Pure Water
Water and Fatigue

In the early 1940s a group of researchers at Harvard tested the effect of dehydration on exercise fatigue in a group of conditioned athletes. The athletes walked on a treadmill at a constant speed in a hot environment under three conditions: no water, water on demand, and water replacement commensurate with water loss. When the athletes were given no water, their body temperatures reached 102 degrees in 3.5 hours, and they were exhausted. When the athletes could drink as much as they wanted, they lasted 6 hours before their temperatures reached 102 and they became exhausted. But when the athletes drank enough water to replace all they were losing as they were exercising, the researchers ended the experiment at seven hours because the athletes felt they could continue indefinitely! Their temperatures remained at a steady 99 degrees.

(G.C. Pitts et al, "Factors Affecting Work Output in Hot Environments" American Journal of Physiology, 1944, 142: 254)