Water & Electrolytes: Disturbance in Fluid Balance
By: Mila Bon, 8 May, 2013
Introduction
Without water, life is impossible. Even single cell organisms need water in order to function. Water is a universal solvent, the most common molecule in the body and commonly known as the most important nutrient. “In the horse, water is essential for fluid balance, digestive function and gastrointestinal health” (NRC, 2009). It also helps to maintain proper moisture level in feces, maintains normal blood volume and aids in the normal function of sweat glands as stated by Dr. Marteniuk of Michigan State University’s ‘Information for Animal Owners’. Horses can tolerate water restriction for extended periods of time, however, a complete lack of water is rapidly fatal (NRC, 2009). Since water plays a major role inside every cell of the body, the interstitial fluid*, circulation, digestive function, gaseous exchange*, cutaneous evaporation*, sweating, electrolyte balance, renal function, fecal formation and milk secretion, some basic knowledge about fluid balance and its more complicated control mechanism are required in order to make proper nutritional choices for the horse as there is no voluntary intake of drinking water without a thirst stimulus. (* Scroll down for Appendix – A - glossary.)
Fluid Balance
Fluid balance or water balance in the horse is achieved by equalizing water losses with water intake. (NRC, 2009). Fluids are lost through feces, urine, respiration, the skin and milk secretion (lactating mares). In order for a horse to voluntarily replace the fluid losses by drinking water and maintain homeostasis*, it requires a thirst stimulus. Direct voluntary water intake through moisture content in forage or feeds does not require a thirst stimulus. Large amounts of water are reabsorbed in the intestinal tract.
Introduction
Without water, life is impossible. Even single cell organisms need water in order to function. Water is a universal solvent, the most common molecule in the body and commonly known as the most important nutrient. “In the horse, water is essential for fluid balance, digestive function and gastrointestinal health” (NRC, 2009). It also helps to maintain proper moisture level in feces, maintains normal blood volume and aids in the normal function of sweat glands as stated by Dr. Marteniuk of Michigan State University’s ‘Information for Animal Owners’. Horses can tolerate water restriction for extended periods of time, however, a complete lack of water is rapidly fatal (NRC, 2009). Since water plays a major role inside every cell of the body, the interstitial fluid*, circulation, digestive function, gaseous exchange*, cutaneous evaporation*, sweating, electrolyte balance, renal function, fecal formation and milk secretion, some basic knowledge about fluid balance and its more complicated control mechanism are required in order to make proper nutritional choices for the horse as there is no voluntary intake of drinking water without a thirst stimulus. (* Scroll down for Appendix – A - glossary.)
Fluid Balance
Fluid balance or water balance in the horse is achieved by equalizing water losses with water intake. (NRC, 2009). Fluids are lost through feces, urine, respiration, the skin and milk secretion (lactating mares). In order for a horse to voluntarily replace the fluid losses by drinking water and maintain homeostasis*, it requires a thirst stimulus. Direct voluntary water intake through moisture content in forage or feeds does not require a thirst stimulus. Large amounts of water are reabsorbed in the intestinal tract.
Fluid Homeostatis
The Hypothalamic-Pituitary (HP) Axis* works as a negative feedback system responsible for i.a. osmolality changes* in extracellular fluid* based on Na+ concentrations. If Na+ concentrations are too high, or too low a homeostatic response will lead to changes in intake or excretion of water and sodium proportional to the hypertonic* or hypotonic* conditions triggering the response as shown in the diagram below. Note that the dashed lines illustrate potential homeostatic responses for which experimental data was not present at that time. (Bourque, 2008).
The main thirst stimuli and indicators of fluid shifts are total plasma protein concentrations, plasma osmolality and plasma sodium concentrations (Nyman and Dahlborn, 2001). The Renin-Angiotesin-Aldosterone-Vasopressin (RAAV) axis* plays a significant role in preserving hemodynamic stability when changes in blood volume, blood pressure, electrolytes and water occur. In dehydrated horses aldosterone (ALD) and agrinine vasopressin (AVP) are increased. (ALD) promotes K excretion and Na re-absorption in a bid to maintain Na levels, increases blood volume and stimulates thirst. (AVP) promotes water reabsorption in the kidneys.
(Muñoz et al., 2010).
