Written by: Shannon Pratt-Phillips, PhD

Learn the science behind electrolytes and how to ensure your horse is getting enough.

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Pam Mackenzie photo

Most people are likely aware that electrolytes help stave off dehydration, and should be supplemented in horses that are in heavy work, especially during hot weather. But, do you know how they really work, or how to make sure your horse is getting enough?

How They Work

Electrolytes are substances that include the ions of sodium (Na+), chloride (Cl-), potassium (K+), calcium (Ca2+) and magnesium (Mg+). They are related to an electrical charge to conduct a current and cause a response in the body. When an atom doesn’t have the same number of electrons as protons it has a charge – if it has one less electron, it has a positive charge and if it has an extra electron, it has a negative charge (and if it has two fewer electrons, then it has a 2+ charge on so on). When these substances are placed in a solvent, such as water, the individual components disassociate (or separate). When table salt (NaCl), for example, is mixed with water, it dissociates into Na+ and Cl-.

Fluids and electrolytes within the body are found both inside the cell (intracellular fluids) and outside of the cells (extracellular fluids). Differing concentrations of electrolytes in the intracellular and extracellular fluids result in cells having an electric charge, as measured by the charge on the inside of the cell relative to the outside of the cell. This is called the membrane potential. Stimuli (such as neurotransmitters or drugs) can open or close channels in the cell membrane causing ions to move into or out of a cell, resulting in a change in electrical gradients. Such changes have the ability to activate other cells or cellular processes, such as muscular contraction.

Several functions of the body are dependent on the movement of these electrolytes across cell membranes. The nervous system transmission, for example, requires the movement of Na+ and K+ across the membranes of neurons. Electrolytes are also very important to maintain hydration status and blood pH. The thirst centre within the hypothalamus in the brain, for example, is sensitive to the amount of salt present in the blood. When the blood is concentrated (less water), specialized cells called osmoreceptors are activated and trigger thirst. Blood pH (acid-base balance) is maintained, in part, through the excretion or reabsorption of ions such as sodium, at the kidney.

Provided in the Diet

The minerals associated with electrolytes are required in the normal diet. They are considered ‘macro minerals’ in that they are needed in relatively large quantities per day, measured in grams per day (in comparison to the ‘micro-minerals’ that are only needed in milligrams per day). Most common equine feeds (forages, cereal grains) contain decent quantities of chloride, potassium, calcium and magnesium, though they tend to be low in sodium. This is why all horses should have a supplemental salt source (such as some kind of salt block or the salt that is already added to commercial feeds).

Electrolytes are also important nutritionally because they are a large component of sweat. Equine sweat is highly concentrated compared to human sweat (if you don’t believe me, taste it!), with concentrations of these ions being up to five times higher in equine sweat. Sodium, for example, is found at 125 mEq/L in equine sweat, but only 50 mEq/L in human sweat; chloride is found at 150 mEq/L in equine sweat, but only 45 mEq/L in human sweat; and potassium is found at 45 mEq/L in equine, but only about 5 mEq/L in human sweat. Thus, horses lose large quantities of these minerals when they are active (or sweating under any conditions, even just in the hot weather). As such, under hot conditions, or when a horse is athletic, he will need to be supplemented with additional electrolytes.

The Need for Supplementation

The extent to which electrolytes need to be replaced depends on their losses, which is related to the amount of sweat lost. Depending on the intensity of the workout, a horse may lose anywhere from five to 15 litres of sweat per hour, or five kilograms to 50 kilograms (light race workout vs endurance race) of body weight as sweat in the event (allowing for fecal and urinary losses). If you have access to a scale (and many equestrian events will have them available), you can weigh your horse before and after the event to determine how much weight he has lost – one kilogram of weight is approximately equal to one kilogram or litre of sweat (again, accounting for fecal and urinary losses). Within each litre of sweat, approximately seven grams of chloride and four grams of sodium are lost (as well as 1.5 grams potassium, and smaller amounts of calcium and magnesium). So, it is possible that after a big event, a horse may lose upwards of several hundred grams of electrolytes. Replenishment of electrolytes post-exercise is equally as important as prevention of electrolyte depletion.

Equine athletes should be given daily electrolytes, and may need further supplementation during and after a big event. The electrolyte requirements for a horse at maintenance and in moderate and intense work are shown in the table “Electrolyte Requirements” and these can come both from the diet and through a daily supplement. In addition to these requirements, a horse may need another 50 to100 grams of electrolyte supplementation with significant work efforts.

Commercial electrolyte supplements offer a good mix of the key electrolytes (Na, Cl and K) as well as magnesium and calcium. They also contain sugar (dextrose), which not only helps with palatability, but may also help facilitate absorption of the electrolytes from the gut into the bloodstream. This is because the transporter in the digestive tract that transports sodium requires glucose (a sodium-glucose co-transporter). Sugar should not be the main component of the mix, however, so you should avoid products where sugar is the first ingredient listed. Alternatively, you can mix table salt (NaCl) and ‘lite’ salt (KCl) in equal ratios, and add to the diet as needed. Some owners add electrolyte mixes to drinking water to help encourage water consumption, but it is suggested to introduce these slowly, monitor consumption (to ensure a horse isn’t turned off by such solutions and to ensure sufficient intake) and to also offer a source of non-supplemented water.

Usually a horse won’t overeat electrolytes, and if you add too much to their food they probably won’t eat it (the food or electrolytes).If you were to artificially give too high a dose (such as via a syringe directly into the mouth) they would likely just drink water to help them flush the excess via the kidneys (and would only be harmful if they have kidney problems)

Consequences of Depletion

The consequences of electrolyte depletion can be significant. If a horse becomes low in electrolytes, he often will not drink, resulting in dehydration. This is because, in part, one of the mechanisms for thirst as described earlier is the increase in concentration of salt in the blood. But if a horse is low on electrolytes (because, for example, he has sweat out most of them) there is reduced stimulus for thirst. A horse may be dehydrated without low electrolyte status and vice versa, but often the two go hand in hand. Further, electrolyte depletion can result in impaired electrical signaling within the body, resulting in conditions such as thumps or neurologic symptoms including incoordination. A horse that is depleted will also not be able to sweat, and as sweat is the major way a horse regulates his body temperature, he can develop hyperthermia. This is not to be confused with a condition called anhidrosis, in which the horse cannot sweat (regardless of electrolyte or hydration status). Prevention of electrolyte depletion is much easier than treatment, which often requires intravenous electrolytes and fluids.