It is not necessarily true that horses on high protein diets have excess energy. The reason for the idea that protein makes horses "hot" is that protein can be used as an alternative energy source. Certain amino acids can be converted to glucose through a metabolic pathway.
The body uses protein primarily to build cell structures and secondarily as a fuel source. In humans, protein contributes about an average of 10% of the total fuel used, both during exercise and during rest.
If a horse consumes a diet rich in carbohydrates such as hay and grain, the horse will burn less protein than a horse eating a protein- and fat-rich diet. Exercise requires glucose, and combined with a high-fat, low-carbohydrate diet, hastens the use of protein for fuel initially. Low- to moderate-intensity exercise of long duration demands large quantities of fuel, including protein. Short-duration, high-intensity anaerobic exercise demands less total protein fuel. The better trained the horse, the less protein used during exercise.
Most equine athletes probably need just a little more protein than a maintenance level or sedentary horse. Endurance horses burn greater quantities of protein for fuel than horses performing exercise of shorter duration. All equine athletes should get sufficient protein, but carbohydrate needs should not be neglected either. Otherwise, they will burn off as fuel the very protein they wanted to retain as muscle.