Diagram by: Bourque, 2008
Water Requirements
Total body water (TBW) of a horse is an average of 62 - 68% (NRC, 2009) of which 21 – 25% is extra-cellular. TBW reduces linearly with age (NRC, 2009). Average maintenance water intakes are about 5L/100kg BW/d and the estimated maintenance requirements for a 500 kg horse that is fed 1.5% hay and kept at thermal neutral temperatures (5 - 20º C) ranges from 21 – 29 L/day (NRC, 2009). Water needs for light activity do not differ much from maintenance requirements, according to NRC (2009), but it is difficult to predict water needs for exercising horses as it depends on environmental condition, exercise intensity, duration, the fitness of the horse and acclimatization to its environment (NRC, 2009). Horses working in milder weather conditions or horses turned out in hot weather can lose approximately 4 L of sweat per hour and a horse sweating heavily can lose up to 16 L per hour (Kellon, 2012).
Electrolyte Requirements
Electrolytes are compounds that conduct electricity. When dissolved in water they become charged particles involved in many basic cell functions like muscle contraction and nerve impulse transmission (Oke, 2001). The main electrolytes are sodium, potassium, chloride, calcium and magnesium (Kellon, 2012). Electrolytes lost in sweat are primarily Na+, CL- and K+, (Kellon, 2012). The negative feedback system of the (HP)axis is based on sodium concentrations in extracellular fluid (Bourque, 2008). When sodium concentration and/or plasma protein concentration increase it will trigger the thirst stimulus (Nyman and Dahlborn, 2001). Maintenance requirements for the main electrolytes as per NRC (2009) are: Sodium (Na) – 0.02 g/kg BW; Chlorine (Cl) – 80 mg/kg BW; Potassium (K)– 0.05 g/kg BW; Calcium (Ca)– 0.04 g/kg BW; Magnesium (Mn)– 15 mg/kg BW.
Disturbances in Fluid Balance / Dehydration in Horses
The definition of dehydration is very vague: “when the loss of body fluids, mostly water, exceeds the amount that is taken in” (WebMD); “Dehydration is the loss of water and salts essential for normal body function” (TheFreeDictionary); “an abnormal depletion of body fluids” (MerriamWebster); “Excessive loss of body water” (MedicineNet). Mild disturbances (< 5% of body weight in water loss) are not easy to detect. Dehydration can further be categorized as a disturbance in volume (isotonic dehydration) or in concentration (hypertonic or hypotonic dehydration). In “Horse Owner’s Veterinary Handbook” Gore et al. (2008) state that dehydration is not recognized until >= 5% of body weight in water loss occurs and that a water loss of 12 – 15% of body weight is life threatening. Sneddon (1992) reviewed the adaptive successes of Arab-based horses in arid climates and concluded that Arab-based horses have the ability to protect plasma volume during dehydration by maintaining plasma protein levels, especially albumin. It was also concluded that Arab-based horses’ red cells are able to resist haemolysis* during a 72 hr. (12%) dehydration/immediate rehydration regime and thus these horses are resilient to a 12% hypertonic dehydration and can be classified as semi-arid rather than wet-temperate ecotypes, according to Sneddon’s findings. Exercise places a great strain on the cardiovascular system as increased blood flow is needed to transport oxygen and nutrients to the muscles and remove metabolic waste. At the same time heat is building which requires increased blood flow to the skin to cool the horse by dissipating heat through sweat. The water loss from sweat reduces the viscosity of the blood and the volume of extracellular fluid which puts even more strain on the cardiovascular system.
Although there are medical causes for dehydration (for example: prolonged diarrhea, fever, infections, renal dysfunction, endocrine dysfunction, use of certain medications like Furosemin etc.) these are outside the scope of this paper which puts the emphasis on dietary causes or dietary treatment options (mainly water and sodium).
