Several years ago one of my horses caught a nasty respiratory virus which he proceeded to give to all the other horses. The two followed this up with a secondary infection which the vet was having a hard time clearing up. After a tracheal wash it was determined that a antibiotic resistance strain was causing the infection (this is truly scary) and he mixed up an antibiotic that did work. All of this caused the horse to develop allergies. The vet said it could be temporary or chronic - and suggested the use of low levels of dexamethasone - a synthetic glucocorticoid. Glucocorticoids are produced by the adrenal cortex. His theory was that higher levels would decrease the inflammation and as such may keep the allergy reaction from becoming chronic.
After a month or so the vet decided that we should take the horse off of dex. He gave me a "weaning schedule". The horse had to be weaned since there was the real possibility that the adrenal cortex had become lazy and would not be able to produce the necessary amounts if we went cold turkey. That is what we did.
What is the moral of this? There is more to conditioning an endurance horse than conditioning the legs, heart, lungs, etc. Biological systems are controlled by hormones. Hormones are those substances, usually a peptide or steroid, produced by one tissue and conveyed by the bloodstream to another to effect physiological activity, such as growth or metabolism. Yes the hormonal system is critical for the performance of an endurance horse. Hormones cause the heart rate to rise in response to exercise or fright. Hormones control the movement of the gut.
An aside, one of the effects of over training is to overly stress the hormonal system so that it cannot do its job. Here is an superbly conditioned athlete whose performance as fallen off for no apparent reason. In many cases it is because the hormonal system cannot keep pace with the physical demands placed on it by the training regiment.
The hormonal system has to also be conditioned to respond to the stress of exercise. For a horse in tip top shape then the hormonal system is capable of controlling the heart rate, the oxygenation of the muscles, the metabolism of energy, etc. The production of these necessary will be sufficient for perk performance of that particular horse once the hormonal system has been trained. I don`t know how long this process takes, I`ll let one of the vets respond here - but I would suspect that it takes several years of conditioning.
Now what happens when we interfere with this conditioning and acclimation process by adding all sorts of "nutrients" - MSM, DMG, etc. Are we really helping or are we inhibiting (with all good intentions) the natural acclimation process brought on by exercise?
As far as PSGAG`s (Adequan and Cosequin), it is a nutrient that supports the regeneration of the cartilage matrix and supports the production of hyaluronic acid. Since the effect of PSGAG is to provide the nutrient for the body to regenerate the cartilage it should not be required be provide on a continuous. Many people use a regiment of an Adequan shot on a periodic basis (say every three months). At any rate it is hard to conceive that the "withdrawal " of Cosequin for a period prior to a ride should cause any harm on the horse. Jeannie Waldorn, DVM has a good article in the 1994 Feb-March EN on this subject.
Of course we have to provide a balanced ration to our horses which means testing for missing minerals and providing these. This biological system called the horse is an incredibly complex system and the ability to adapt to exercise is truly amazing - but there are no short cuts. My concern of using many of these nutricuticals is they may be providing a crutch and minimizing the adaptation to exercise of the biological system as a whole. Thinking that we need to use DMG, MSM, etc. or our horses can not do a ride may very well be (as Dane said) "the only person we are fooling is the person in the mirror".