Sweating & Electrolyte Losses

Published in Horses and People Magazine

Exercising horses in Summer time

By Mariette van den Berg, BAppSc. (Hons), MSc (Equine nutrition)

During exercise, heat is produced as a by-product of skeletal muscle contractions. To prevent overheating and maintain core body temperature, horses have to lose this generated heat. One of the most important means to get rid of this heat load is by evaporation of sweat. When horses sweat, large quantities of water and electrolytes are lost. These fluid and electrolyte losses have to be replaced to maintain normal body function. Excessive sweating can cause an electrolyte deficiency which may lead to muscle fatigue, muscle cramp, reduced stamina and poor post exercise recovery, if not balanced.

In summer time, warmer environmental conditions can place more stress on the exercising horse as the heat production by the muscles can become greater than the heat loss. The following article will discuss temperature regulation during exercise, water and electrolyte losses  in horses during exercise, strategies that can be used to replace fluids and electrolytes and how to prevent heat stress.

Water requirement

Water intake is determined by the amount of body water lost through faeces, urine, respiratory gases, sweat and milk (lactating mares). Horses receive much of their water requirements by eating fresh forages and feeds. Horses that consume dry conserved forages (hay) will have increased water needs because fiber can absorb large amounts of water in the large intestine. Horses at maintenance fed a dry diet need about 5.0-5.5 litres/100kg body weight. Intense exercise and warmer environmental conditions are the other two major factors that increase water requirement. During exercise in warmer conditions heat production increases and sweating becomes the most important means for regulating the body temperature. It is therefore essential to balance water loss when horses are exercised in hot weather.

Electrolytes

The main electrolytes in sweat include the minerals sodium (Na+), chloride (Cl-), potassium (K+), calcium (Ca++) and magnesium (Mg++). These minerals, called ions, conduct energy when they are dissolved in a solution (e.g. blood, urine and body fluids. Na+ and Cl- are most abundant in the extracellular fluid (outside the cell) and are involved in the acid-base balance and osmotic regulation of body fluids. K+ is most abundant in the intracellular fluid (inside the cell) and is important for optimal muscle, heart and kidney function. Na+ and K+ also play an important role in establishing proper electrical gradients across cell membranes. Ca++ and Mg++ are vital for many functions in the body including skeletal growth, healthy function of the heart, muscle, nerves and blood clotting. Electrolyte imbalance therefore may lead to impaired muscle and nerve function.

The kidneys play an important role in maintaining a correct electrolyte balance in the body. Kidneys filter electrolytes in the blood and excrete excess amounts in the urine to preserve the balance. The kidneys control Na+ excretion and wastes only a very little amount in the urine keeping the Na+ concentration in the body relatively constant. K+ and Ca+ are generally ingested in large quantities with food and are therefore also excreted by the kidneys in substantial quantities on daily basis.

Temperature regulation during exercise

Muscles cells convert chemical energy available from food into mechanical work. The efficiency of this energy conversation is only 20-25%. About 75-80% of the energy of muscle contraction is released as heat. The rate of heat production by working muscles is proportional to the intensity of the exercise. The faster the horse runs or works, the greater the rate of heat production. During exercise, heat production can increase 40-60 times above resting values.

All mammals have various mechanisms to maintain their body temperature and protect it from rising when exposed to the warm environmental conditions or falling when exposed to the cold conditions. During exercise, the muscle and body temperature increases and to prevent overheating horses have to get rid of this generated heat. There are four basic means by which a horse can dissipate this heat and cool its body;

• Radiation: movement of heat energy at the body surface
• Convection: transfer of heat by movement of liquid or gas between areas of different temperature; e.g. heat transfer between the skin surface and surrounding air and heat movement within the bloodstream.
• Conduction: transfer of heat between surfaces that are in contact with each other; e.g. heat from muscles can be transferred to bloodstream and redirected to the skin by the circulation.
• Evaporation: conversion of a liquid to a vapour; e.g. evaporation of sweat at the skin surface and panting/blowing (respiratory heat loss).

For exercising horse the most important method to lose heat is through evaporation. Horses produce large quantities of sweat to cool their bodies. Depending on the intensity of exercise, horses rely around 70-85% on sweating and 15%-25% on respiratory heat loss to cool down.

Evaporative heat loss is most effective when the air is very dry. Dry air can accept moisture in the form of water vapour. The effectiveness of evaporative heat loss reduces when the relative humidity (air saturated with water vapour) increases. When horses are exercised in cool humid conditions heat loss can still take place by convection. In hot and humid conditions, both evaporative and convective heat loss may be diminished. It seems in hot and humid conditions that horses sweat more then on a day with lower humidity. In fact, the sweating rate is similar only in hot and humid conditions sweat can’t be quickly evaporated and remains wet on the skin surface and drips from the body. Sweat that is dripping from the body doesn’t cool the body enough, but it does remove water and electrolytes. Therefore, heat loss in hot and humid conditions becomes greater challenge for exercising horses.