Causes of Dehydration in Horses
Clinical Signs of Dehydration in Horses
Mild dehydration (less than 5%) is hard to detect. The easiest test a horse owner can perform to detect dehydration is the ‘skin-turgor test’ (NRC, 2009) or ‘hydration-pinch test’ (MacInnes and Davis, 2011) is performed at the point of the shoulder on an adequately hydrated horse, the skin returns to normal in less than a second (MacInnes and Davis, 2011). In a dehydrated horse, the skin, when pinched, will not immediately snap back, but will ‘tent’, so in some literature this is also called the ‘skin-tent test’. In the capillary-refill test performed on an adequately hydrated horse, after pressing on the gums above the corner incisor, the colour will return to normal within 2 seconds (MacInnes and Davis, 2011). Urine will be concentrated and darker in colour, but even with total water deprivation will not be less than 0.5 L/100 kg BW per day unless the horse suffers from renal failure (NRC, 2009). The horse is able to absorb substantial amounts of water and electrolytes from the large colon during prolonged exercise and by doing so will get a “tucked up” appearance to the abdomen (Schott, 2011). Thumps is a serious sign of isotonic dehydration and exhaustion, although the condition itself is not. It’s comparable with the hiccups in humans (the horse’s flank twitches and makes an audible “thump”), but when it occurs veterinary care is required (MacInnes and Davis, 2011). Thumps is caused when too much chloride is lost through sweat. Chloride and bicarbonate are the only anions (negatively charged ions), so in order to maintain electro-neutrality when Na+ is reabsorbed under influence of aldosterone (secreted during moderate exercise - Muñoz et al., 2010) and CLˉ is depleted, HCO3ˉ is no longer excreted by the kidneys and too much bicarbonate causes alkalosis* which in turn can cause colic or thumps. Tie up is an even more serious form of isotonic dehydration. The muscles cramp up and the horse becomes reluctant to move and might seize up completely (MacInnes and Davis, 2011).
Prevention and Management of Dehydration in Horses
As the system for fluid homeostasis is based on a negative feedback system, some imbalance has to occur before action is taken to restore the balance. According to Coenen (2005) a positive water balance is achieved after feeding and watering, and water retention increases postprandially* for 4-5 hours, but the rate of fluid loss through sweat during a 2-hour exercise period results in a negative water balance. A basic ration that depresses the acidic effect of metabolism, lowers heat production itself, and lowers cortisol liberation would be desirable as cortisol readily increases during moderate exercise. As the weakening capacity to eliminate the heat which is produced by strenuous exercise is a performance-limiting factor, Coenen (2005)hypothesized that fat as energy source would reduce heat of metabolism, but fermentable fiber is still required in order to prevent a collapse of the microbial system in the hindgut. Fiber sources like the highly fermentable pectin in beet pulp has the added benefit of increased water consumption (Coenen, 2005). Ideal would be to replace water and ions at the same rate as they are lost in sweat, but during exercise gastrointestinal function is compromised by many factors that have priority over gastric emptying and intestinal absorption, which makes it difficult to actually absorb the water and electrolytes offered (Ecker, 1995; Frape, 2010). Care has to be taken with offering high concentrations of electrolytes to avoid hypernatremia (Schott et al., 1999; Hess et al., 2007). Hess et al. (2007) hypothesized that potassium supplementation affects plasma K+ and may lead to neuromuscular hyperexcitability. They also hypothesized that diets high in calcium (1.5% instead of 1%) would lead to higher plasma Ca++ and the results of the study during an 80 km endurance race confirmed this, however, the test group that was supplemented with potassium during the race was also supplemented with 1.5% calcium and thus the neuromuscular hyperexcitability due to high K+ and low Ca++ did not occur, but Hess et al. (2007) concluded that higher plasma Ca++ and lower K+ during exercise could help reduce the risk.
Conclusion
Water is the most important nutrient, but without balanced electrolytes (and a properly working endocrine system) it is impossible for the horse to maintain fluid homeostasis. Whether regarding an idle horse or sport horse, maintaining fluid balance is important to sustain all bodily functions. Horses evolved to withstand cold better than heat and in natural settings usually rest during the hottest part of the day. In natural settings horses only spend 1% of the time galloping or trotting (Lee, 2011) which is only 15 minutes out of the day. It is therefore not difficult to understand why heat gain is a major limiting factor to exercise and why thermo-regulation in horses did not evolve to be a very efficient cooling system for endurance exercise and/or hot and humid conditions, but often results in moderate to severe dehydration.
Important Considerations: Take Home Points
1. Thirst, lack of thirst, slow capillary refill, ‘tenting skin’, concentrated urine, sunken eyes, excessive sweating and thumps,
low pulse : respiration rate are signs of dehydration.
2. Always provide access to clean, fresh palatable water.
3. Salt is very important in maintaining water balance.
Salt blocks alone are often not sufficient for exercising horses, provide loose salt or mix it with the feed.
