The horse is a surprisingly sturdy creature and a marvel of engineering. He is a large system, one that possesses tremendous strength and endurance but that is able to function on just vegetable material and water. Left to their own devices, horses do pretty well. Of course, horses are not left to their own devices. Once man found out that horses made great work animals, play partners and companions, it was inevitable that things would begin to happen to the horse.
Man seems to have a peculiar inability to leave well enough alone.
Good enough rarely is. In most cases, this desire to help the horse is a good thing. For example, when your horse has an infection, helping him by giving him antibiotics to kill the infection is a fairly straightforward concept. There is a difference, however, between helping a horse get back to what he was and trying to make a horse be better than what he is. For example, a wide variety of nutrients can be demonstrated to be necessary in the normal diet of the horse.