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Heat Production, Fluid Balance, Hydration and Electrolytes in Endurance Horses

Or: You can lead a horse to water……

The issue of hydration, heat production (and loss) and electrolytes is a complicated subject, and refers to many vital body processes and functions which horse owners must consider every time the horse performs exercise, especially when the duration and intensity of exercise increase, and equally, when environmental factors including heat and humidity increase.

Heat production (and loss) has serious implications from simple factors such as;
(1) feeding (what is the food source, and how much of it is fed, how often),
(2) drinking (how much water is consumed, how often, how much sweat is produced)
(3) exercise intensity and duration
(4) environmental conditions (heat and humidity, terrain)
(5) electrolyte, fluid and mineral status of the horse

The first very important compound to consider is water. This means body water or fluid intake and loss, because this determines the body fluid balance (or the hydration status) of the horse. There is a phenomenal amount of information available about electrolyte supplements and replacement programs for performance horses, but very little progress has been made to explain in plain language exactly why water and electrolyte balance are so vitally important to horses.

The aim of this article is to try to examine and explain why water is so critical to heat loss and body function, and what the critical factors are for you to observe, so you can manage fluid balance, electrolyte balance and heat production/loss properly.

So, why all the fuss about heat production and hydration in horses?

During exercise athletic horses get their energy for muscle contraction from food. The food is broken down and stored as energy (glycogen and fat), and this stored energy must be turned into mechanical energy when the muscles contract to move the horse along. How this occurs is a separate story relating to energy production and muscle function.

This energy conversion process is relatively inefficient, where about 80% of the energy derived from energy stores in the body is actually lost as heat. Horses, like all other performance animals and man, must lose this heat, or life threatening increases in body temperature may occur. (of course, life threatening increases in body temperature will occur much quicker in weather conditions that are hot and humid, and when workload is very high, as the horse cannot lose heat as efficiently in these conditions).

As an indication of the amounts of heat produced by exercising horses, Susan Garlinghouse, Endurance News, June 2000, outlined that, during a 50 mile endurance ride the average horse will produce enough heat to not only melt a 150lb (68kg) block of ice, but also to bring that amount of water to a boil! Other authors say that the heat produced in a 160km endurance ride is sufficient to boil 770 litres of water. No matter who is right, the horse produces tremendous amounts of heat while exercising, particularly over long term exercise such as endurance.

The primary method of losing heat in horses is by evaporation of sweat. When evaporation of sweat is able to continue, this is a very efficient means of maintaining the core body temperature within a very narrow limit, even in adverse climatic events, or long endurance work. But – when heat production exceeds the ability of the body to lose excess heat (as in exercise-induced heat stress in high humidity/high temperature climatic conditions, or when they have conditions such as anhidrosis), a wide range of physiological problems rapidly occur.

It is absolutely essential that riders, particularly in long, endurance events, recognise the signs of impending heat stress.

Evaporation of sweat is the primary method of heat loss in horses. (Horses and humans are the only two species that sweat as the primary method of heat loss – sheep, dogs and pigs rely primarily on panting). The water evaporating from the surface of the body results in heat loss. The actual amount of heat loss occurring in this way is determined largely by the temperature and humidity of the surrounding air. For the scientific minded readers, the heat lost from vaporisation of 1gm of water is 598 calories (2501 J) at 0 degrees C.

Now for the interesting bit: when the vapour pressure at the skin surface is very close to the vapour pressure in the air (as in very humid conditions), sweat no longer evaporates, it simply drips off the skin, and no heat is lost to cool the horse. Sweat requires evaporation – not just dripping off the skin, to provide cooling. At this stage, body temperature will rise because cooling is no longer occurring by evaporation.

As a second method of heat loss, horses do lose significant amounts of heat as they breathe. This again is governed by temperature and humidity, as well as the rate of exercise and respiration. The more the horse breathes heavily (panting, like dogs), the more heat it will lose from this respiratory route.

Another interesting fact relating to horse sweat is that it contains an unusually high level of a protein called latherin. This is responsible for producing the lather seen on horses after exercise, and it has actions to promote the spread of sweat over the skin area, hence increasing evaporation and, hopefully, cooling.