4. Cold water temperature reduces intake, while in the winter water requirement might be higher due to increased hay intake to
maintain body temperature (Marteniuk)
5. At 20:1 moisture, a horse has to eat 500 litre of fresh snow to melt it into 25 Litre of water.
6. Sweating is controlled by thermo-regulation, independent of hydration status or electrolyte homeostasis (NRC, 2009).
7. Horses should be allowed to drink frequently during extended work and should be allowed 2 minutes for each occasion (Frape, 2010)
8. After exercising on a hot day, horses prefer water of around 20º Celsius (NRC, 2009)
9. Drink volume after exercise is higher from buckets than it is from automatic waterers (Nyman and Dahlborn, 2001)
References:
Bourque, C.W., 2008. Central mechanism of osmosensation and systemic osmoregulation. Nature Reviews Neuroscience 9, 519-531 (July 2008)
Butudom, P., Axiak, S.M., Nielsen, B.D., Eberhart, S.W., Schott II, H.C., 2003. Effect of varying initial drink volume on rehydration of horses. Physiology & Behavior 79 (2003) 135– 142
Coenen, M., 2005. Exercise and stress – Impact on adaptive processes involving water and electrolytes. Livest Prod Sci. 92, 131-145
Ecker, G.L., 1995. Fluid and ion balance in the performance horse. The performance horse: Focus on endurance I and II, Equine Research Centre (1995), University of Guelph
Frape, D., 2010. Equine Nutrition and Feeding. Wiley-Blackwell, West Sussex, UK.
Hess, T.M., Greiwe-Crandell, K.M., Waldron, J.E., Williams, C.A., Lopes, M.A., Gay, L.S., Harris, P.A., Kronfelder, D.S., 2007. Potassium-free electrolytes and calcium supplementation in an endurance race. Comparative Exercise Physiology 5 (1): 33-41
Kellon, E., 2012. Equine Electrolytes, Exercise, and the Heat. Uckele Health and Nutrition, Aug 2012, Article #29617.
Lee, J., Floyd, T., Erb, H., Houpt, K. 2011. Preference and demand for exercise in stabled horses. Applied Animal Behaviour Science 130 (2011) 91-100
Leibsle, S., 2012. Cold Weather Colic. AAEP Health Articles.
MacInnes, R., Davis, H. 2011. Equine Emergency First Aid Manual, 2011. Equi-Health Canada, Okotoks, AB, Canada.
Marteniuk, J. Winter dehydration in horses. MSU College of Veterinary Medicine.
Muñoz, A., Riber, C., Trigo, P., Casteljón, F.M., 2010. Clinical Applications of the Renin-Angiotensin-Aldosterone-Vasopressin Axis in the Horse and Future Direction for Research. Journal of Equine Veterinary Science, Vol 30, No 11 (2010)
National Farm Animal Care Council (Canada). DRAFT – Code of practise for the care and handling of equines (nov. 2012).
Nutrient Requirements of Horses. Sixth revised edition, 2009. National Research Council of the National Academies Press, Washington, DC, USA.
Nyman, S., Dahlborn, K., 2000. Effect of water supply method and flow rate on drinking behaviour and fluid balance in horses. Physiology & Behavior 73 (2001) 1 – 8
Oke, S., 2010. Electrolytes for Horses. Factsheet, TheHorse.com 2010.
Schott II, H.C., Du Sterdieck, K.F., Axiak, S.M., Woody, K.A., Eberhart, W., 1999. Oral Electrolytes Stimulate Water Drinking by Dehydrated Horses. AAEP PROCEEDINGS 9 Vol. 45 / 1999
Sneddon, J.C., 1993. Physiological effects of hypertonic dehydration on body fluid pools in arid-adapted mammals. how do arab-based horses compare? Comparative Biochemistry and Physiology -- Part A: Physiology (February 1993), 104 (2), pg. 201-213
The Hypothalamic-Pituitary (HP) Axis* works as a negative feedback system responsible for i.a. osmolality changes* in extracellular fluid* based on Na+ concentrations. If Na+ concentrations are too high, or too low a homeostatic response will lead to changes in intake or excretion of water and sodium proportional to the hypertonic* or hypotonic* conditions triggering the response as shown in the diagram below. Note that the dashed lines illustrate potential homeostatic responses for which experimental data was not present at that time. (Bourque, 2008).
