This article is from the Misc Bicycles FAQs, by various authors.
Current research indicates that the most likely reason
electrolytes are eliminated in sweat is to compensate for their rising
concentrations in the blood. The blood / electrolyte ratio rises due
to a net fluid loss caused by drinkng less than is eliminated in
sweat. Supplementation with electrolyte-containing "sports drinks"
only raises blood salt levels higher and increases volemic (as opposed
to osmotic, or intra-cellular) dehydration.
Many commercial sports drinks offer little more value than diluted
fruit juice. Of all the studies done on sports drinks and performance
only one has shown any effect. The American Dietetic Association
(1981) showed a decrease in the performance of athletes drinking fluids
containing electrolytes when compared with straight water.
Electrolytes pull fluids out of the bloodstream and deposit them into
cells where they are far less effective in compensating for fluid
losses brought about by sweating and physical effort.
Overly sweet fluids too, can hurt your performance. Sugar
concentrations over 100 calories per water bottle slow the absorption
of fluid and, except for fructose, can cause an insulin reaction. The
high sugar concentration of some commercial drinks merely lessens their
effectiveness in preventing dehydration.
On the other hand, one of the best sources of quick energy is the
water bottle, especially for shorter races. Watch a pro peleton in
Europe: in the final kilometers you will see riders pulling small
flasks out of their pockets and drinking the contents. These flasks
usually contain a dextrose/glucose solution that quickly and
temporarily raises blood sugar. The extra sugar can be a tremendous
advantage in the sprint.
There is controversy in sports-medical literature these days about
the optimal sugar concentration of fluid taken during exercise. Most
of these studies fail to consider that a less than maximal rate of
absorption may be appropriate if much-needed calories are also
absorbed. Despite the debates, most cyclists learn by trial and error
what fluids work best and in what quantities.