When a hot horse is exercising, the horse increases the blood flow to the skin, in addition to sweating and panting. This increased skin blood flow helps to take heat away from the body centre, and increases the skin temperature. This allows some convection of heat away from the body. In more severe heat stress the increased blood flow to the skin also provides the additional heat required to evaporate sweat, and supplies fluid for sweat production. Conversely, in cold climates reduced blood flow to the skin reduces skin temperature, thus reducing heat loss from the body.

Horses are capable of coping with environmental temperature extremes from 50 degrees C in the Middle East and inland Australia, to lows of about -40 degrees C in Canada, and can still maintain normal body temperature within a very narrow normal range.

What else happens in hot horses?

In response to heating, the heart output increases because the heart must maintain blood pressure when all the additional blood is sent to the skin to lose heat. This means that heart rate will increase in hot conditions during exercise – important for endurance riders to recognise. Heart rate is one excellent indicator of the degree of heat stress.

During exercise, the heat produced within the body can be very significantly more than the heat load of the environment, even in very hot conditions. Without effective sweating, respiratory heat loss and increased skin blood supply to lose all of this metabolic heat, the horse body temperature could rise 1 degree C with every 5-8 minutes of exercise (McConaghy, F. Thermoregulation: The Athletic Horse. Hodgson & Rose , W.B. Saunders)

How much heat is produced during exercise?

We have seen the mention of the block of ice earlier. Now consider that a thoroughbred racehorse racing at about 16-17 metres/ second requires the maximal oxygen uptake of about 85-90 litres oxygen per minute, so the horse will lose some heat through rapid respiration. This level of exercise is associated with heat production of 450Kcal per minute. If this heat was not lost through normal sweat and skin loss as described, the body temperature would increase 1 degree C for every minute of exercise.

A galloping racehorse can lose between 4 and 7kg of bodyweight during a fast work period. About 90% of this is water.

In contrast, a working standardbred will lose between 5 and 15kg bodyweight during a 1,600 metre race. Again, 90% of this is water.

An endurance horse exercising at a speed of 4-5 metres per second consumes about 25 litres of oxygen per minute (this is about 40% of its maximal uptake), and produces a heat load of about 100kcal per minute. This would result in an increase in body temperature of about 0.25 degrees C per minute of exercise (or 15 degrees C per hour) if no heat loss could occur.

In an 80km endurance ride, it is expected that a horse can lose between 30-50kg bodyweight in water

Even though heat production does not look to be as high in endurance exercise, heat loss is more important than with gallopers or sprint events because of the long duration of exercise. A thoroughbred will only race for 1-3 minutes generally, and this would result in heat production of about 1350 to 2250 kcal (450 kcal/minute). An endurance horse competing for 6 hours at a heat production rate of 100 kcal per minute is producing vastly more heat which must be dissipated over the 6 hour period.

How much heat is lost in sweat?

If 1 litre of sweat evaporates, about 580kcal of heat is lost from the horse. This is the amount of heat generated by 1-2 minutes of maximum sprint exercise, or 5-6 minutes of endurance exercise.

In order to efficiently lose the vast heat load generated during endurance exercise, a large volume of sweat must be produced, and evaporated. To be precise, McConaghy, 1994, reports that one hour of endurance exercise (submaximal work) would produce 6000kcal, and to lose this heat through sweat by evaporation would require 11 litres of sweat at the minimum! This volume of sweat would increase as environmental effects such as temperature and humidity increased, and as work load increased.

How much sweat can be produced?

Sweat loss of horses exercising in the heat has been measured at 10-15 litres per hour. Trials have been conducted to measure the sweating rates of horses by weighing them before and after exercise.

Moderate exercise (3.5 metres/sec) for 6 hours resulted in a 5-6% loss of body weight (27kg). During the first 100km of the Tevis Cup endurance ride the mean weight loss was 17.5kg (4% bodyweight), with the maximum weight loss from one horse being 45kg (this represented 10.5% of the horse’s total bodyweight). Remember here that these losses were recorded after the horses had an opportunity to drink at rest stops, etc, so this is the fluid loss that they were unable to replace effectively.

Thoroughbreds racing over 1-2 miles may lose up to 10 litres of sweat during the warmup, race and early recovery period.

