Supplementing Fluid and Ion Losses in the Endurance Horse

An Update on Supplementing for Fluid and Ion Losses in the Endurance Horse

Gayle L. Ecker

Problems Faced by the Endurance Horse

When a horse is exercising, fuel is broken down to release energy for muscle contraction. However, because this process is inefficient, about 75-80% of this energy is not used and is released as heat. Some of this heat builds up in the body, thereby increasing body temperature. During strenuous exercise, if enough heat is not dissipated, the temperature could rise as much as 0.25°C per minute. This would increase the temperature to a potentially lethal level of over 40°C in about 12 minutes! Heat can be gained from the environment, such as the radiant heat from the sun. This applies especially to dark coloured horses (solar radiation, and heat reflected from the ground).

Heat is lost from the horse by conduction, radiation, convection (when air is moving over the horse) and evaporation (when sweat on the skin turns to water valor, heat is taken from the skin). Evaporation of sweat is the main method of heat loss for the horse during exercise (this is aided by convection as the horse moves through the air), although there is also heat loss through the respiratory system. The heat produced in the muscles is carried out to the surface of the skin via the blood. When the body temperature is raised, blood flow or cardiac output is redirected to the skin where the heat is released by conduction and convection to the skin. When conditions of high temperature and exercise are present, there is a "competition" for the limited supply of blood; the muscles need blood to bring oxygen and fuel and to take away the metabolic "wastes" including the heat produced.

With increases in body temperature, the sweat gland secretes a watery fluid onto the skin. When the sweat evaporates into a valor, heat is taken from the skin. Sweat that merely drips off removes little heat. Evaporation of sweat is reduced when relative humidity is high and when the skin is covered, reducing airflow over the skin (such as with saddles and blankets). Air currents moving over the skin help increase the rate of heat release and the frequent application of water on the skin and scraping off helps to take heat away.

Body surface vs. body mass
Heat dissipation depends on the available surface area relative to the body mass (BM) of the animal. For a human with a mass of 60 kg, the surface area (SA) is approximately 1.7 square meters, resulting in a ratio of 35 to 1. For a 500 kg horse, the SA is about five square meters, and this results in a ratio of 100 to 1 (think of the heat producing muscle mass during exercise that contributes to total BM). Considering that the saddle and girth cover more of the surface area (and the back is an area of high density for sweat glands), this further reduces the mass specific area for heat dissipation. Hair density is also a factor that can further reduce the heat loss. A tight girth may interfere with the circulatory system of areas of the skin, so the girth should be loosened when possible too.

Breed differences for heat dissipation are evident. Heavily muscled horses (high BM:SA) generally have more problems dissipating heat than lean, fine-boned horses (low BM:SA). Due to heavy genetic selection, it is also possible that Arabians have further differences that result in the endurance capacity noted for this breed. It is likely these differences that account for the fact that 90% of the horses listed in national endurance standings are of Arabian descent.

A Review of Fluid Compartments in the Horse

Water, water everywhere?
There is approximately 300 L of water in the average 450 kg horse, approximately 66% of BM. The water can be considered to be in two compartments, the water inside the cells (intracellular fluid, ICF) and the water outside the cell walls (extracellular fluid, ECP). The ICF accounts for 200 L, most of that is in the muscle. The ECF is approximately 100 L, and includes the water in the blood stream (plasma 25 L), intestinal and lymph fluid (45 L) and the gastrointestinal water (30 L). The ECF is high in sodium and chloride while the ICF is high in potassium. The water can move freely between compartments and remains in osmotic equilibrium. The functioning of the tissues depends on the osmotic balance between compartments. A change in the water or ion content of one compartment affects the other and may affect the various biological processes (i.e., the loss of potassium (K) in the sweat depletes the K in the muscle; this may affect the contraction process).

