You shouldn’t relate overheating to dehydration. You overheat when you run too fast. That’s the key. You don’t overheat because you become dehydrated. The brain’s too clever. If you’re not going to drink, the brain will slow you down, and that will lower your body temperature, not raise it. So, we’ve got some great studies where we look at people running half marathons, marathons, short ultramarathons, and long ultramarathons. The longer the race, the lower the temperature, because they are running slower - Tim Noakes in Outside Online.
MY COMMENT: Noakes is stirring up a hornets nest. I didn't realize 4 of the 6 researchers who published the ACSM Guidelines on Dehydration had ties to Gatorade. According to Noakes, "Of the six authors, four – Drs. Maughan, Burke, Eichner and Stachenfeld – have direct and longstanding involvement with Gatorade and the Gatorade Sports Science Institute (GSSI), but only three (Drs Maughan, Eichner and Stachenfeld) deemed it necessary to disclose in the Position Stand the existence of that relationship. The two remaining authors – Drs. Sawka and Montain – are employed by the United States Army Research Institute for Environmental Medicine (USARIEM)" -sweatscience