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Gayle Ecker and Michael Lindinger

For Many of us, it is easy to marvel at the fluid grace found in the movement of the equine athlete, yet it may surprise most to learn that from an "energy-conservation" point of view, the horse is quite inefficient. Approximately 70 to 80% of the energy produced is not useful for the cells and must be lost as heat. In order for the horse to maintain a healthy functioning body, this heat must be dissipated from the body or serious heat problems will develop. As a horse exercises, large amounts of heat are produced which must be dissipated through sweating (evaporative cooling). With prolonged periods of sweating, significant amounts of electrolytes are lost through the sweat which can lead to health problems for the horse. When heat and humidity are present, the heat released through sweating is compromised, causing the horse to sweat more and therefore lose more electrolytes.

The major route for heat loss, about 65% of the total heat load, is through evaporation of the sweat. The sweat gland uses fluid from the blood stream to secrete fluid out to the surface of the skin where the sweat spreads out and evaporates, taking heat away from the body. Under certain conditions, such as heat and humidity or lack of a breeze, the efficiency of the evaporation process is decreased. If heat can not escape, such as when the horse is blanketed or in a closed trailer, the horse is unable to reduce the body heat efficiently. The surface area available for sweating is also important. A higher surface area means more sweat glands are available for sweating. Compared to humans, horses have less surface area relative to their body mass than humans. When saddles and heavy blankets are left over the back area, this reduces the area available for sweating and therefore heat dissipation. It is important to realize that the saddle area has a high density of sweat glands and therefore should be uncovered whenever possible in hot weather conditions. A second route for heat loss is through the respiratory system, which is why you may see horses hyperventilating or "panting" when the body temperature is high.

Sweat collected from the horse has been reported to be hypertonic, which means that there are higher concentrations of electrolytes in the sweat than in the blood. Although there are problems associated with sweat collection procedures, researchers have measured the electrolytes in sweat and these averages are presented in Table 1. Note the large difference between the sweat and plasma concentrations of potassium (K+) and chloride (Cl-). Whenever the horse is sweating, there is loss of electrolytes from the blood and all cells in the body. With high rates of sweating or prolonged periods of sweating. The losses of electrolytes can be quite high and lead to problems. Sweat rates of horses may reach 12 L per hour or more during heavy exercise in hot and humid conditions, although rates of 3 to 6 L are more common.

Table 1. Average concentrations of sweat 7 plasma electrolytes (mM/L) in exercising horses.
	Na+	K+	CL	Ca++	Mg++
Sweat	159	40	165	6.2	4.6
Plasma	140	4	100	3.0	0.8
Note: These numbers represent averages taken from the results of several studies.

In order to lose body heat, the blood flow to the skin is increased, where the blood vessels of the skin are dilated and produce that "lacy" appearance. But if the horse is exercising or recovering from exercise the muscles still require high blood flow, not only to bring fuel to the muscle, but also to take away the metabolic wastes and the heat produced by the muscles. Therefore not enough blood may go to the skin to allow adequate rates of evaporative cooling, and heat strain will result unless exercise intensity is reduced. In order to help supply the blood to both areas, the body diverts blood away from the digestive tract and other "nonessential"tissues. As fluid is lost from the body due to sweating, the blood becomes "thicker," indication that the body has become dehydrated and this can be measured as a decrease in body weight. If this continues, the perfusion to the muscles, brain and lungs becomes inefficient. If the water is not replaced, this could lead to a life-threatening situation for the horse, as the kidney has increasing problems filtering the blood, and the lack of blood supply to the gut can cause the gut to shut down.

In the summer and fall of 1991, research was carried out to study the fluid and electrolyte changes in horses competing at Ontario endurance rides. A total of four rides, with distances from 50 to 75 miles in length were chosen, and riders permitted us to sample blood at rest, at the midpoint of the ride, at the finish and at one hour of recovery. The total amount of blood taken was less than 0.1% of the total blood volume, not enough to be noticed by the horse! The blood was analyzed for plasma electrolytes including: sodium (Na+), K+, CL and ionized calcium (Ca++), and also plasma proteins, glucose, and hematocrit. Horses were divided into two groups based on speed, with group A averaging 14 kph or higher, and group B averaging less than 14 kph. Blood levels between stages and between groups were compared for statistically significant differences. Based on estimated resting body weights and average blood volumes, the losses of electrolytes were calculated on selected horses and compared to supplements received.

