Horse Health

Supplementing the Sweat

If you are thinking ahead to the imminent summer heat by looking at electrolyte supplements, good for you. Horses lose massive amounts of electrolytes (and water) in sweat, putting them at high risk for dehydration, impaired performance, heat stroke, GI issues and impaired organ function.

An electrolyte is simply a mineral present in the blood in a charged electrical state—positive or negative. The major electrolytes of nutritional importance are sodium, potassium and chloride. In addition to the sweat losses with exercise, requirements also include sweat losses at rest in the heat and baseline losses in urine and manure (digestive tract secretions).

Sodium is most problematic because dietary levels are usually very low. In hot weather the baseline requirement is at least 20 g/day for a 500 kg horse, which is equivalent to approximately 2oz of salt. Salt (sodium chloride) is the best choice for a sodium supplement since chloride level in the diet is highly variable and could also be low. Levels in grass hay range from a low of 0.169% to 1.035% in the Dairy One database which contains over 40,000 samples.

After meeting the baseline requirements you need to compensate for sweat losses. Sweat contains approximately twice as much chloride as sodium and twice as much sodium as potassium. Your first step in selecting a supplement is to find one that approximates these electrolyte ratios although if your horse is not exercised heavily in the heat you can allow lower levels of potassium since the equine diet always has excess potassium. In other words, sodium can be greater than 2X potassium. The relative electrolyte amounts in horses is very different from human sweat, so using human products like powdered Gatorade is not advisable.

If the product label only lists salt rather than sodium and chloride separately you can calculate the amounts using figures of 40% sodium in salt and 60% chloride. For example, if salt is 50%, sodium would be 0.4 x 50% = 20% and chloride 0.6 x 50% = 30%.

Once you have found a product with the correct ratios of sodium, potassium and chloride, the next step is to figure out how much you would have to feed. Product labels don’t always make this easy! Law requires they always be listed as a %. A product that is 10% potassium will provide 0.1 x 28.4 (28.4 g/oz) = 2.85 g of potassium per oz. You will need to know the weight of the serving size in grams (g) if it is different from 1 oz. A half ounce serving would be 14.2 g (28.4/2) and at 10% potassium would provide 0.1 x 14.2 = 1.42 g of potassium.

A horse that is sweating only lightly will lose about 5 grams of potassium an hour, but if sweating heavily loses over 20 g/hour. There is a HUGE variation in the levels of electrolytes provided in a manufacturer’s “recommended dose”. For example, potassium ranges from less than 1g to over 9g per serving, so you have to do your homework and compare your horse’s actual needs to what each serving of the various products provides.

To summarize correct electrolyte supplementation:

  • Meet baseline sodium and chloride requirements first, using 2oz of salt per 500kg of body weight.
  • Look for a product that provides approximately twice as much chloride as sodium and at least twice as much sodium as potassium.
  • Once you have a supplement with the correct proportions of electrolyte ingredients, calculate how many servings you need using the guideline of 5g of potassium for every hour of light sweating up to 20 g/hour with profuse sweating (500kg horse). Note: Because you made sure the proportions were correct first you only need to calculate servings for one of the electrolytes. The others will follow along in correct amounts because the ratios are right.


All content is for informational purposes only. Contact your local veterinarian if you have any questions regarding the health of your animals.

 About the Author

Dr. Eleanor Kellon is a renowned expert on equine nutrition and related health issues. She offers private nutritional consultations and online courses through Equine Nutritional Solutions. Find out more at, and read more of her articles at