ridecamp@endurance.net: glycerol and exercise in humans

glycerol and exercise in humans

Beth Glace (lb@nismat.org)
Tue, 18 Feb 1997 14:32:29 +0000

To clarify, glycerol is not an electrolyte. An electrolyte by
definition is a substance which in solution can carry an electrical
charge. Glycerol is a "sugar alcohol". Although its use by
triathletes is not widespread, research has focused on its use as a
means of hyperhydrating. Exercise in the heat can lead to a severe
challenge to fluid balance, which in turn dramatically limits
performance. We know, for example, that relatively minor changes in
blood volume drastically reduce aerobic capacity in humans. A 3%
decrease in body weight produced by exercise in the heat can lead to
an 8% decrease in VO2max, or maximal oxygen consumption. Furthermore,
the rate of sweating during exercise in the heat can exceed the rate
at which fluids can be absorbed from the intestinal tract, thereby
making it nearly impossible to maintain hydration.

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.
Sports Nutritionist

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