Standardbreds jogging for 25 minutes during summer conditions may lose up to 25 litres of sweat over several hours of jogging and cooldown period. It is well recognised that standardbreds will lose significantly more sweat than thoroughbreds, due to their longer training periods. It takes about 2 hours for these horses to cool down thoroughly.

All of this means that, for endurance events in particular, protracted submaximal exercise results in huge heat loads which must be dissipated from the body. You can see from the above example that sweat production at the rate of 11 litres per hour is needed to evaporate and lose the heat produced in endurance work at normal temperatures.

This volume of sweat production obviously results in tremendous loss of both fluids and electrolytes. Endurance horses in particular, forced to exercise for long periods in hot environments, can become severely dehydrated extremely quickly.

The total body water of a normal 450-500kg horse is about 300 litres. About 2/3 of this is found inside cells, and 1/3 is in the fluid surrounding cells and in gut fluid. Sweat comes from the fluid outside individual body cells, and when a lot of sweat is produced, fluid leaves the cells to equalise the fluid volume outside the cells. So sweating for long periods reduces fluid levels both inside cells and in tissues around cells. A water loss of 40 litres, as seen in endurance activity represents a loss of over 15% of total body water

Horses have a substantial reserve of water in the large intestine, particularly if they have adequate roughage in the diet. They can absorb up to 20 litres of water from the gut to make up for lost sweat.

At the same time as fluid is being lost in sweat, there are substantial losses of the mineral electrolytes chloride, potassium, calcium and magnesium from blood plasma, because these minerals are in sweat.

Major variations in the fluid and electrolyte balance of horses are known to adversely effect athletic performance. This is not in doubt. They may even result in a life threatening metabolic state if not recognised, particularly in endurance animals.

Electrolytes are essential for control of activity in cell membranes, muscle contractions, nerve conduction and enzyme reactions. They are thus central to all forms of exercise.

Dehydration also results in a significant reduction in the efficiency of evaporation and cooling, because the skin blood flow is reduced, and sweat production rate decreases. When dehydrated horses lose the ability to sweat, this condition is known as anhidrosis. Dehydration is the single most important factor in the development of heat stress in working horses.

Dehydration also affects energy utilisation, through accelerated depletion of muscle glycogen stores, with an increased production of body heat, plus an increased heart rate, as well as an increased production of lactate through the increased glycogen utilisation. To maintain the same work intensity the dehydrated horse relies more and more heavily on anaerobic metabolism, which contributes to more rapid development of muscle fatigue and metabolic diseases such as colic or tying up.

If this process of energy depletion, electrolyte loss, acid-base imbalance and dehydration continue, it can alter the function of both the gastrointestinal tract and the nervous system, reducing the ability of the horse to voluntarily replace fluid losses by drinking. Progressive dehydration reaches a stage where the horse has no thirst reflex, and will show no interest in drinking under any circumstances. At this stage the horse is in a metabolic crisis and requires immediate veterinary attention.

All of these factors negatively influence recovery rate, as well as performance, and the efficiency of muscle contraction.

In general terms, dehydration losses of 3-4% of bodyweight lost as fluid have an adverse effect on performance, even though there may be no obvious signs of dehydration.

Horses with an 8% dehydration have a slower capillary refill time of 2-3 seconds, poor skin test reflex, dry mucous membranes, dry faeces, and a high heart rate.

A horse with 10% dehydration is in very serious trouble, requiring immediate extreme veterinary attention. At 12% dehydration the horse is close to death.

Now just think about what we have written about sweat production rates, and then look at the dehydration percentages mentioned above;
For a 500kg horse, 4% dehydration is 20kg bodyweight loss
8% dehydration is 40kg bodyweight loss
10% dehydration is 50kg bodyweight loss
12% dehydration is 60kg bodyweight loss, and death.

Now remember that we have shown that endurance horses in particular can lose sweat at a rate of 11 litres per hour. It doesn’t take long to lose 50kg if the fluid loss cannot be replaced regularly and effectively.

From the example above: what is the actual difference between a horse with 4% dehydration, and one with 10% dehydration and metabolic distress? The difference is 30kg bodyweight loss, or 30 litres of fluid roughly. This difference at the most is 3 hours of sweating in an endurance ride.