Ion Extracullular Fluid Intracelllular Fluid
Na Sodium 142 mEq/L (138-146 mM) 10 mEq/L
K potassium 4 mEq/L 140 mEq/L
CL chloride 103 mEq/L (103-112 mm) 4
CA calcium 2.4 mEq/L (1.0-1.4 mm) >0.00001 mEq/L
MG magnesium 1.2 mEq/L (0.06 mm) 58 mEq/L
HC03 bicarbonate 28 mEq/L (24-32 mm) 10 mEq/L
Glucose 90 mg/dL (7-95 mg/dl) 10-20 mg/dl
PH 7.4 (7.3-7.5) 7.0

Routes of Water and Electrolyte Losses

Where does it go?
The main route of water and electrolyte loss during exercise is through the sweat. Sweat rates may reach 10-15 L/h in the horse, whereas maximal sweat rates are 2-3 L/h in the human. During endurance rides, it is likely that the sweat rate would be in the 6-10 L/h range, depending on conditions. There are also fluid and ion losses through the respiratory system and in the urine, but relative to the sweat produced over long periods, sweating is the main route of water and electrolyte loss. The purpose of the sweat gland is to secrete a fluid onto the surface of the skin, so that the water may evaporate and heat loss from the skin will occur. The sweat gland in the horse lies in the dermis alongside a hair follicle. It does not possess the coiled duct section that reabsorbs ions in the human. This mechanism is very important in the human to reabsorb the electrolytes before they are lost. Why this does not happen in the horse is unclear. Differences in sweat concentrations between breeds have not been systematically studied and very little has been published.

During prolonged exercise, the rise in core temperature triggers the sweat glands to secrete a hypertonic fluid (more concentrated than the plasma) to the skin surface. Table 2 demonstrates the ion concentrations in equine sweat, and contrasts this to the sweat ion concentrations in the human. Electrolyte supplementation based on sweat concentrations has been suggested, however, sweat ion concentrations are not constant over time, therefore this may not provide optimal replacement.

Source NA (mm) CL (mm) K (mm) CA (mm) Duration
Carlson and Ocen,
1979 (excr)
132±43 174±66 53±21 6±3 ---
Carlson and Ocen,
1979 (epi)
130±11 189±11 25±5 3±2 ---
Rose et al.
1980 (excr)
249±9 301±20 78±8 --- ---
Snow et el.
1982 (exer)
159 165 32 --- 4-5 hour
80 km
Kerr and Snow,
1983 (epi)
194±6 207±9 24±2 --- 3 hour
Kerr and Show,
1983 (heat)
149±4 194±8 50±8 --- 5-6 hour
Humans Nelson,
9-65 6-43 4 negligible ---

Table 2: Sweat ion concentrations in the horse and in humans (bottom row).
Note: exer = exercise-induced sweat; epi = epinephrine infusion sweat.

How Do We Cool the Horse?

Sweating is the main route of heat loss for the exercising and recovering horse, therefore it is important to do what we can to enhance this process. The sooner the horse cools down after it arrives the better. As long as thermoregulatory demands are being made on blood volume and distribution, there may be less blood available to the gut for adsorption of water and nutrients. High body temperatures while exercising have been shown (in other species) to increase the reliance on glycolysis (using carbohydrate sources as fuel), rather than on using fats for energy which is preferred for endurance exercise. To help the horse cool out, remove the saddle and pads when possible (or at least loosen the girth) as this is an area of high density of sweat glands. Apply repeated applications of water and scrape it off as this will help take more heat off the skin, and reduce the amount of heat needed for cooling. If the horse is going to be exercising in the heat, clip the hair-coat short down the neck, chest and shoulders, down the tops of the legs (leave hair where leather may rub for protection). Place a wet towel over the neck or poll area to allow water to run down the neck, taking heat with it. Find some shade with a bit of breeze, if possible. There has been a lot of controversy about using cold water to cool out a horse. Some recent work on event horses has shown no adverse effects when using ice water over these horses. However, these horses are generally hotter than endurance horses, and ice water over the skin shows only a temporary cooling effect before the blood temperature heats the skin up again.

