Electrolytes and Dehydration

What are Electrolytes?

Electrolytes are positive or negative charged elements that are called “ions” when solved in a watery solution (cations+ and anions-). The most important ions from a nutritional point of view: sodium (Na+), potassium (K+), Chloride (Cl-), Bicarbonate (HCO3-), Calcium (Ca2+), Magnesium (Mg2+) and phosphate (HPO42-). These elements are present in the blood and the fluid within and surrounding the cells of the (horse) body. What role do electrolytes have in the body?
Sodium, Potassium and Chloride are needed for the regulation of the horse’s water balance. Electrolytes play also a role in maintaining the balance between positive and negative elements, and the generation of electrical impulses in nerves and muscles. They may also play a role in enzyme activity. A sodium deficit may lead to a lower performance, reduced feed- and water intake and excessive licking behaviour. A potassium deficit may lead to muscle problems, lethargy, weakness, a lowered feed and water intake and weight loss.

When may an electrolyte deficit occur?
An electrolyte deficit may occur when the horse takes in electrolytes below its requirement. Horses that are subjected to low intensity exercise require electrolytes at about maintenance requirement. This requirement should be fulfilled! The daily ration of stabled horses (no pasture access) sometimes lacks sufficient sodium. It is advised to provide a salt- or mineral lick to these horses. This should always be accompanied with free access to clean drinking water.

If your horse produces large amounts of sweat during an event, electrolyte supplementation may be indicated. The electrolyte requirement especially increases when the horse is subjected to long term intensity exercise. The requirement is influenced by temperature and to a certain extent also by the humidity.

The horse body aims to maintain a constant body temperature. A large part of the energy that is used by the body is converted to heat. The conversion of energy to mechanical energy of locomotion in horses is at best 20% efficient, such that most of the chemical energy is converted to heat. Thus, contracting skeletal muscles produces large amounts of heat at high rates, and the rate of heat production increases with increasing exercise intensity. Heat that is produced in the muscles (especially during exercise), is transported by the blood to the skin. By sweating the horse is able to reduce its thermal load.

The sweat rate of horses has been estimated to be about 6.5- 9 litres /hour at speeds observed in the endurance discipline. During prolonged periods of exercise horses may loose considerable amounts of water by sweating. This may result in dehydration. A loss in body weight up to 7 to 11% may occur (i.e. a 7 -11 % water-loss-induced decrease in body weight). Dehydration decreases performance, muscle problems and finally exhaustion. A body weight loss indicating 12-15% dehydration can be fatal. With lower intensity exercise the water loss will be lower but this can also negatively affect performance. In practice, a dehydration of 4 to 5% or greater can be detected by delayed recoil of a fold of pinched-up skin, which is best observed in the skin over the shoulders. The skin over the neck is looser and considered less accurate indication for dehydration. In the normally hydrated horse, a pinch of shoulder skin should return to its original position within 1 second, and capillary refill time should be less than 3 seconds.

Prolonged exercise does not only result in water losses but also in Sodium, Chloride and Potassium losses. Excessive sweating may lead to a deficiency of these nutrients and to a minor extent on losses of calcium, magnesium and phosphorus. Horse sweat differs from human sweat. Human sweat is isotonic (i.e. the sweat concentration is similar as in the blood) while horse sweat is “hypertonic” (i.e. a higher concentration of electrolytes in sweat than in blood). This is indicated in Table 1.

In practice, electrolyte products often contain a small amount of glucose or dextrose. This is not done with the aim to provide the horse with energy but to enhance electrolyte and water transport. An electrolyte product should have a good solubility, taste and highly available sources of calcium and magnesium should be used. Recent research has indicated that electrolyte supplementation enhanced glycogen resynthesis and faster restoration of plasma hydration status in horses subjected to a simulated speed and endurance test of a 3-day event.

How do I prevent an Electrolyte deficit?

In contrast to many nutrients there are no body stores of water and electrolytes other than those carried in the gastrointestinal tract. Any access absorbed is excreted in the urine. Thus, body water an electrolyte deficiency cannot be prevented by giving them before they are lost. However, deficiencies can be prevented by replacing them as they are lost.

Below a few practical tips are listed to prevent a water or electrolyte deficiency (and therefore preventing loss in performance and muscle problems):

  • Make sure the horse has free access to clean drinking water at all times
  • Compensate the electrolyte loss when the horse has been subjected to heavy exercise (note: especially in humid conditions!)
  • Electrolytes given a few hours before prolonged exercise may be of value if adequate water is also provided and the horse is adequately hydrated. Do not provide excessive amounts of elektrolyten.
  • It may be advantage to have a (hind)gut filled with water and forage, so forage intake may be stimulated before a race and good quality forage used during a ride.
  • Some horses do not use their salt or mineral lick. Mixing electrolytes with the feed may be an option
  • An optimal rate of intake of electrolyte solutions is obtained when the solution is about ca. 20˚Celsius
  • Pastes can be a convenient way to provide electrolytes provided that the horse has access to and drinks water: hypertonic pastes given to dehydrated horses can lead to serious problems.

Lewis, 1995. Equine Clinical Nutrition; feeding and care. Williams & Wilkins, Media, PA (USA)
NRC, 2007. Nutrient requirements of horses. The National Academies Press
Lindinger, M.I. 2008. Sweating, dehydration and electrolyte supplementation: Challenges for the performance horse. Proc. 4th European Equine Health & Nutrition Congress. Wageningen.