Some researchers have addressed this problem by attempting to hydrate
to supra normal levels prior to exercise. Glycerol, given in humans
at a rate of approx 1 gram/kg body weight, plus large quantities of
water is one method that has been studied. The consesus of most of
these studies is that glycerol can reduce the amount of fluid lost in
the urine; the water retained is not stored in the blood but seems to
be contained in the interstitial spaces, or the spaces between cells.
This seems to result in greater rates of sweating and perhaps better
dissipation of body heat. However, a lot of the studies did not
measure exercise performance so it is not clear that glycerol
ingestion improves endurance capacity in humans. Since you have to
have subjects voluntarily consume large amounts of fluid with the
glycerol in order to super hydrate, glycerol may have limited use in
horses since "you can't make them drink". I am not sure if anyone has
looked at its use in horses and I am not familiar enough with horse
physiology to venture more than a guess as to its usefulness.
Beth Glace, M.S.