The main thirst stimuli and indicators of fluid shifts are total plasma protein concentrations, plasma osmolality and plasma sodium concentrations (Nyman and Dahlborn, 2001). The Renin-Angiotesin-Aldosterone-Vasopressin (RAAV) axis* plays a significant role in preserving hemodynamic stability when changes in blood volume, blood pressure, electrolytes and water occur. In dehydrated horses aldosterone (ALD) and agrinine vasopressin (AVP) are increased. (ALD) promotes K excretion and Na re-absorption in a bid to maintain Na levels, increases blood volume and stimulates thirst. (AVP) promotes water reabsorption in the kidneys.
(Muñoz et al., 2010).
Diagram by: Bourque, 2008
Water Requirements
Total body water (TBW) of a horse is an average of 62 - 68% (NRC, 2009) of which 21 – 25% is extra-cellular. TBW reduces linearly with age (NRC, 2009). Average maintenance water intakes are about 5L/100kg BW/d and the estimated maintenance requirements for a 500 kg horse that is fed 1.5% hay and kept at thermal neutral temperatures (5 - 20º C) ranges from 21 – 29 L/day (NRC, 2009). Water needs for light activity do not differ much from maintenance requirements, according to NRC (2009), but it is difficult to predict water needs for exercising horses as it depends on environmental condition, exercise intensity, duration, the fitness of the horse and acclimatization to its environment (NRC, 2009). Horses working in milder weather conditions or horses turned out in hot weather can lose approximately 4 L of sweat per hour and a horse sweating heavily can lose up to 16 L per hour (Kellon, 2012).
Electrolyte Requirements
Electrolytes are compounds that conduct electricity. When dissolved in water they become charged particles involved in many basic cell functions like muscle contraction and nerve impulse transmission (Oke, 2001). The main electrolytes are sodium, potassium, chloride, calcium and magnesium (Kellon, 2012). Electrolytes lost in sweat are primarily Na+, CL- and K+, (Kellon, 2012). The negative feedback system of the (HP)axis is based on sodium concentrations in extracellular fluid (Bourque, 2008). When sodium concentration and/or plasma protein concentration increase it will trigger the thirst stimulus (Nyman and Dahlborn, 2001). Maintenance requirements for the main electrolytes as per NRC (2009) are: Sodium (Na) – 0.02 g/kg BW; Chlorine (Cl) – 80 mg/kg BW; Potassium (K)– 0.05 g/kg BW; Calcium (Ca)– 0.04 g/kg BW; Magnesium (Mn)– 15 mg/kg BW.
Disturbances in Fluid Balance / Dehydration in Horses
The definition of dehydration is very vague: “when the loss of body fluids, mostly water, exceeds the amount that is taken in” (WebMD); “Dehydration is the loss of water and salts essential for normal body function” (TheFreeDictionary); “an abnormal depletion of body fluids” (MerriamWebster); “Excessive loss of body water” (MedicineNet). Mild disturbances (< 5% of body weight in water loss) are not easy to detect. Dehydration can further be categorized as a disturbance in volume (isotonic dehydration) or in concentration (hypertonic or hypotonic dehydration). In “Horse Owner’s Veterinary Handbook” Gore et al. (2008) state that dehydration is not recognized until >= 5% of body weight in water loss occurs and that a water loss of 12 – 15% of body weight is life threatening. Sneddon (1992) reviewed the adaptive successes of Arab-based horses in arid climates and concluded that Arab-based horses have the ability to protect plasma volume during dehydration by maintaining plasma protein levels, especially albumin. It was also concluded that Arab-based horses’ red cells are able to resist haemolysis* during a 72 hr. (12%) dehydration/immediate rehydration regime and thus these horses are resilient to a 12% hypertonic dehydration and can be classified as semi-arid rather than wet-temperate ecotypes, according to Sneddon’s findings. Exercise places a great strain on the cardiovascular system as increased blood flow is needed to transport oxygen and nutrients to the muscles and remove metabolic waste. At the same time heat is building which requires increased blood flow to the skin to cool the horse by dissipating heat through sweat. The water loss from sweat reduces the viscosity of the blood and the volume of extracellular fluid which puts even more strain on the cardiovascular system.
Although there are medical causes for dehydration (for example: prolonged diarrhea, fever, infections, renal dysfunction, endocrine dysfunction, use of certain medications like Furosemin etc.) these are outside the scope of this paper which puts the emphasis on dietary causes or dietary treatment options (mainly water and sodium).