How the Research was Carried Out

Blood, sweat and tears!
The study included over 240 horses competing in 13 rides of 30-100 miles (48-160 km), with mean temperatures from 6-32°C, variable weather conditions (from the miserable to the sublime!), and mean speeds of 6-11 mph (9-17 kpm). Blood samples were collected and BM obtained at rest, mid-ride, finish and one hour of recovery. Due to the limitations inherent in field studies, assumed values for average blood volume and ECFV were used.

Lessons From the Ride of Champions (ROC) and Other Rides.

1. The most significant water losses occur even during the first 20 miles of a 100 mile ride. At the first vet check of the ROC, water losses were measured at 5-28 L. Water losses occur early and often are not replenished. Losses of 22-27 L, were common at the mid-ride and losses of 8-30 L were found at the end of the ride. Virtually all horses were in a water deficit situation all through the ride. Out of over 240 horses, only one had regained its preride body weight after 100 miles. What is not yet known (and can only be done with controlled research in the lab) is the distribution of water between gut, ECF and ICF.

2. Losses of ions occur early in the ride and in general persist for the rest of the ride, with little recovery after 60 minutes of rest, despite fluid and electrolyte supplementation.

3. Elite horses maintained fluid and ion balance better than other horses at the ROC.

4. Preloading with electrolytes may help offset losses, at least in the early part of the ride.

5. Loss of BM is correlated to ride speed at the ROC but not for the whole population.

6. On rides of 40 miles, electrolyte supplementation seemed to have no effect on fluid and ion balance during mild temperatures.

7. Increasing speed, temperature and the presence of muddy terrain are correlated to increased losses of ions.

How are We Doing With Electrolyte Supplementation?

Theory vs. practice
The theory behind fluid and electrolyte supplementation is to ingest solutions at the same rate as they are being lost. However, from human studies, we know that the electrolyte solutions themselves have an impact on the function of the gastrointestinal tract.

Riders participating in the study were questioned regarding electrolyte supplementation during endurance rides. A descriptive summary of the results follows. On rides less and 80 km (50 miles), less than 50% of riders reported administering electrolytes to their horse. On rides of 80 km (50 miles) or greater, 97% of riders used electrolyte supplements. A survey of 95 riders revealed little consistency in how or which electrolyte supplements were used. Riders mentioned 17 different brand names and at least 23 variations of "home recipes." Only 37% of horses received electrolyte supplements the morning of the ride. Of this group, 62% of these were horses at the Race of Champions (ROC100), therefor only 23% of horses not competing in the ROC100 received electrolyte supplements the morning of the ride. On rides of 80 km (50 miles) or longer, 63% of horses received electrolyte supplements before or at the first vet check. It was common to find combinations of brand name electrolytes and home recipes being used. Dosage, rate and method of administration and other factor varied considerably. It is clear from the BM losses on horses and the decreases in plasma ion concentrations that electrolyte supplementation is inadequate in the large majority of horses studied to date.

Can We Have A Recipe?

Effects of temperature, speed, distance and terrain
1. losses of sodium and chloride increase with increasing temperature.
2. losses of sodium and chloride increase with increasing mean ride speed.
3. losses of ions are increased in the presence of muddy terrain.
4. losses of total body water are not statistically correlated to temperature, speed, distance or terrain.

It is difficult to give actual amounts of electrolytes based on the current research as there are so many variables which could affect this. For instance, the initial nutritional status of the horse and the presence of food materials already in the gut. Some feeds are more easily broken down than others and the electrolyte content can differ substantially. The training level of the horse relative to the mean speed it maintains and whether it is trained to drink in the early stages of the ride will also have an impact. Therefore it would be premature to give actual amounts of electrolytes for supplementation. Based on the wide inter-individual variation, ideally each horse should be evaluated; now, at best, most horses will receive either too little or too much. What is needed is for owners/riders to carefully and repeatedly evaluate water and ion losses in their horses under different conditions to get a feel for the optimum amount of supplementation.