The message is that horses in endurance activity in hot or humid conditions in particular, must drink at every opportunity, and must be cooled down with water at every opportunity. These are the obvious measures. But there is more you can do:

How much actual electrolyte salt is lost in horse sweat?

The primary electrolytes lost in all horse sweat are sodium, chloride and potassium. Excessive loss of these electrolytes results in muscle weakness and fatigue, and decreases the thirst response to dehydration.
If you are considering an electrolyte supplement, it is important to understand just how much of these essential minerals are lost when horses sweat. The three primary electrolytes are listed below, with actual volumes of the electrolyte lost with varying levels of sweating (Pagan, Kentucky Equine Research, Inc., Versailles, KY. Feeding Management of Horses Under Stressful Conditions).

Total daily electrolyte requirements (grams/day) as a function of sweat loss

The amount of sweat lost depends obviously on factors including duration and intensity of exercise, temperature and humidity.

In general, horses exercising at low intensity (12-18km/hour) lose between 5-10 litres of sweat per hour, those doing higher intensity work (30-35km/hour) will sweat up to 15 litres an hour.

Pagan reports that, at the 1996 Olympic Games in Atlanta, horses lost an average 18.4kg bodyweight just during the speed and endurance phase of the 3 day event. This is a sweat loss of about 15 litres.
Using the sweating rates above, an endurance horse will lose between 45-60 litres of sweat during a 160km ride. This actually represents electrolyte losses of between 460-690g, and additional losses of 9-14g calcium and 5-8g magnesium.

The message here is simple: large amounts of essential electrolyte minerals must be supplied daily to horses doing long distance work, sweating heavily, and in hot, humid conditions. There is little chance that these considerable levels of minerals will be available from the feed alone.

To examine the figures above in a slightly different context, consider the following;

A Standardbred (during a race) will lose an estimated 16-46g Sodium, 6-17g potassium, and 31-88g Chloride per race.
A Thoroughbred (during a work period) will lose an estimated 16-23g Sodium, 6-8g Potassium, and 31-44g Chloride
An endurance horse (during a 100km ride) will be expected to lose 33-132g Sodium, 12-48g Potassium, and 63-252g Chloride

Given these levels of loss during competition, electrolyte availability (and hence the ability to sweat and cool down, or lose body heat) can become a very real problem when the rate of electrolyte loss exceeds the replacement rate. Unfortunately, horses do not store electrolytes from one day to the next, so “loading” electrolytes for days before competition is of little value. At the same time, daily supplementation of high levels of electrolytes to meet the workload demands is highly recommended.

A research team at Michigan State University, led by Dr. Hal Schott, reported in 1998 that when horses exercised over a 60km endurance ride without electrolyte supplementation, they lost about 50 pounds of fluid as sweat, and replaced only about 50% of this loss by drinking 25 pounds of water during the ride. However, when the horses ran with electrolyte supplementation dosed before and during the ride, the horses drank between 5 and 6 gallons of water (40-50 pounds), replacing almost all of the fluid lost in sweat. This trial used greater amounts of electrolytes than are typically used by competitors (150g sodium chloride and 75g potassium chloride), and the team reported that these large amounts of electrolytes were still less than the horse lost in 50 pounds of sweat, yet they found no adverse effects. In fact, the supplements were successful at ‘tricking” horses into drinking a greater total amount of water, and in getting horses to drink earlier during competition. Ranvet also has some interesting articles on endurance feeding.

We briefly mentioned previously that roughage stored in the large intestine improved intestinal water and electrolyte storage. What could be simpler than preparing for endurance work by feeding the most forage you can in the week before an event. Forage will take several days to reach the large intestine, and up to 20 litres of water and electrolytes can be stored in this roughage in the gut, ready to be drawn on during endurance work. The water stored in the hindgut is almost equal to the amount that makes the difference between 4% dehydration with no clinical signs, and 10% dehydration with real performance problems.

