Working Up a Thirst

Working Up a Thirst

By Gayle L. Ecker

Water and Electrolyte Losses During Endurance and Eventing Competition

Did you know that if a horse did not lose body heat when exercising, the body temperature could rise as much as 0.25 C each minute during endurance-type exercise? This would quickly take the normal temperature of about 37.5 C up to lethal temperatures of over 40.5 deg C in only 12 minutes! Fortunately, for the horse (and the owners), heat is lost or dissipated from the body to prevent this from occurring most of the time. In order to lose that amount of heat during exercise, the horse depends mainly on the process called sweating. This is a process whereby fluid (composed of water and salts or electrolytes like sodium (Na+), chlorine (CL-), potassium (K+) and calcium (Ca++)) from the bloodstream is secreted to the surface of the skin. Once the fluid is on the skin, the water evaporates, taking heat away from the body. If the horse continues exercising for prolonged periods, the loss of water and electrolytes from the body will cause health and performance problems. Initially, the fluid comes from the bloodstream, but as duration of exercise is prolonged, fluid is lost from the tissues, i.e., the muscles, as well. Humans have been using electrolyte solutions in the form of sports drinks for many years to combat the water and electrolyte losses due to sweating. Far less research had been done on exercising horses.


Three years ago, a research program was initiated to collect information on endurance and eventing horses to help understand the losses of water and electrolytes during exercise. If the losses can be quantified, then better strategies can be developed for 1) prevention and treatment of health and performance problems; and 2) replacement of water and electrolyte losses.


Endurance Research

The study included over 240 horses at 14 different events including competitive trail and endurance events, and distances of 48 to 160 km (30 to 100 mi.). Blood samples were collected at rest, during the ride, at the finish, and after 1 hour of recovery. At the same time, body mass (BM) was measured on a portable equine scale (donated by the Canadian Morgan Horse Foundation). Information was also collected on the use of electrolyte supplementation during and after the ride.


Water Losses

Water losses ranged from 8 to 30 L at the end of endurance rides. A surprising find was that, during the 160 km Race of Champions (ROC), a mean loss of 18 L occurred in the first 32 km (20 mi.)! Individual horses lost as much as 28 L..This means the horse is now in a water deficit state and must try to "play catch-up" while continuing to exercise for another 128 km (80 mi.). The highest water loss recorded for a horse that finished (although slowly) was over 48 L. However, many horses were pulled due to dehydration related problems with water losses much smaller than that.


Figure 1 shows the mean water losses recorded at various distances for all the rides. It is of interest to note that horses classified as elite horses (consistently top finishers in 14 kph mean ride speed) had lower water losses than other horses at the mid-ride. Over 40% of the elite horses fell into the low to moderate BM losses (% BM loss) category, but only 20% of non-elite horses had low weight losses. The high BM loss category (10% BM loss) included over 16% of non-elite horses, but only 6% of elite horses.


Electrolyte Losses

Similar to water losses, these losses occur early in the ride with little change (on less strenuous rides) or with further losses (for rides at higher speeds, tough terrain, or high heat and humidity) over the last half of the ride.


Horses that were pre-loaded (given an electrolyte dose prior to the start of the ride) at the ROC had significantly lower losses of NA+, CL-, and a trend for lower losses of K- and Ca++ at the 32 km vet check. Analysis of data on horses competing in rides 60 km under temperate conditions (less than 21 deg C) showed no difference between horses receiving electrolyte supplements during the ride vs. those that did not receive electrolyte supplements when comparing water or electrolyte losses. The presence of muddy terrain increased the losses. This occurred despise the use of electrolyte supplementation, similar correlations were found for speed as well.


Results of Survey on Use of Electrolytes

Riders participating in the study were questioned regarding electrolyte supplementation during endurance rides. A descriptive summary of the results follows. On rides less than 80 km (50 mi) 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% were horses at the ROC, therefore only 23% of horses not competing at the ROC received electrolyte supplements the morning of the ride. On rides of 80 km (50 MI) 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/home recipes being used and dosage, rate and method of administration and other factors varied considerably.


Practical Applications

The loss of body water may compromise the ability of the horse to thermoregulate (sweat) and may impose extra strain on the cardiovascular system. It is important to reduce the water deficit whenever possible to minimize the effects of dehydration on performance and health. This means monitoring the water intake. This is easy for the pit crew who can use buckets of known size, but a little more challenging for the rider between vet checks. Keep in mind that any grass or hay should be wet, and any grain rations should be made up as a "gruel-like" mixture with water, chopped apples and carrots, apple sauce or other additions to keep it appetizing (You may have to feed this to your horse at home every once in a while as many horses refuse to eat unusual concoctions on race day). It may not be advisable to mix the electrolyte supplement into the gruel as it may make the mixture unpalatable.


Replacement of electrolyte losses should start early in the ride, or possibly before the start, if the ride is going to be fast, over tough terrain, or during heat and humidity. Horses that were pre-loaded at the ROC had lower losses of NA+, CL- and a trend for lower losses of K+ and CA++ at the 20 mile vet check, and therefore, the ion deficits at this stage were less than other horses. Smaller, more frequent doses starting early in the ride should be more effective than large doses of hypertonic (concentrated) solutions once the horse is in a water deficit situation. Administering the electrolyte dose immediately following water consumption should aid in absorption of electrolytes and this should help prevent water being pulled into the gut, as can happen with hypertonic solutions.


Horses need enough time early in the ride to drink, and secondly, enough time throughout the ride to allow the gut to absorb the water, electrolytes and nutrients. If the time allowed is short and the horse is still trying to cool itself, the blood flow to the gut may still be low, and little uptake of gut contents will occur. If the horse is panting, it is unlikely that it will drink or eat until respiratory rates decrease.


The horse will lose less water and electrolytes if cooled down quickly at the vet checks. Cooling the horse with repeated applications of cool water (i.e., then scraping then reapplying the water again) works well to quickly cool down the horse, but monitor the rectal temperature to insure that you know when the horse is cooled adequately. Remove the saddle when ever possible. The blankets restrict heat loss from the back and this is a large area for heat dissipation.


After care of the horse is as important as care during the ride. A quick recovery means the horse will be stronger for the next ride. The data showed that with very few exception, little change occurred during the 60 minute recovery for water and ion balance. Once the horse's temperature is back to normal (and it would be wise to monitor this), then normal blood flow to the gut should be re-established (check the gut sounds). Provision of water, electrolytes and small amounts of glucose, before providing hay rations, may help during the recovery period to restore the water and ion deficits. Even riders, who have become dehydrated during the ride, have remarked on how they suddenly began to feel much better following water intake. And anyone who has seen the amazing transformation of a horse that is exhausted and dehydrated to an interested, bright horse following the administration of IV fluids should be aware of how important it is to restore the balance.




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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