Causes of Dehydration in Horses
- Hypertonic Dehydration –Inadequate water intake. May be due to unavailability of palatable water, water too cold, frozen, different taste or smell, too much competition for water. Research has shown that a low flow rate of automatic waterers, whether it is a pressure valve system or a float valve system and the shallow depth of the the bowl are contributing to reduced water intake (Nyman and Dahlborn, 2001). Lactating may cause hypertonic dehydration as well.
- Hypotonic Dehydration - Loss of electrolytes, primarily sodium, are greater than water losses. May be caused by Lack of sodium in the diet, renal dysfunction, sometimes diarrhea.
- Isotonic Dehydration – Equal loss of water and electrolytes. May be caused by endurance exercise, excessive sweating, hot and humid conditions, diuretics like Furosemin (Lasix), prolonged diarrhea, enterocolitis*, intestinal obstruction. By means of sweating water and electrolytes are lost, which have to be replaced to maintain fluid homeostasis, but insufficient thirst due to the loss of electrolytes may cause dehydration as equine sweat remains isotonic during prolonged exercise, which results in a rather slow rise of plasma osmolality. The rise in plasma tonicity* is what triggers the thirst stimulus (Schott et al., 2011.)
Clinical Signs of Dehydration in Horses
Mild dehydration (less than 5%) is hard to detect. The easiest test a horse owner can perform to detect dehydration is the ‘skin-turgor test’ (NRC, 2009) or ‘hydration-pinch test’ (MacInnes and Davis, 2011) is performed at the point of the shoulder on an adequately hydrated horse, the skin returns to normal in less than a second (MacInnes and Davis, 2011). In a dehydrated horse, the skin, when pinched, will not immediately snap back, but will ‘tent’, so in some literature this is also called the ‘skin-tent test’. In the capillary-refill test performed on an adequately hydrated horse, after pressing on the gums above the corner incisor, the colour will return to normal within 2 seconds (MacInnes and Davis, 2011). Urine will be concentrated and darker in colour, but even with total water deprivation will not be less than 0.5 L/100 kg BW per day unless the horse suffers from renal failure (NRC, 2009). The horse is able to absorb substantial amounts of water and electrolytes from the large colon during prolonged exercise and by doing so will get a “tucked up” appearance to the abdomen (Schott, 2011). Thumps is a serious sign of isotonic dehydration and exhaustion, although the condition itself is not. It’s comparable with the hiccups in humans (the horse’s flank twitches and makes an audible “thump”), but when it occurs veterinary care is required (MacInnes and Davis, 2011). Thumps is caused when too much chloride is lost through sweat. Chloride and bicarbonate are the only anions (negatively charged ions), so in order to maintain electro-neutrality when Na+ is reabsorbed under influence of aldosterone (secreted during moderate exercise - Muñoz et al., 2010) and CLˉ is depleted, HCO3ˉ is no longer excreted by the kidneys and too much bicarbonate causes alkalosis* which in turn can cause colic or thumps. Tie up is an even more serious form of isotonic dehydration. The muscles cramp up and the horse becomes reluctant to move and might seize up completely (MacInnes and Davis, 2011).