Can Horses be Acclimatized?

This is a very important question for the horse and as yet, there are no clear answers. Acclimatization refers to changes in the physiology of the body that enable it to adapt or cope with new conditions, in this case exercising in the heat. Humans can acclimatize to exercising in hot conditions by exposing themselves to hot temperatures for five to seven days. There are still further changes that occur after that, but the majority occur in only a few days. These changes include a more dilute sweat, starting to sweat earlier, therefore keeping body temperature from rising as quickly, a larger blood volume that helps in thermoregulation and cardiovascular stability, and an enhanced endurance for exercise in the heat relative to the endurance capacity before the acclimation process (about 85% of capacity under normal conditions). There are research institutions currently studying the impact of passive and active heat acclimation in horses (Newmarket and Guelph) and the results of those studies should provide more information. For now, the advice being given by the FEI is to acclimatize the horse to hot temperatures for three weeks prior to competing. If you are not able to acclimatize your horse, make sure that it is very fit, as a high level of fitness helps the horse tolerate the heat stress more effectively. Also, the riders and pit crews should be even more vigilant in monitoring the temperature of the horse and assisting with cooling out as necessary. Remember that the rectal temperature is generally about 2°C below the muscle temperature. The time course of changes will vary with intensity and duration of exercise and environmental conditions. Any extra weight should be removed from the horse to lessen the amount of effort which will help reduce the buildup of heat. A report from ELDRIC suggest that for every extra 10 Kg on the horse, the pace slows 1 kph.

Are There Any Conclusions?

Unfortunately, we still have a lot to learn about fluid and ion balance during exercise and the "trial and error" approach continues. Many factors are still unknown. However, from the data collected from this study and several others, and from the many discussions with veterinarians and riders, the following suggestions and comments may be useful:

1. Losses of water and ions occur early in the ride and may be significant even after 20 miles of a 100 mile ride. As speed, temperature increase, in general, so do the losses. The presence of muddy terrain will also increase losses. This suggests that total quantities of water and electrolytes may have to be increased from the usual dose when these conditions are present.

2. Small, frequent doses of diluted solutions (isotonic) may be more beneficial than concentrated, infrequent doses. Dosing of electrolytes should occur early in the ride, before the midpoint. Dosing of electrolytes should be given preferably after the horse has had a drink of water. Preloading may be beneficial to offset losses. Do not give concentrated solutions (hypertonic) to a horse that is already dehydrated.

3. Make sure the horse is fully hydrated at least 12 hours before starting the ride. Dehydration can occur due to trailering, refusal of water that tastes different or is in different container. Increase water ingestion by wetting hay, adding water/diluted apple juice to grain mixtures. Horses may drink more if water is offered in buckets. Favoring the water at home and at the ride site with a bit of apple juice may help to mask the different taste in water from a new location.

4. The addition of glucose to the solution does not seem to increase absorption as it does in humans. Glucose content should be kept low to avoid glucose ups and downs in the blood, but may play a more important role in events lasting four hours. Decreases in blood glucose are common in the last half of the ride and are generally associated with fatigue and slowing of pace.

5. Do not assume that because the horse was pulled in the first 20-30 miles of a 100 mile ride that it is not dehydrated. Horses had fluid defects of 20 L or more after only 20-30 miles.

6. Following the ride, ensure that water ingestion continues. Virtually no horse in the study was fully hydrated even after one hour of recovery and access to water. Continue encouraging water and ion consumption. Avoid the horse ingesting large amounts of dry matter following the ride if dehydration is evident (i.e., wet the hay and if giving grain, make it into a slurry). Replenish fluid and ions first (can include small amounts of glucose), then feed the horse.

7. Even the best supplementation program will not help the horse if it is unfit, if there is a "hard" rider aboard and the horse is not ridden to its level of ability. The training and feeding program is what takes the horse to the event, the cooling and supplementation and smart riding gets him through it.

