Why do horse’s sweat?

What is sweat?

Sweat consists of fluid (water) and a very important group of dissolved minerals and compounds collectively known as electrolytes. Often also referred to as ‘salts’, the minerals include sodium, chloride, potassium, calcium, phosphate and magnesium and compounds such as glucose, bicarbonate, and ammonia.

Electrolytes play a vital role in maintaining many of a horse’s bodily functions including correct hydration levels, electrochemical balance (important for correct nerve transmissions and ability for the muscles to contract), and rigidity of the cells.

So why not just table salt?

Horse sweat is a highly concentrated electrolyte solution and while sodium could be considered the most important electrolyte due to its function in increasing a horse’s water uptake, all electrolytes are lost in sweat to some degree. Each electrolyte plays a vital role in the functioning of a horse’s body, particularly when undertaking intense exercise. Therefore, it is very important that the correct balance of electrolytes is available to the horse to ensure it can continue to function successfully when under exertion.

Sodium

Sodium is essential for nearly every process in the body. Because of this, the horse’s body is very good at preserving sodium and will sacrifice other electrolytes, such as potassium, to maintain sodium levels.

Why is providing fresh water so important when supplementing with electrolytes?

Sodium helps to control the amount of water in a horse’s body at any one time. Water is attracted to sodium and will follow it around the horse’s body. The horse’s body will work hard to keep the sodium/water concentration constant. When we correctly supplement sodium, we increase the sodium concentration to water and this engages the horse’s thirst drive, encouraging it to increase its water intake. This is vital in maintaining hydration.

Horse sweat is very concentrated, making an overall change in the blood and tissue electrolytes concentration very subtle. These subtle changes in concentration may be inadequate for the horse’s ‘thirst sensors’ to detect large losses of fluid (in the form of sweat) and hence fail to drive thirst. If the body does not detect that it has lost large volumes of fluids and electrolytes this can lead to further dehydration. By supplementing with electrolytes high in sodium the balance is redressed and thirst is driven.

Chloride

Chloride is a negatively charged ion that is attracted to positively charged ions such as sodium and potassium. Chloride is the electrolyte that is lost in the greatest amount in heavily sweating horses. Chloride and bicarbonate play important roles in maintaining the acid/base balance within the body. Loss of chloride can upset this balance leading to changes in blood pH (hypchloraemic alkalosis).

Such changes have a detrimental effect on normal body functions and, if severe, can even lead to coma and death.

Potassium

Potassium is essential for the maintenance of internal pressure in the cells and, in association with sodium, can influence nerve and muscle functions. In an attempt to preserve sodium, the horse’s body will excrete potassium instead. Horses’ diets often do not contain enough potassium. A performance horse requires approximately 50g of potassium per day whilst an average ration of hay may only provide 20 to 30 grams.

Calcium

Calcium is an important electrolyte and a major component of bones and teeth. It is required for the maintenance of normal heart rhythm, blood clotting, muscular contractions, and other metabolic activities.

Phosphate

Phosphate is a key component in energy production and protein metabolism as well as playing its part in the structure of bones.

Magnesium

Magnesium is a co-factor in numerous enzyme systems and is involved in energy production, muscle contractility and nerve transmission. Horses low in magnesium often become “nervy” due to an absence of the stabilising effect of magnesium on nerves.

Bicarbonate

Bicarbonate assists the body by mopping up excess acids, such as lactic acid produced during intense exercise, before it can cause damage to the muscles. It is known as a ‘buffer’. A buffer is a weak acid or alkali that assists the body in regulating pH.

Glucose

Glucose is a simple sugar used as an energy source within the body. Together glucose and sodium are actively pumped into the body by a transporter pump to aid/speed their absorption process. The practice of adding glucose to electrolytes preparations assists with the rapid absorption of sodium.

*Owners are advised to seek advice from relevant authorities and their veterinarian before supplementing with electrolytes.

What happens to a horse’s body when it sweats?

The Sweating Process

As the concentrated sweat solution leaves the body and evaporates, the horse’s electrolyte supplies are depleted. If the horse does not have enough stores, or if the electrolytes lost are not replaced immediately, reduced functionality occurs leading to poor performance and lack of stamina.

Sweat loss is determined by three factors:

1. Exercise intensity
2. Exercise duration
3. Climatic conditions
   The longer a horse sweats, the greater its electrolyte losses.

Acidosis and Alkalosis

A horse’s blood has a pH of 7.4 – 7.45, making it very slightly alkaline. It is very important that a horse’s blood pH is kept within these levels for the horse to function correctly. Fortunately, a horse’s system is very good at maintaining this, but to do so it requires the right electrolytes and fluids to be available.

The pH Scale

Alkalosis
Hypochloraemic alkalosis occurs in horses exercising at moderate intensity over a longer period of time, that is, half and hour or more, and therefore encountering overall heavy sweat loss. A large quantity of sweat loss equals a large level of electrolyte loss, particularly chloride.  This results in the blood becoming more alkaline.  Alkalosis results in poor performance, blowing or thick windedness, nervousness and increased bicarbonate levels in the blood. 

Acidosis
Horses exercising at a very high intensity for a shorter period of time will lose less sweat, but will produce excess lactic acid in their muscles which can lead to acidosis. This lactic acid is transferred to the blood causing it to become more acidic. Acidosis results in muscle fatigue and soreness, lack of stamina and can lead to ‘tying up’.

Endurance horses are heavy sweaters and can lose up to 60 litres of sweat in a hard race.

Electrolytes play a vital role in the horse’s body function and all are lost in the horse’s cooling processes in differing amounts depending on the activity being undertaken. This is why it is necessary to supplement with a complete electrolyte that contains the right balance for the requirements of the horse.

Recovery

After a horse finishes a period of intense exercise, its heart rate is still elevated and heat still needs to be expelled. Any lactic acid in the system needs to be recycled and toxins and free radicals excreted. Lactic acid is produced when the body burns energy over the amount generated aerobically (with oxygen). This is termed an oxygen debt and has to be paid back at some point. It is paid back during the recovery phase by converting the lactic acid back to pyruvate, a process that requires oxygen. In this situation, the horse will continue to breathe heavily even after finishing the exercise. This process is termed ‘repaying the oxygen debt.’ The heart rate will remain elevated as the oxygen debt is repaid and also the horse will continue to sweat until it has regained control over its elevated temperature.

Whilst and after the oxygen debt has been repaid and the body heat is removed, the kidneys will be filtering the blood and removing toxic metabolites, the process of remodelling and repairing any damage will start and the replenishment of energy reserves will begin.

Any nutrients lost in this process, including vitamins, water and electrolytes, need to be replaced through the diet or supplementation.

Reducing Recovery Time

B vitamins play an important role in the recovery of horses that have had a period of intense exercise, or suffered from stress or illness. B vitamins aid in building appetite as well as providing muscle and blood-repairing properties.

Vitamin E and selenium are powerful antioxidants that assist horses which are prone to tying up by neutralising the damaging free radicals that are naturally produced during hard exercise. Lactic acid build-up and the formation of free radicals are thought to contribute to tying up. This can be countered with the aid of buffers and mild diuretics. Citrate buffers are a natural bicarbonate precursor. Adequate levels of these stored in the horse’s system enable it to very effectively neutralise lactic acid before it can cause damage to the muscles.