- Hypertonic Dehydration – Thirst, fatigue, lower urine output, dry tacky mucus membranes, delayed capillary refill, reduced skin turgor, sunken eyes, restless, decreased feed intake, weight loss, impaction colic, collapse
- Hypotonic Dehydration – Fatigue, Lack of thirst, rapid faint pulse, weakness, renal dysfunction, collapse, muscle spasm, shock
- Isotonic Dehydration – Lack of thirst, reduced skin turgor, coldness, fatigue, concentrated urine, sunken eyes, muscular tremors, colic, thumps, muscle spasm tie-up, shock
Prevention and Management of Dehydration in Horses
As the system for fluid homeostasis is based on a negative feedback system, some imbalance has to occur before action is taken to restore the balance. According to Coenen (2005) a positive water balance is achieved after feeding and watering, and water retention increases postprandially* for 4-5 hours, but the rate of fluid loss through sweat during a 2-hour exercise period results in a negative water balance. A basic ration that depresses the acidic effect of metabolism, lowers heat production itself, and lowers cortisol liberation would be desirable as cortisol readily increases during moderate exercise. As the weakening capacity to eliminate the heat which is produced by strenuous exercise is a performance-limiting factor, Coenen (2005)hypothesized that fat as energy source would reduce heat of metabolism, but fermentable fiber is still required in order to prevent a collapse of the microbial system in the hindgut. Fiber sources like the highly fermentable pectin in beet pulp has the added benefit of increased water consumption (Coenen, 2005). Ideal would be to replace water and ions at the same rate as they are lost in sweat, but during exercise gastrointestinal function is compromised by many factors that have priority over gastric emptying and intestinal absorption, which makes it difficult to actually absorb the water and electrolytes offered (Ecker, 1995; Frape, 2010). Care has to be taken with offering high concentrations of electrolytes to avoid hypernatremia (Schott et al., 1999; Hess et al., 2007). Hess et al. (2007) hypothesized that potassium supplementation affects plasma K+ and may lead to neuromuscular hyperexcitability. They also hypothesized that diets high in calcium (1.5% instead of 1%) would lead to higher plasma Ca++ and the results of the study during an 80 km endurance race confirmed this, however, the test group that was supplemented with potassium during the race was also supplemented with 1.5% calcium and thus the neuromuscular hyperexcitability due to high K+ and low Ca++ did not occur, but Hess et al. (2007) concluded that higher plasma Ca++ and lower K+ during exercise could help reduce the risk.
- Hypertonic Dehydration – Always provide access to fresh, clean, palatable water and free choice salt, as a salt block or loose salt and if necessary added to the feed. Limited water intake reduces food intake (NRC, 2009). “Good-quality water must be free of harmful germs, foreign material, excessive minerals, environmental pollutants and unusual flavours” (Marteniuk). Horses may eat snow on occasion, but snow should not be considered as a water source (NRC, 2009; Marteniuk). Recommended water temperature is 5 – 8º C in cold ambient temperatures and around 20 º C after exercise on hot days. On long transports regular stops should be made to provide water.
- Isotonic Dehydration – Horses need the time to adapt to large changes in temperature which may take up to 3 weeks (Frape, 2010). Training increases the maximum oxygen uptake abilities and core temperature is proportional to the percentage of VO2max (Frape, 2010). The upper critical temperature for adult horses is 25º C. Above this temperature horses thermo-regulate through sweating and/or breathing more rapidly. (NRC, 2009). “Ensure the horse consumes his baseline sodium, potassium, and chloride requirements first and then use electrolyte supplements as needed to replace sweat” (Kellon, 2012). Choose an electrolyte product that has a potassium-sodium-chloride ratio of 1:2:4 as this is the ratio in which it is lost in sweat (Kellon, 2012). Strategies to make horses drink more during endurance activities are for instance oral electrolyte pastes before and during the ride. Tests have shown that horses treated with 5 oz of NaCl and 2.5 oz of KCl paste (to replace 25 liters of sweat) were drinking more water and started drinking earlier after exercise than without the paste (Schott et al., 2011). Field studies however showed that this extreme high dose does not work for all horses as some horses did not drink enough to replace the 30 liters of sweat loss and consequently showed plasma sodium concentration values exceeding 150 mmol (hypernatremia). This resulted in a recommendation of replacing only 30 – 50% of the estimated electrolyte loss (Schott et al., 2011). Most commercially available electrolyte pastes contain less than 10 g of NaCl and KCl combined and many contain carbohydrates and trace amounts of other minerals for which there is no supporting data. The losses of Na+, Cl2- and K+ are 100 fold greater. Recovery from furosemide-induced dehydration by oral administration of NaCl was found to be more effective than KCl (Schott et al., 1999). Schott et al. (1999) observed during simulated endurance exercises that water intake mostly occurred within 60 seconds of stopping the treadmill. No adverse effects were noted with the availability of unlimited water immediately after exercise.
Conclusion
Water is the most important nutrient, but without balanced electrolytes (and a properly working endocrine system) it is impossible for the horse to maintain fluid homeostasis. Whether regarding an idle horse or sport horse, maintaining fluid balance is important to sustain all bodily functions. Horses evolved to withstand cold better than heat and in natural settings usually rest during the hottest part of the day. In natural settings horses only spend 1% of the time galloping or trotting (Lee, 2011) which is only 15 minutes out of the day. It is therefore not difficult to understand why heat gain is a major limiting factor to exercise and why thermo-regulation in horses did not evolve to be a very efficient cooling system for endurance exercise and/or hot and humid conditions, but often results in moderate to severe dehydration.
