More specifically, a 2-gallon dose of a hypotonic electrolyte supplement can help keep fluids in the muscular cells and lead to longer exercise times, while maintaining high sweating rates—meaning the horse’s cooling system is still working as it should, said Michael Lindinger, PhD, president of The Nutraceutical Alliance, in Burlington, Ontario.

Horses lose large quantities of sodium, potassium, and chloride, as well as smaller amounts of magnesium and calcium, through their sweat. Unlike human sweat, horse sweat includes greater quantities of these electrolytes than what’s found in the circulating blood itself. That means to create this relatively concentrated sweat, horses’ bodies must draw out significant amounts of ions from the blood. To prevent that stock from running out, they dip into the cellular reserves in the skeletal muscles and other soft tissues, Lindinger explained.

Tracing Highlighted Electrolytes Through Horses’ Systems

In a series of studies, Lindinger and his fellow researchers have followed the paths of radioactive electrolytes through horses’ bodies to see where they go from the time the solution enters the stomach. The low trace amounts of radioactive elements are not harmful to the horse, but they work as a marker that allows scientists to trace the paths the electrolytes take through the body.

In their most recent study, the team worked with four mares owned by the University of Guelph ranging from age 5 to 12, including a Thoroughbred, a Standardbred, and two mixed–breed horses. They trained the horses to work on a treadmill.

About three hours after their morning meal, the horses received a radioactive substance that allowed researchers to measure fluid around muscle cells. An hour after that, they received either water or an electrolyte solution containing radioactive sodium and/or radioactive potassium through a nasogastric tube, in one of three quantities: 1 liter, 3 liters, or 8 liters (1/4, 3/4, or 2 gallons, respectively). One hour later, the horses worked on the indoor treadmill, in front of a blowing fan, to about 50% of their full capacity as determined by their respiratory oxygen uptake and heart rate. All the horses received all the treatments, with a two- to three-week break in between.

The researchers found that when the horses received electrolyte supplementation before exercise, they lost less fluid and fewer electrolytes from extracellular fluid compartments in the soft tissue during exercise and recovery compared to when they received pre-exercise water alone, Lindinger said.

The horses continued to sweat at regular rates—as measured via sweat collection bands around the horses’ barrels and changes in body weight—and analyses revealed the sweat included high levels of electrolyte ions. Despite those losses, the pre-exercise electrolyte supplementation into the stomach was associated with good replenishment of the ions in and around the muscle cells within two hours of supplementation.

Nearly all the sodium stayed in the compartment just outside the muscle cells, whereas the potassium mostly went inside the cells, he said. Meanwhile, the supplemented horses could work significantly longer on the treadmill than those given water alone.

The findings indicate that giving a large volume of a balanced electrolyte supplement prior to exercise or travel can help maintain whole-body fluid and ion balance while supporting muscular function despite intense sweating, Lindinger said.

Sweat Happens—Even When Hair Is Dry

People don’t always realize how much their horses sweat—mainly because the sweat evaporates almost as quickly as it comes out of the skin, he said. Because the horse’s body heat is getting extracted by using that energy to evaporate the sweat water, owners and riders often underestimate the amount of fluid and electrolytes horses lose.

“People grossly underestimate how much sweating a horse undergoes,” Lindinger told The Horse. “So even when we don’t notice that there’s sweat leaving the horse or dripping off the skin of the horse, the horse is still losing a tremendous amount of fluid. The horse can lose about 5 liters (1 1/4 gallons) of fluid an hour, including 4 liters from the skin and another liter from the respiratory tract, and we wouldn’t even notice that.”

Seeing sweat dripping off a horse doesn’t mean the cooling system is working better, he added. “High body temperatures resulting from exercise, heat exposure, and transport result in very high sweating rates that can exceed the rate of evaporation, and it is the evaporation of sweat that provides the cooling effect,” he explained. “Unfortunately, the sweat dripping off the horse does not provide a cooling effect, and the sweat is being lost for nothing. The sweat just ends up being produced in excess. It’s just, you know, collateral damage of the way the horse has been designed in relation to what we are asking the horses to do, really.”

Water Isn’t Enough: Why Ions Matter

Water alone is not sufficient to rehydrate horses properly, Lindinger said. While horses can get electrolytes from their environment (feed and forage), sport horses—especially racehorses and endurance horses, but also any competitive sport horse and any horse that travels long distances to get to an event—need additional supplementation. Because they lose more electrolytes in their sweat, relatively speaking, than humans do, it’s even more important that owners ensure horses get the electrolyte balance they need.

Although electrolyte supplementation after physical effort can be helpful, this new study reveals that added electrolytes prior to intense sweating can offset electrolyte losses within the soft tissues, leading to better health and performance, Lindinger said.

Many horses don’t like the taste of electrolyte-supplemented water, but they can adapt to it, he added. By slowly introducing electrolytes at low doses and building up over time—outside of high-intensity exercise periods—horses can learn to drink the solutions prior to exercise and travel.

The study, “Preloading large volume oral electrolytes: tracing fluid and ion fluxes in horses during rest, exercise and recovery,” was published by the Journal of Physiology in August 2021.