Glucose

Glucose was included in this study because it is a metabolic fuel source that is used by the tissues. Certain tissues, such as the brain and retina use glucose exclusively as a fuel, so it is vital that glucose levels be maintained. When other metabolic stores in the body are being used up, such as muscle glycogen, or, if the intensity is too great and the body can not burn fat quickly enough to meet the energy demand then the glucose coming from liver glycogen is used from the blood. Stores of glycogen can be mobilized to provide more glucose, but it may not keep up to the prolonged energy demand, and blood glucose will decrease. There was great deal of variation in the levels of glucose, due to a variety of factors such as diet and stress level. Generally though, glucose levels will increase during the early part of the ride as glucose is mobilized, then there is a decline in glucose levels towards the finish below the pre-ride concentrations as liver stores start to become depleted. By one hour of recovery, glucose levels regained normal resting values due primarily to the decreased energy demand, and possibly to feed intake. In the longer rides the top finishing horses were well trained and highly fit. Their higher glucose levels, compared to their less well-trained counterparts, indicate an improved ability to burn fat for energy, thus sparing liver and muscle glycogen stores to some extent. In contrast, in the shorter rides the faster horses also tend to be the highly trained athletes, moving at a rapid pace, which requires that an increasing amount of the energy demand by exercising muscles be met from glucose oxidation. Therefore muscle and liver glycogen stores are used more rapidly and as liver glycogen becomes low, plasma glucose levels decrease. This decrease has to be associated with a decrease in exercise intensity, i.e. running speed. With the slower speed more reliance can be placed on fat for an energy source. Fat can not provide energy at as fast a rate as glucose, but it provides more energy on a mass basis.

Lactate

When a horse is exercising at high intensities, the energy demand from the muscles is higher than can be supplied by fat metabolism. The muscle glycogen (actually clusters of glucose molecules attached together in a complex chain stored in the muscle which can be used as fuel) is broken down or metabolized to provide energy for the muscles in a process called anaerobic glycolysis. This route for energy production is very fast but produces lactate (lactic acid) which increases the acidity of the muscle cells and blood. Endurance horses generally maintain speeds which are "aerobic" or below the intensity which products lactate. There may be parts of the trail, such as going up hills, or a sprint to the finish where the lactate levels will increase, however, in the well-trained horse the lactate is quickly metabolized (and actually used to provide energy) when the intensity decreases. Blood lactate levels were generally well within resting levels on the endurance horses, as by the time the horses cleared the vet checks, any remaining lactate should be metabolized.

Sodium

The greatest store of NA+ is found in the fluid outside the cells, also called the extracellular fluid (ECF) space which includes blood plasma. Generally during exercise and dehydration sodium levels are maintained within normal limits, because NA+ is being lost together with water from the plasma. When accounting for dehydration, however, the losses of NA+ become apparent.

Chloride

Chloride is found in high levels in the ECF and also in the red blood cell. A significant amount of CL can be lost in the sweat. As water is lost through sweating, there is also a loss of chloride, and it is common to see plasma concentrations of CL decrease over an endurance ride, especially during hot weather. CL losses balance the NA+ and K+ losses (charge balance of electrolytes).

Potassium

Compared to other electrolytes, K+ is present in low concentrations in the blood, but in high concentrations in the red blood cell and in muscle. During exercise, K+ moves from muscle to plasma. In fact, during high intensity exercise, plasma K+ may increase to levels as high as 10 mM/L but the K+ is quickly moved back into the muscle during the first few minutes of recovery. During endurance exercise however, a significant amount of K+ can be lost in the sweat and decreases in plasma concentrations are reflective of body losses. Decreases in muscle and plasma K+ are associated with the development of fatigue and exertional rhabodomyolysis (also called "tying up," a stiffness which develops in the muscles). It has also been seen that there may be a further decrease in plasma K+ following the ride. This could be due to movement of K+ back into the muscles. It could also be due to dilution of the blood if the horse is drinking only water with no electrolytes or, if the horse is feeding, K+ and H2O may move transiently from the plasma into the gut.

Calcium

Loss of Ca++ can occur with prolonged sweating, however there can also be movement of Ca++ into muscle during prolonged exercise. There is also a high store of Ca++ in the bones, which can be mobilized under the influence of the parathyroid hormone. All four rides demonstrated a decrease in the plasma levels of Ca++ over the ride, and generally these levels were not restored with one hour of recovery.

Low plasma concentrations of Ca++, and also low K+ and CL, have been associated with the development of "thumps" in some horses.

Plasma Proteins

On all the rides, there was an increase in plasma proteins. This is an indication that fluid was lost from the blood plasma and the horse has become dehydrated. Many horses do not drink enough during the ride (particularly during the first 15 to 30 miles) to replace what is being lost in the sweat. As there was little difference between finish line values and one hour recovery values, the water deficit was slow in being replenished to pre-ride levels. In general, the horses were given ample opportunity to drink on these rides, not only along the trail, but also at the vet checks, yet as a group, they were unable or unwilling to restore the water deficit, imposing a voluntary dehydration. Several studies have shown that unless trained otherwise, horses and other mammals will not drink until dehydration equal to at least a 10% loss in plasma volume has occurred, i.e. about 3 L in endurance horses. Protein from plasma can also be lost in sweat and this is seen as foaming in untrained horses or those not acclimated to heat.

If a horse had an average sweat loss of 10 L/h for 4 hours, which could occur on an endurance ride, the following amounts of fluid and electrolytes would be lost from the body through the sweat: 40L water, 6.3 moles NA+, 1.6 moles of K+, 6.6 moles of CL, 0.24 miles of Ca++, and 0.18 moles of magnesium (Mg++). To replace this loss through electrolyte supplementation would require approximately 2 1/4 cups of Half and Half Salt (half NaCl and half KCl) for the NA+ , K+ and CL losses.

