Electrolytes, the tiny molecules that we can’t live without. The astoundingly delicate balance of water and electrolytes in the horse’s body is essential to keep them healthy and performing well. 

What are electrolytes?

Water accounts for the majority of the horse’s bodyweight, as in humans. This water is present both inside the cells (in intracellular fluid), and outside the cells (in extracellular fluid). It moves between the intracellular and extracellular compartments as needed, to keep the water balance (or osmolality) steady. Electrolytes are the salts dissolved in this water. The main ones are: potassium in intracellular fluid, and sodium and chloride in extracellular fluid. Others are phosphorus, calcium and magnesium. Electrolytes are key for normal cell functions, and the proportions of water and electrolytes in (and around) the cells affect how well these processes work. The proportions can alter with exercise, but also with illness. 

It’s important for horse owners to be aware of electrolytes and what maintaining the normal electrolyte balance means for their horse, particularly in the case of performance horses.

Why worry about electrolytes?

Well, they are essential for essential everyday processes, such as muscle contraction and generating energy for use in the cells. This is especially important when we think about our performance horses. Knowing that electrolytes are needed to sustain activity, we can understand how electrolyte alterations can impact performance. We can also then appreciate the importance of keeping the horse’s electrolyte and water balance in check. Additionally, when the horse is unwell we often see abnormal changes in electrolyte levels. This may be from excess losses, dehydration or reduced intake. This is the reason for supplementing the horse with intravenous fluids in many medical disorders, to restore correct water and electrolyte balance in the body. 

It has also been thought that an altered intracellular level of calcium could play a part in recurrent exertional rhabdomyolysis (‘tying up’), a painful condition that causes muscle damage and can lead to kidney failure. 

Sweating and exercise

During exercise the horse can lose litres of water, particularly during long periods of low intensity exercise like endurance rides – the animal can lose up to 15% of its bodyweight in water. The water lost through sweat evaporates from the skin surface, taking heat with it. This is an important mechanism by which the horse regulates its temperature during and after exercise. 

The loss of water through sweating, and its use by cells during exercise, gradually depletes the water volume of blood. In response, along with increasing heart and breathing rate, the horse reduces blood flow to organs such as the gut. This shunts blood towards essential structures like the muscles. The extent to which the horse must increase its heart rate, breathing rate and shunt blood, affects the rate at which it recovers from exercise. So, the horse that was well hydrated beforehand (meaning it had an optimal circulating blood volume and normal intra- and extracellular water proportions), will recover faster than a dehydrated horse. This highlights the importance of keeping a competition horse well hydrated at all times… But especially during warmer months, intense competitions and warm climates. 

If the horse was dehydrated to begin with, this can have serious implications because the water in the extracellular fluid will be depleted, and water in the intracellular fluid may move out to replace it. If cells lose their water to the extracellular fluid, they become too dehydrated to work. 

Sweat isn’t just water; sodium, chloride, potassium, as well as magnesium, calcium and some protein, all move with the water to the skin surface and are lost through sweating. Combined with consumption of electrolytes by the muscles and other tissues during exercise, the result is gradual electrolyte depletion. 

What does this mean for the horse?  

The horse maintains its water and electrolyte balance by eating and drinking. Many horses will be keen to drink during and after competition, but water alone won’t provide the electrolytes they need to replace. In order to supply these in a timely manner, the horse can be fed or syringed electrolyte formulations which will help restore the balance. Every effort should be made to tempt horses who are reluctant to drink during competitions. Otherwise, dehydration and electrolyte depletion can lead to serious consequences such as neurological signs, and the expected exhaustion, dehydration and poor performance.

It’s important to recognise that whilst a salt lick can be offered to help with day-to-day regulation of the electrolyte balance in any horse, giving replacement electrolyte supplements in every meal or before exercise in a normal, hydrated horse is generally not necessary. This is because when the balance is right, any additional electrolytes will just be excreted and lost in the urine. The time for administering these to performance horses is generally after exercise. This allows replenishment in the period of recovery and when water and salt losses are replaced.

A word on the ‘Thumps’

Otherwise known as synchronous diaphragmatic flutter (SDF), depleted electrolyte levels are the cause of this odd phenomenon. In a horse that has the thumps, the diaphragm contracts in time to the heartbeat because electrolyte abnormalities cause altered activity in the nervous supply to the diaphragm. Some horses might need calcium supplemented fluids to help stop it. 

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