Some other “heat” issues

All owners are aware of the disadvantage of high protein feeds, and even high grain diets, where excess heat is produced while the diet is being digested. This excess metabolic heat is produced because of the complicated pathways required to digest protein and make it available as energy, for example. This energy production pathway is relatively inefficient, as the breakdown of protein produces 4-6 times more metabolic heat waste than does the utilisation of equivalent amounts of carbohydrate or fat.( High protein meals could be a distinct advantage in very cold climates where maintaining body temperature could be a problem)

Fat and oils, in fact, are very useful in increasing the energy density of a ration without increasing the volume, because fats contain roughly 2.5 times the energy of comparable carbohydrates. When utilising fats for energy (only for aerobic energy production), the fats help to conserve the muscle glycogen stores for periods of maximal anaerobic exertion, thus preserving muscle glycogen stores and delaying the formation of lactic acid and muscle fatigue.

You will often see fats and oils, and even high fat food such as rice bran, marketed as “cool” energy because they do not result in high levels of metabolic heat being produced during their conversion into energy for muscle contraction.

What are the implications of all this for endurance horse trainers?

Encourage the horse to drink (and possibly eat a little) at every available opportunity. Take particular care to ensure water intake at rest stops, and work assiduously to cool horses down during vet checks and rest stops.

Don’t feed large meals during the ride – encourage small, regular meals high in fibre. Ride preparation should include maximum forage intake in the week or so before a long ride, especially in anticipated hot or humid conditions. This allows the hindgut to store the maximum amount of water and electrolytes. Consider this practice as filling the extra fuel tank.

Daily electrolyte supplements are essential to maintain adequate electrolyte balance. Consider supplementing electrolytes the night before hard rides, along with a forage meal and a good ration of water. Electrolyte supplements may be oral powders to add to feed, and there are now some excellent oral paste syringes for use during long rides, which may be used at drinking stops, rest periods, vet checks, etc, as required. These concentrated oral pastes will trigger the thirst reflex, and trainers should ensure that adequate fresh, cool, clean water is readily available after electrolyte administration. It is recommended that the horse be allowed to drink before oral electrolyte pastes are used, as this dilutes the paste contents and avoids large fluid shifts required if fluid must be diverted into the gut to dilute the electrolyte concentrate.

If you wait until horses are dehydrated before supplementing electrolytes, a large percentage of the dehydrated horses will not drink. These horses are not recovering well, and immediate veterinary attention is required.

Pre-ride electrolyte loading is an acceptable practice so horses commence the ride with adequate water and electrolytes. Supplement electrolytes the night before and again the morning of the ride, and ensure that enough water is available to drink immediately, particularly if using paste supplement concentrates. Supplement again at each vet check and at water stops on the ride whenever necessary. Paste products are lightweight and ideal to carry for this purpose.

Feeding practice is important. Rather than providing large meals several times daily, it is far better to space regular small meals, as this avoids the often large fluid shifts that occur with large meals when fluid moves from blood to the gut to produce saliva and other gastric fluids needed to process large meals. (In a 450kg horse the fluid shift from blood to the gut after a large meal can be up to 12 litres of fluid). This fluid does find its way back into the blood plasma after a few hours of digestion, but it obviously results in a short, temporary fluid deficit (or dehydration) immediately after large meals. In those few hours an endurance horse can do a lot of work since its last meal at a vet check or rest stop. This could mean the difference between success and failure. To avoid this situation, simply feed small meals often, even if horses are encouraged to nibble grass or small treats of roughage at regular intervals every few hours, where possible.

Keep the protein component of the ration low during rides to reduce excess metabolic heat production. Mature working horses only require about 8-10% crude protein in the diet. It is not necessary to supplement additional protein, rather energy is the limiting nutrient in most performance horses. Remember that processing excess protein for energy production is an inefficient pathway which produces a great deal more metabolic heat, and requires additional energy for it to occur anyway.

Additional reading

References:
Hodgson, DR, RJ Rose. The Athletic Horse, Principles and Practices of Equine Sports medicine. WB Saunders Company, Philadelphia, 1994

Carlson,GP. Thermoregulation, fluid and electrolyte balance. In Snow DH, person, SGB, Rose RJ (Eds): Equine Exercise Physiology. Cambridge, Granta Editions, 1983, pp291

Lindinger, M.I., and G.L. Ecker. 1995. Ion and water losses from body fluids during a 163km endurance ride. Equine Vet J., Suppl. 18:314-322