Hydration and Exercise in the Heat
Drinking fluids during exercise in hot weather actually does very little to prevent the body’s core temperature from rising.
Evidence from recent studies suggests that the nervous system regulates body temperature and performance during exercise in the heat through a mechanism called regulatory anticipation. Essentially, the brain allows the body to work only hard enough to reach his highest safe core body temperature, which is more or less the same in all humans.
Therefore, as long as they are working at maximum capacity–as one does during a race–runners competing in the heat will reach the same core body temperature whether drinking has a cooling effect or not, because inasmuch as it does have a cooling effect, the runner’s brain will simply allow him to run a little harder so that he still reaches the same body temperature.
A more recent study performed by researchers at the University of Cape Town South Africa. Found that cyclists performing a time trial in a hot environment on several occasions, consumed fluid at a different rate in each. The authors of the study found that the rate of fluid intake had no effect on core body temperature, but it did affect performance. The cyclists performed best when they drank at a freely chosen rate.
So while drinking while running in the heat will not cool you down, it will speed you up. Specifically, drinking during hot-weather runs will keep your blood volume at close to normal levels, which in turn keeps your sweat rate high. And since oxygen is delivered to the muscles through the blood, maintaining your blood volume through drinking also enables your heart to deliver more oxygen per contraction, so you perform better than you can if you allow your body to become too dehydrated.
Source: Active.com, “The Truth About Hydration in the Heat,” by Matt Fitzgerald