What About the Rider?

A short note about fluid and electrolyte supplementation for riders.

Much is written about fluid and electrolyte supplementation for the horse, but little seems to be published in the same journals for fluid supplementation for the rider. Endurance riders are prone to fluid and ion losses similar to their horses. Such losses can lead to fatigue and reduced performance on the rider's part. A fatigued rider will experience decreases in their ability to ride well and to concentrate. This may result in further strain on the horse, and possibly more distance being asked of the horse because of getting lost on the trail. Many riders spend a great deal of time on the ground running beside their horse to lighten the load. With hyperthermia and the associated fluid losses, the rider's endurance will be decreased just as it is with the horse. Keeping the rider "cooled out" is equally as important. Wearing loose, cotton-type clothing (or some of the newer fabrics that wick the moisture away from the skin) will help the sweat evaporate and speed up the heat loss. Light coloured clothing will reduce the heat absorbed from the sun. When hot, remove the helmet when possible and wash the neck and face repeatedly with cool water. It is difficult to make any specific recommendations for endurance riders as the trained state, acclimatization to exercise in the heat, and intensity and duration while running beside the horse vary greatly. Fluid losses in a runner may be 2 L/h or more in warm weather. Conscientious pit crews can help the rider monitor fluid intake by keeping this number in mind. A rough guideline might be 1/2 cup water every 30 minutes. Just as you've heard about the horse, "keep the kidneys flowing!" Sports drinks (non-carbonated), diluted fruit juices and fruit are good to consume during the ride. Add in small frequent feedings of carbohydrate foods like sandwiches (no large amounts of food ingested at one time). Do not consume large quantities of high glucose products like chocolate bars, as these will lead to "peaks and valleys" of blood glucose that may cause headaches, stomach upsets, and fatigue. Consuming high fat or high protein during the ride would not be advisable. Keep in mind that radical changes in diet can have devastating effects on the rider's gastrointestinal tract too.

The inclusion of 4 to 8 g of carbohydrate per 100 ml of water is suggested for marathon runners to help provide an energy source. The addition of carbohydrate helps increase the absorption of electrolytes and becomes more important in events lasting over two hours, when stores of carbohydrate in the liver are decreasing. Glucose polymers such as maltodextrin are attracting greater attention as they provide more energy per unit and there is less reduction of gastric emptying reported by some researchers. Marathon runners have been advised to ingest 100-150 ml of the solution every 15 minutes. Under very high temperature conditions, water consumption up to 300 ml every 15 minutes may be necessary. It is easy to include a bathroom scale with your equipment and this can help the rider monitor large fluid losses (a loss of only 3% of body weight can affect your exercise performance: about 2 kg for a 60 kg person or about 5 lb. for a 160 lb person).

High concentrations of carbohydrates and electrolytes in the drink can slow down the absorption and delay rehydration. Water loss is the main problem so make sure to ingest water by itself if using a sports drink (i.e., one bottle of water, one bottle of sports drink). Water is absorbed the quickest and is the main problem for running events less than 1-2 hours for runners.

The thirst response for humans (and horse) is a poor indicator of the need for water. Therefore, the rider must be encouraged to ingest fluids frequently during the ride and even after the ride. It may take over 24 hours for a person to replenish fluid losses if attention is not paid to amount consumed. The use of a scale is a useful indicator of dehydration status. By monitoring your weight after the ride, you can get a reasonable estimate of the fluid deficit.

Acclimatization to exercise in the heat will help the rider cope with the demands of heat and exercise. Approximately 5-10 days of exercise under conditions of heat (or "overdressing") will increase one's ability to cope with the heat and exercise more efficiently. Humans can "preload" with water too, just like some do for their horses. Drink about two cups of water over the 1/2 hour before the ride starts if the conditions promise to be hot and humid.

The contents are copyrighted but may be copied,
on condition that the Equine Research Centre be
acknowledged for the use of its information.

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

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