Updated: May 8
A pile of new evidence shows that all cell replication in the immune system requires glutamine. Unfortunately, this non-essential amino acid cannot be made by immune cells. It is made almost exclusively by muscle cells. But the immune system uses a ton of it. So your muscles have to supply large amounts of glutamine continuously to the immune system.
There lies the problem. As we know, glutamine is also the anti-catabolic agent in muscle, that is, the compound that helps preserve muscle during and after exercise. The heavier the horses' training, the heavier the stress on muscles, and the greater your muscle use of glutamine. There is now considerable evidence that traumatic catabolic conditions, such as the effects of intense training, overwhelm the body's ability to produce glutamine. So both muscle cells and immune cells get inadequate supplies, with a consequent loss of muscle and strength and a decline in immunity. The effect is so strong that researchers now call glutamine a "conditionally essential amino acid, and others suggest that skeletal muscles should be included as an essential part of the immune system.
But don't just supplement tons of glutamine to try and make up for the deficit. Supplemental glutamine loads the body with toxic ammonia. Ammonia is a definite downer for performance! The right stuff to use is alpha-ketoglutarate, the ammonia-free carbon skeleton of glutamine. Branched amino acids help too, because they work in the body to provide a substrate for glutamine.