Fluid and electrolyte losses during exercise

Fluid and electrolyte losses are determined by two factors; the duration and intensity of the exercise and the environmental conditions. During steady exercise in cool and moderate environmental conditions (15-20°C) horses will lose about 5-7 Litres of sweat per hour exercise. The sweating rate increases up to 10-15 Litres per hour exercise in warmer conditions (35°C). Athletic horses start to develop problems when dehydration approaches 6 to 7% of body mass in water loss. Rehydration is therefore very important to keep your horse health and cool.

Equine sweat is hypertonic, which means that is more salty than body fluids. Horses lose large quantities of electrolytes when they sweat. The composition of electrolytes in equine sweat may vary between trained and untrained horses and the time of exercise.  Approximate concentrations are depict in table 1.

Table 1. Electrolyte composition equine sweat

Replacing fluid and electrolyte losses

Fluid and electrolyte replacement is very important for optimal body health, performance and recovery. Horses should always have excess to fresh and clean drinking water. However, after exercise water intake should be limited to few sips to cool the horse. Large amounts of cold water immediately after a workout may lead to colic.

It is essential to supplement electrolytes daily in the diet of performance horses. The amount of electrolytes offered will vary with the degree of fitness, work level and the environmental conditions. Non-working horses consuming hay and fresh pasture will have a K+ intake that is usually higher than the daily requirements. However, Na+ and Cl- intake is often insufficient. Even if the horses are relatively inactive they are still losing small amounts through sweat and urine, therefore some form of salt supplementation is a necessity. For non-working horses a salt-block is sufficient to balance Na+ and Cl- levels. Exercising horses that are offered a salt-block may not always have enough voluntarily intake of salt. It is therefore advised to provide salt in the diet of the athletic horse. A combination of salt and Lite salt ( potassium chloride) in 50:50 mix at a rate of 10g/100kg body weight can be offered daily. For heavy sweating horses it is recommended to feed higher levels of K+. Potassium chloride or a commercially electrolyte replacer (20% K+) will help to correct the electrolyte losses during exercise. In addition to salt and K+ an electrolyte supplement should contain Ca++ and Mg++. Both minerals are important in preventing muscle cramping.

Preventing heat stress

The normal body temperature of horses is approximately 37.0-38.0°C. Body temperatures of 40°C is not uncommon after intense exercise or competitions. At this body temperature horses will benefit of being cooled down to prevent hyperthermia (overheating). Proper management assist the horse to quickly and safely lower its body temperature under conditions of exercise in the heat. These include repetitive bathing with cold water, frequently providing small amounts of cool drinking water and appropriate electrolyte supplements. During rest pauses, horses should be protected from the sun by natural shade or shade structures. The use of misting fans in the shade will decrease the air temperature and will provide air flow to facilitate cooling. Competitions in hot and humid weather should be held as early in the day as possible when temperatures are lower and solar radiation may be less severe. If weather conditions are very hot and humid, consider to stop all training if more than one hour duration. Always monitor vital signs of your horse to identify heat stress. These include increased heart rate, increased rate of breathing (blowing), increased body temperature, low sweating rate and fatigue (muscle weakness).

Heat acclimatization and physical conditioning will assist with increasing tolerance to work in the heat. Horses that are unconditioned and overweight have higher risk of overheating when they are worked too hard in warm environmental conditions. Fat acts like an insulator, trapping the heat in the body. Physical condition will improve the temperature regulation during exercise. Muscles will work more efficient and produce slightly less heat. The blood circulation is also better able to move heat away from muscles to the skin surface. Moreover, training will also increase efficiency of sweating.

Nutrition can also play an important role in keeping your horse hydrated and cool. Horses can retain up to 60 litres of fluid in the large intestine which functions as a fluid reserve during exercise. Dietary fibre is very important in diet of the horse to maintain gut fill and proper gut function, but it also increases the size of the gut fluid reservoir capacity, which is very important for exercising horses, in particular in endurance-type exercises.

High energy-dense feedstuffs are important in the diet of the athletic horse because daily nutrient requirements increase when horses are intensively trained in hot weather conditions. Grain and cereal based products are commonly fed to increase energy levels in the diet of performance horses. The major concern with high grain intake is that it may lead to digestive and metabolic disorders such as acidosis, laminitis, tying up and colic. Oil is an excellent energy source and can replace a part of the carbohydrate diet or protein diet. The other advantage of oil supplementation is that it reduces the metabolic heat generation in both rest and during exercise. Horses can utilize fat very efficient generating less heat.  Up to 10% can be added to the diet to safely increase the energy density.

Preventing heat stress in Summer is not only important for your horse but also for yourself. To avoid dehydration, drink plenty of water and isotonic sports drinks to replace electrolytes. Keeping your horse and yourself hydrated and cool is important for optimal health and performance.

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