The plethora of protein-rich muscle-building supplements available for humans might lead you to believe that muscle can be built by diet alone. But of course that is a silly assumption because if that were the case, we’d all have toned, muscly bodies. As many are all too aware, muscle building requires the muscles to contract i.e. exercise, or work. There is also a genetic component to the maximal muscle size possible.
Muscle building is technically called hypertrophy, and it results from the muscle adapting to strength training, or resistant exercise. Muscle also adapts to endurance exercise (relatively low intensity, long duration aerobic exercise, which can be sustained for periods of over several minutes), but mostly via enhanced energy metabolism rather than size of fibres, or building/bulking up.
Muscle contraction is a process requiring energy. So the most important dietary factor for muscle building is energy, which comes mostly from carbohydrate in horse diets. Protein is required to actually build the growth in the muscle, but taking in more protein than required will not build more muscle than was stimulated by the contraction. So a certain amount of protein is important, but eating or feeding more than is necessary will not increase gains further, but instead will be wasteful and expensive, and cause an increased heat increment during exercise, which could be harmful to horses exercising in hot, humid climates.
The fact that muscle is composed mostly of protein probably contributed to the myth that high intakes of protein are required for muscle building. In reality, both human and equine athletes generally ingest more protein than they need. Athletes taking anabolic steroids would require higher protein requirements than normal, but thankfully such drugs aren’t in common use in the world of equine athletes.
There is a genetic component to the maximal muscle size possible.
Non-working horses of 500 kg require about 690 g crude protein daily (about 7% in the total diet) and hard-working horses require about 940 g (about 9.5% in the total diet). Just as important as the total amount is the quality, or the content of essential amino acids. Non-working horses (500 kg) need about 30 g lysine daily, whereas hard working horses of similar bodyweight need about 40 g lysine daily. It should be noted, however, that relatively little is known about individual essential amino acid requirements of horses at different ages or in different workloads and much more research is necessary before we have a good understanding.
Horses and human athletes on low energy and/or low carbohydrate diets e.g. those trying to lose body fat could benefit from extra supplementary protein because their intakes may be suboptimal.
Ingesting good quality protein after a bout of exercise might well be worthwhile, however, since this is the period of increased uptake. So a protein-rich meal or a well formulated amino acid supplement post-exercise might be beneficial to horses in both strength and sprinting, and endurance/stamina disciplines.
The horse’s main superficial and deep muscles, viewed from above.