heat exhaustion wrote:
The negative effects of heat go way beyond risk of dehydration. When it is warm, your body spends significant more energy to cool itself - or more precisely, to get rid of heat produced by running in this case. Among other things, you will send more of your blood supply to the surface of your skin to lose heat. That is cardiac output going towards cooling your body rather than to muscles to deliver oxygen so that you can run faster.
Just to emphasize this: yes, dehydration isn't the only or (in most cases) the main reason heat slows you down. If you run in a heat chamber while drinking exactly as much as you sweat, or better yet hooked up to an IV that keeps your hydration perfectly stable, you'll still be far slower in the heat. In addition to the redistribution of blood flow, there may be central effects: once your body reaches a certain temperature, you'll be unable to continue. The interesting (and unanswered) question is exactly how/why that happens and which parts of your body matter most. In addition to core temperature, skin temperature and brain temperature also seem to matter, perhaps in different ways (e.g. skin temperature might influence sense of effort, while brain temperature might affect muscle activation).
Anyway, that's all speculation in terms of the mechanisms. But however it works, you can't eliminate the effects of heat simply by drinking.