Important Considerations: Take Home Points
1. Thirst, lack of thirst, slow capillary refill, ‘tenting skin’, concentrated urine, sunken eyes, excessive sweating and thumps,
low pulse : respiration rate are signs of dehydration.
2. Always provide access to clean, fresh palatable water.
3. Salt is very important in maintaining water balance.
Salt blocks alone are often not sufficient for exercising horses, provide loose salt or mix it with the feed.
4. Cold water temperature reduces intake, while in the winter water requirement might be higher due to increased hay intake to
maintain body temperature (Marteniuk)
5. At 20:1 moisture, a horse has to eat 500 litre of fresh snow to melt it into 25 Litre of water.
6. Sweating is controlled by thermo-regulation, independent of hydration status or electrolyte homeostasis (NRC, 2009).
7. Horses should be allowed to drink frequently during extended work and should be allowed 2 minutes for each occasion (Frape, 2010)
8. After exercising on a hot day, horses prefer water of around 20º Celsius (NRC, 2009)
9. Drink volume after exercise is higher from buckets than it is from automatic waterers (Nyman and Dahlborn, 2001)
References:
Bourque, C.W., 2008. Central mechanism of osmosensation and systemic osmoregulation. Nature Reviews Neuroscience 9, 519-531 (July 2008)
Butudom, P., Axiak, S.M., Nielsen, B.D., Eberhart, S.W., Schott II, H.C., 2003. Effect of varying initial drink volume on rehydration of horses. Physiology & Behavior 79 (2003) 135– 142
Coenen, M., 2005. Exercise and stress – Impact on adaptive processes involving water and electrolytes. Livest Prod Sci. 92, 131-145
Ecker, G.L., 1995. Fluid and ion balance in the performance horse. The performance horse: Focus on endurance I and II, Equine Research Centre (1995), University of Guelph
Frape, D., 2010. Equine Nutrition and Feeding. Wiley-Blackwell, West Sussex, UK.
Hess, T.M., Greiwe-Crandell, K.M., Waldron, J.E., Williams, C.A., Lopes, M.A., Gay, L.S., Harris, P.A., Kronfelder, D.S., 2007. Potassium-free electrolytes and calcium supplementation in an endurance race. Comparative Exercise Physiology 5 (1): 33-41
Kellon, E., 2012. Equine Electrolytes, Exercise, and the Heat. Uckele Health and Nutrition, Aug 2012, Article #29617.
Lee, J., Floyd, T., Erb, H., Houpt, K. 2011. Preference and demand for exercise in stabled horses. Applied Animal Behaviour Science 130 (2011) 91-100
Leibsle, S., 2012. Cold Weather Colic. AAEP Health Articles.
MacInnes, R., Davis, H. 2011. Equine Emergency First Aid Manual, 2011. Equi-Health Canada, Okotoks, AB, Canada.
Marteniuk, J. Winter dehydration in horses. MSU College of Veterinary Medicine.
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APPENDIX – A
Alkalosis Interstitial fluid Gaseous exchange Cutaneous evaporation Homeostasis Hypothalamic-Pituitary (HP) Axis Pituitary Gland Osmolality changes Extracellular fluid Hypertonic Hypotonic Haemolysis Hypovolemia Anaerobic exercise . Aerobic exercise Postprandially Enterocolitis Isotonic Renin-Angiotesin-Aldosterone Vasopressin (RAAV) axis Plasma tonicity |
GLOSSARY
When the body’s pH changes towards alkaline Fluid between the cells Exchanging CO2 for O2 in the body Evaporation through the surface of the skin Maintaining stability Hypothalamus – part of the brain that regulates all basic life functions Part of the brain that produces regulatory hormone factors Changes in fluid concentration Fluid inside the body, but outside of the cells Higher osmotic pressure (higher concentration of solutes) Lower osmotic pressure (lower concentration of solutes) Recycling of old red blood cells Low blood volume Short and intense, fuel for the muscles is generated mostly without the presence of oxygen Low to medium intensity, Fuel for the muscles is generated in the presence of oxygen After eating Inflammation of small and large intestines Same concentration of solutes as the blood Preserves haemodynamic stability to changes in blood volume, blood pressure, electrolytes and water Plasma concentration |