulting in high rates of heat storage when exercise is continued. With sweating rates of 15-20 L/h dehydration can rapidly ensue unless effective strategies are used to replace water and electrolytes lost through sweating. Effective electrolyte supplements provide a balanced mixture of water, sodium chloride, potassium chloride, magnesium and calcium as readily dissolved salts, and dextrose to enhance intestinal absorption of sodium and water. The effective of electrolyte supplementation can be non-invasively monitored in horses using multi-frequency bioelectrical impedance analysis.(Sweating, dehydration and electrolyte supplementation: challenges for the performance horse. Lindinger, M. I.HYPERLINK "http://www.cabdirect.org/search.html?q=do:"Proceedings+of+the+4th+European+Equine+Health+and+Nutrition+Congress.+Wageningen+University+and+Research+Centre.+Wageningen,+Netherlands,+18-19+April,+2008""Proceedings of the 4th European Equine Health and Nutrition Congress. Wageningen University and Research Centre. Wageningen, Netherlands, 18-19 April, 2008 2008 pp. 46-56
Role of Electrolytes
Sodium (Na+): Sodium plays a vital role in maintaining the fluid balance between the various body compartments. Increasing the Na+ concentration on one side of the membrane causes the water to follow. For example, with sweating during exercise, plasma Na+ usually increases, which results in a flow of water from cells to the plasma. When Na+ is lost from the body, this affects the balance of ions on both sides of the cell membrane and hence water distribution is altered and nerve and muscle function is affected.
Potassium (K+): The highest concentration of K+ in the body is found inside the cells. The level of K+ inside and outside the muscle and nerve cell, helps to regulate how easy it is to stimulate the cell. If the muscle K+ falls, the muscle can become weak and there may be reduced GIT mobility. During exercise, the muscles may not receive enough blood flow and cramping of the muscle may result in cases of extremely low plasma K+. Fatigue is also associated with low levels of K+.
Chloride (Cl-): The majority of the body's Cl- is found in the ECF (approx. 80%). As Cl- is lost from the body through sweat, the kidneys reabsorb bicarbonate to maintain ionic balance and this contributes to the development of alkalosis (increasing pH) in exercising horses.
Calcium (Ca++) and Magnesium (Mg++): Most horse owners are aware that calcium is required in the diet for growth and maintenance of the bones but it also plays an important role in muscle contraction and nerve function. Decreases in muscle and plasma Ca++ contribute to neural and muscle problems, such as cramping and thumps. Mg++ is also lost in sweat during competition, and is an important mediator for cellular function.
Electrolyte movement during exercise
Increases in plasma [K+] occur quickly with the onset of exercise due to release from the working muscles. Following prolonged exercise, it is common to see decreased plasma [K+], as the muscles have taken up K+, and K+ has also been lost through sweating. At the onset of exercise, there is also movement of Na+ and Cl- from the plasma to the exercising muscle. There is a major difference in the composition of sweat from the horse as compared to a human. Horse sweat is hypertonic (concentrated in ions). The sweat gland of the horse does not possess the coiled duct section that reabsorbs ions as is present in the human. Reasons for this are unclear. Electrolyte and water losses have been explored in many review articles. The most consistent finding has been a decrease in plasma [Cl-], plasma [Na+] remained unchanged, increased or decreased. Plasma [K+] generally decreased or remained unchanged. [Ca++] and [Mg++] remained the same or decreased. The inconsistencies in the findings are likely due to varied conditions surrounding the competitions.
Weather conditions as well as terrain, speed, distance and individual differences may account for the observed variations. Electrolytes and the thirst response
We have all heard the old saying, "You can lead a horse to water, but you can't make him drink." Under normal circumstances, water is lost from the body and this causes the plasma Na+ to increase. It is this increase that causes the thirst response. During periods of heavy sweating, as can occur during athletic exertion, water and salt are lost, therefore the plasma Na+ concentration may not increase despite the high water loss. Electrolyte and fluid replacement A good quality diet for maintenance horses, or those undergoing low level or short-term work, can generally readily replenish electrolyte losses through sweat. However, horses in training several days a week, and regularly involved in intense or prolonged activity, may become chronically deficient in electrolytes. Horses involved in long-term, higher intensity work may lose electrolytes at a rapid rate and the sudden decrease in electrolytes may cause muscle problems and heat injuries.
Replacement of electrolytes and water will offset the losses incurred by sweating and will assist in maintaining homeostasis (balance), which is necessary for optimal functioning of the cells and hence performance.
Theoretically, the ideal supplementation routine would replace the water and ions at the same rate and in the same quantities as lost but for many reasons beyond the scope of this manuscript this is not possible or practical
Human sweat is hypotonic and a low Na+ content solution,
to which glucose can be added, may restore sweat losses rapidly
during competition. In contrast, horse sweat is hypertonic, with a
higher content of Na+, C1- and K+ (McCutcheon et al. 1995).
Using hypertonic solutions with a high content of those
electrolytes would theoretically restore the fluid and electrolytes
required. However, Sosa Le6n et al. (1995) demonstrated that
hypertonic solutions were not well absorbed in the small
intestine in dehydrated horses and found that the closer the
tonicity of the oral rehydration solutions to that of plasma the
better the absorption and retention of fluids.( Electrolyte vs. glucose-electrolyte isotonic solutions for oral
rehydration therapy in horses
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Restoration of water and electrolyte balance in horses after repeated exercise in hot and humid conditions
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The Implications of Heat Generation and Electrolytes in Feed and
Competition in Athletic Horses
J. W. Backhouse
Potassium regulation involves two control systems: external and internal balance.
External balance is determined by the difference between potassium intake and excretion.
In horses a sweating rate of 15 L/h has been reported
(Hodgson et al, 1993, Scott et al, 1996).