Chemically, glycerol is a very large molecule that is not broken down by
the body, and it is too large to easily pass through cell membranes. This
means that once it is in the bloodstream it remains there until it is
filtered out by the kidneys.
When a large, non-absorbable molecule such as glycerol is in the
bloodstream it exerts what is known as an osmotic pressure. That is, a
certain quantity of water is also drawn into the blood stream in order to
dilute the molecule. If not, the concentration of solute (particles)
would be too great and the blood viscosity would increase along with many
other physiological changes that would be devastating.
In order to dilute the glycerol and keep blood at normal osmolality, water
is drawn out of the tissues and into the blood stream. This can result in
TISSUE dehydration, although there is increased fluid in the blood vessels
themselves. This is the exact clinical application of glycerol. When
there is fluid on the brain, or fluid in the lungs, under certain
conditions glycerol is administered to "dehydrate" these tissues.
Now, if large quantities of water are given with the glycerol the tissues
themselves will not necessarily become dehydrated. However, the increased
fluid in the bloodstream is not only diluting the glycerol, it is also
diluting the electrolytes and proteins in the blood stream. Water alone
cannot be administered. There is an increased need for electrolytes.
There are formulas for the amounts in humans, but I would not venture to
apply them to endurance horses who are already trying to make up deficits
due to their unique physiology and exercise performance.
Other points to be made, the increased fluid load in the blood vessels
along with the glycerol is eventually filtered out through the kidneys.
There is a great increase in urine output, which carries with it extra
sodium and potassium. The increased urine output is why this compound is
classified as a diuretic. The additional losses of electrolytes in the
urine will worsen the deficits the horses suffer.
Finally, the increased fluid load in the vasculature will elevate blood
pressure, and put an increased load on the heart as the volume of blood
the heart must pump with each beat is now increased. Granted, we want to
maintain normal hydration in our horses and conserve water, but to
"hyperhydrate" could have consequences we can only imagine.
There is much more involved here. Sweating is increased in human athletes
that have consumed glycerol. But humans have the ability to secrete sweat
that is almost pure water. Therefore, they can actually increase their
ability to cool themselves and just sweat out excess water.
Horses can not sweat plain water. Potassium is very concentrated in horse
sweat as well as certain proteins. That is why they lather. One of the
most important routes by which horses lose potassium and become deficient
in electrolytes is through their sweat during endurance exercise. In
fact, potassium in the sweat can be 50 times higher than in the blood
stream. If we force them to sweat more, they may be more efficient at
cooling, but again we are inducing even greater electrolyte losses.
Bottom line, human exercise studies can NOT be applied to horses. Be very
L. M. Crespo, Ph.D.
Associate Professor and Chair
Department of Pharmacology