Horses sweat at very high rates which is an effective cooling system, but the downside is the loss of electrolytes and body fluids.
All muscular contraction creates heat, be it walking around the field grazing or running a race. Some of this heat is stored by the body for bodily functions and for keeping warm. However, it is vital that the excess body heat be lost or dissipated if the animal is to maintain or control its body temperature.
Excess heat can be lost in a number of ways, in the horse it is primarily through sweating (85%) and to a certain extent through respiration (15%). Dogs on the other hand dissipate heat primarily through respiration, as in panting and sweat very little.
The horse has the ability to sweat at much higher rates than any other animal, they can sweat up to three times as much as humans do. Horses are capable of sweating at rate of up to 10-15 litres/hour. As an example, in a light trot/canter workout up to five-seven litres can be lost and in a short duration high intensity workout up to 10 litres of sweat could be lost.
Although this provides for a very efficient cooling mechanism it has its shortfalls. Equine sweat is hypertonic, meaning that it is more salty than body fluids, unlike human sweat, which is hypotonic. This causes the horse to lose higher quantities of electrolytes through sweat than we do. The horse also loses copious amounts of body water compared to what we lose, leaving the horse in danger of suffering from both dehydration and electrolyte imbalance/loss.
Electrolytes are minerals that are dissolved in body fluids. They are vital for a wide range of body functions such as regulation of all body fluid levels into and out of the cells, nerve impulse transmission and muscle contraction, as well as pumping of the heart, movement of food and water through the gut and the filtering of waste products through the kidney and liver.
The fine balance between water and electrolyte concentration is essential for appropriate muscle contraction and also in the cooling the horse. An imbalance or a depletion of electrolytes can lead to premature muscle fatigue, reduced stamina, muscle cramps, poor post exercise recovery, tying up and thumps.
The composition and the amount of the sweat lost varies between trained and untrained horses, duration and type of exercise, environment conditions and at different periods throughout a bout of exercise. For example, during the first 15 minutes of exercise sodium concentrations increase, potassium concentrations fall and chloride concentrations remain unchanged. In general horse’s sweat contains approx 56% chloride, 27% sodium, 15% potassium and smaller amounts of calcium and magnesium.
Each electrolyte has its own particular function:
Sodium helps balance the body water levels and maintain blood pressure.
Chloride is required to maintain the balance of acids and bases (alkalis).
Potassium helps balance the fluid inside the cells and is vital for optimum muscle, heart and kidney function. Potassium loss decreases muscle strength, tone and ability to contract.
Calcium is essential for maintaining normal, controlled skeletal and heart muscle contractions and excessive losses can result in a condition called ‘heaves’. Calcium also builds bones and teeth and contributes to normal function of heart, nerves, muscle and blood clotting.
Magnesium is important for normal muscle function, bone formation and reducing nervous behaviour.
An imbalance or a loss of particular electrolytes can affect the horse in a variety of ways. For example, the loss of sodium, chloride, potassium and magnesium greatly disrupts the functioning of nerve and muscle tissue, the loss of both sodium and potassium rapidly decreases thirst and appetite and so actually delays rehydration and the loss of calcium and magnesium can lead to sensitisation of the phrenic nerve causing ‘thumps’.
An electrolyte imbalance/loss can occur when horses sweat at more that 10 litres/hour or during extended periods of profuse sweating. Replacing lost electrolytes post exercise is vital for optimum health and performance, delayed onset to fatigue and quicker post exercise recovery. It is important however that the electrolyte replacement product used contains electrolytes in similar proportions to those lost in sweat and that adequate amounts are fed so as to match amounts lost in sweat. Rate of supplementation will vary with the degree of fitness of the horse, work levels required and environmental conditions. Electrolyte supplements should also contain carbohydrates such as glucose or sucrose which are readily available sources of energy and help in the absorption of electrolytes. Free access to water should always be provided.
When administering electrolytes to replace the losses that occur with sweating it is vital that they be administered with water. Electrolytes not used with water can in fact dehydrate the horse further as it causes the water ‘to go the wrong way’. Administering large single doses of electrolytes in the form of an oral paste for example can cause the gut to absorb water from the surrounding blood vessels to dilute the concentration within the gut, further worsening the dehydration within the muscle cells, for the short term at least.
Likewise, trying to rehydrate a horse with water alone will also delay the hydration process. The body monitors and controls fluid volume by responding to the sodium concentration of body fluids. A decrease in sodium concentration due to electrolyte loss will signal to the horse that it is not thirsty. Giving water to a horse that has just undergone a lot of sweating will dilute the sodium concentrations further and cause the horse to stop drinking. It will also signal to the kidneys to excrete more water so as to bring the plasma sodium concentration back to normal, causing further dehydration. It is important to add electrolytes, in particular sodium and chloride, to water when giving to a horse post exercise. The use of electrolytes during the cooling down process has been known to decrease recovery times from 12-24 hours to only 45 minutes.
Sodium and chloride are the two major electrolytes lost in sweat and are easily supplemented by adding table salt. However this may not be sufficient to meet the electrolytes lost during periods of hard work so it is important to use electrolytes specific to performance horses. Sodium bicarbonate products are specific to sick or ill animals and not performance horses as bicarbonate is not lost in sweat.
It is not possible to ‘load’ electrolytes by feeding electrolytes on a daily basis as the body will excrete what it doesn’t need, however pre-loading of electrolytes 1-4 hours before competition can increase time to fatigue by up to 23%.
Diet can affect electrolyte balance and amount of sweat lost. Cereal diets are acidic and valuable electrolytes are lost in processing this diet, namely calcium. Calcium is important in both nerve and muscle function. It is advisable for young horses prone to tying up to be given electrolytes daily. Excess protein in the diet increases blood ammonia causing nerve irritability and a disturbance in carbohydrate metabolism. Increased urea levels causes an increase in urine production leading to an unnecessary loss of body fluids and higher ammonia levels in stalls.
Equine Exercise Physiology – David Marlin and Kathryn Nankervis
Sweating, dehydration and electrolyte supplementation: Challenges for the performance horse – Michael I. Lindinger