In order to understand what this means in terms of electrolyte losses, let's take an actual example of a horse competing at a 60 mile endurance ride. Before starting the ride, we determine that hour horse weights 381 kg. We then take a blood sample and measure the hematocrit, the plasma proteins, and the electrolyte levels in the blood before the ride (Table 2).

At the end of the ride, we again weigh the horse and take another blood sample. Our horse lost 18 kg or 5% of its body weight by the end of the ride. We then look at the change in the plasma proteins and the levels of electrolytes in the blood to determine the actual losses of electrolytes. We find that this horse lost the following amounts of electrolytes when compared to pre-ride status: NA+ 1.9 moles, CL 1.53 moles, K+ 0.06 moles and 0.05 moles of Ca++. Relative to initial levels of electrolytes in the ECF, this represents a loss of 17% NA+, 19% CL, 25% K+ and 37% Ca++. This horse finished the ride but developed "thumps" at the finish line due to the electrolyte losses. The horse was receiving an electrolyte supplement during the ride, however, still experienced substantial losses and was unable to maintain plasma electrolyte levels or hydration.

Conclusions from the study

Exercise for prolonged periods will result in significant losses of fluid and electrolytes. These losses may lead to serious problems for the horse and can be like-threatening if not corrected through fluid and electrolyte supplementation.

There is a considerable variety of supplementation regimes in use by endurance riders, yet plasma levels of electrolytes suggest that substantial losses are occurring anyway. Some riders mix up their own recipes, however many prefer the convenience of commercially available electrolyte products. Most of these commercial products are designed for animals suffering from various conditions such as colic, diarrhea, or shock, and it seems likely that the needs of the healthy exercising horse are quite different for not only quantity, but also the ratio of electrolytes in these supplements. Many horses experience problems related to electrolyte and fluid depletion despite receiving electrolyte supplements. It is clear that further research is needed to accurately quantify the losses. This research will develop guidelines for electrolyte supplementation to improve the health and performance of our equine athletes.

Factors Affecting Electrolyte Requirements in the Exercising Horse

If the horse is exercising at high intensity and/or for long duration in hot and humid weather conditions, if the horse is not well trained and/or not acclimated to hot and humid conditions or if dehydrated and electrolyte depleted before the ride (due to prior exercise, trailering, colic, etc.), then fluid and electrolyte supplementation should be increased beyond the normal levels. NOTE: Water alone is not sufficient! Water intake may only dilute the blood further following electrolyte losses. Supplements containing sodium, potassium, chloride and calcium should be used in conjunction with fluid intake. Development of more definitive guidelines for electrolyte supplementation is the focus of current research. Glucose in small amounts (i.e. about 2% solutions, or 6 tablespoons per gallon) enhances the uptake of electrolytes from the gut in humans, therefore, it may be beneficial to include small amounts of glucose in the solution. Solutions with higher levels of glucose should be avoided as these will cause increases in insulin secretion, and greater swings in glucose and K+ levels.

Current and Future Research

Research is currently underway to study the changes in electrolyte levels at three types of competitions which represent increasing levels of exercise intensity: endurance rides with distances of 50 to 100 miles, competitive rides with distances of 25 to 40 miles, and during the cross-country jumping portion of the horse trials (one-day eventing). Blood samples and body weights are taken before, during and after the event in order to study the loss of fluid and electrolytes and to determine the shifts that occur between the various compartments in the body.

The program currently underway with the Young Riders will not only determine the fluid and electrolyte losses during the Horse Trials, but will also give riders valuable information on the fitness level of their horses and suitability for attempting advanced levels. If funding is available, further work will be done in this area to compartmentalize the losses; that is, to determine what losses are occurring from inside the cells vs. outside the cells. Then, how the various electrolyte supplements work to restore the losses due to exercise or other conditions will be investigated. Factors such as concentration, method of administration, intensity of the exercise following administration and hydration status of the horse are likely to affect the rate of uptake and effectiveness of the electrolyte supplement and these will be investigated as well.

A Note of Thanks: Without the cooperation of owners and riders, this research is not possible. I would like to thank all participants for the wonderful support shown at the events and for volunteering to become a part of the "research team." A special note of thanks to John Neille, the Young Riders and the Ontario Equestrian Federation for their assistance in developing this program, and to the Ontario Competitive Trail Riders Association members for their support and cooperation. I would also like to acknowledge the financial contribution of the Ontario Horse Trails Association in support of the Young Riders. The acquisition of the portable equine scale was made possible by the Canadian Morgan Horse Foundation. My heartfelt thanks to all of you for your important contribution to research.

An active search for corporate sponsorship to fund portions of this research is underway. Any inquiries may be made to Gayle Ecker at the Equine Research Centre or to Mike Lindinger at the School of Human Biology, University of Guelph.

 

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The Equine Research Centre
University of Guelph
Guelph, ON N1G 2W1