Understanding electrolytes, why and when we give them, is absolutely key to any endurance program. To better understand this, we must understand fluid balance in the horse, metabolism, sweating, and the added offense of environmental factors (particularly heat and humidity).

As the saying goes, you can lead a horse to water… 

First, I am going to reiterate the fact that high protein feeds and high grain diets produce excessive heat during digestion/metabolism (discussed in ‘Nutrition and Feeding the Endurance Horse’). Protein metabolism, in particular, produces far more heat than the metabolism of carbohydrates or fat and, therefore, is an inefficient energy source in the endurance horse’s diet. However, this may be helpful in very cold weather where maintaining heat may be difficult.

How do horse’s dissipate their heat production?

The horse has two primary means to dissipate heat production:

  1. Sweating
  2. Respiration/heart rate

The evaporation of sweat is the primary method the horse uses to dissipate heat. The horse becomes heat-exhausted when he loses the ability to keep up dissipating the amount of heat produced (such as in high humidity). Of interest is that when the vapor pressure of the skin surface is close to the vapor pressure in the air, such as in humid conditions, sweat no longer evaporates. When the sweat no longer evaporates, the horse is no longer dissipating heat through sweat evaporation. Sweat requires evaporation for heat dissipation. Sweat dripping off the skin is not evaporation. Also of interest is the fact that horse sweat has a high amount of latherin, which is a protein. It is this protein in horse’s sweat that is the cause of lather and also promotes the spread of sweat over the skin to increase evaporation. This is why lather is one of the first visible signs of heat stress in a horse, meaning he is producing a large amount of heat and is having a hard time dissipating this heat.

The horse will also dissipate some body heat by increased blood flow and panting. Increased blood flow means increased heart rate. A well-conditioned horse that is panting may still recover his heart rate within criteria, but he is panting to achieve this and, therefore, this is a sign of excessive work and the rider should pay close attention.

How do we replace all the fluid loss caused by heat dissipation?

We probably don’t. We can expect any endurance horse to at least lose some fluid during a 50 or 100-mile event. BUT we need to understand what is needed, when and why, what signs to recognize and what they mean. The key is to keep the horse hydrated and not let him become dehydrated. Once he is dehydrated, it is a chasing game.

The volume of sweat our horses lose obviously results in a great loss of body fluid. And, in this loss of body fluid, is a great loss in electrolytes. Horses cannot store electrolytes so it really is not beneficial to load your horse with electrolytes days in advance. It is important, however, to keep electrolyte balance at all times as much as possible. The large intestine can store a large amount of water, so one way to try to keep excess water and electrolytes in your horse is by feeding adequate hay/roughage. Beet pulp is a great source of roughage and a great water “container.”

When a horse sweats, this fluid has to come from somewhere, right? The blood plasma carries a lot of electrolytes throughout the body. The mineral electrolytes lost in the horse’s blood plasma through sweat are chloride, potassium, calcium and magnesium.

When we lose electrolytes in the blood plasma, the blood volume decreases. When the blood volume decreases, blood flow to the skin decreases. When blood flow to the skin decreases, sweat production decreases. When sweat production decreases, the horse loses his ability to cool down. You’ve heard of the term, anhidrosis – this is the inability to sweat.

While dehydration is causing the loss of electrolytes and the blood pressure to drop, the muscles are beginning to use their store of glycogen, which increases heat production and increases the heart rate (also more heat), and then lactic acid production due to the glycogen utilization… now we have a dehydrated horse and a muscle-fatigued horse. If kept up, this horse will soon suffer from tying up or colic (this is discussed in greater detail under ‘Colic and the GI System’ and ‘Tying up and Muscle Fatigue’).

If these processes discussed above continue, both the GI tract and the nervous system are affected and the horse can lose his ability to voluntarily drink and, in severe cases, the horse loses his thirst reflex and refuses to drink under any circumstances. Obviously, this horse needs immediate veterinary attention.

Signs of dehydration to look for:

  • Slow capillary refill time (2-3 seconds) – this is an indicator of dehydration by decreased blood flow to the skin.
  • Increased skin tent – when pinched, the skin remains pinched or is slow to “bounce back.”
  • Dry or pasty mucous membranes.
  • Dry feces – small, tight stools
  • Increased heart rate

The signs listed above are common in a moderately dehydrated horse. It is important to understand, it does not take a whole lot more stress for the horse to continue to decline and require immediate veterinary attention.

Above, we discussed the main electrolytes lost in blood plasma, which decreases the blood flow to the skin. The main electrolytes lost in sweat are sodium, chloride and potassium. Excessive loss of these results in muscle fatigue and decreased thirst reflex to dehydration.

How much electrolytes do I supplement for my horse?

This depends on how much sweat is lost. How much sweat is lost depends on environmental factors (heat/humidity), duration of exercise, and intensity of exercise. It is hard to give a median here without doing some blood chemistry. There are rides that offer free blood samples from time to time. In most studies, potassium frequently came up low. Potassium loss causes decreased muscle strength and tone.

Remember, without proper water and electrolyte balance, the horse loses his ability to cool himself, and a horse can only efficiently sweat (and, thus, cool himself) if he is not dehydrated. And, of course, a horse cannot stay well hydrated without proper water intake and electrolyte balance (in addition to your B group vitamins). Electrolytes are lost in sweat, urine and feces and include sodium, potassium, magnesium, calcium, chlorides, sulphates, posphates, and bicarbonates.

  • Sodium and chloride are the main electrolytes lost in sweat. Sodium helps keep water levels balanced and blood pressure maintained. Chloride helps with acid/base balance.
  • Potassium balances cell fluid levels and is vital for muscle, heart and kidney function.
  • Calcium assists heart, nerve, muscle, and blood clotting functions.

Deficient levels of potassium and sodium causes a decrease in appetite as well as thirst.

Deficient levels of calcium causes many heart and muscle disorders including muscle fatigue and “heaves.”

Key points to remember:

  • Electrolytes are not stored by the body.
  • Keep your horse hydrated – don’t wait until your horse is dehydrated before supplementing with electrolytes.
  • Remember how your horse needs to dissipate heat and keep in mind your environmental factors in this equation.
  • Know your electrolytes and what their functions are; be sure to give adequate amounts of sodium (often combined with chloride), potassium, calcium, magnesium, and phosphorus.
  • And, don’t forget your B Group vitamins (see ‘Supplements’) – they are equally essential in the endurance horse.

If you would like a customized program for you and your horse, please contact me to discuss your next step in success!

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