Runners are not designed well to deal with heat or limited water - we can survive for a moderate amount of time without food, but water is a close second to oxygen on the list of essentials to survive. Fluid is lost through sweating, breathing and urinating - for runners, particular attention has to be paid to the sweating factor. As soon as runners start to run, they start to dehydrate with about 75 per cent of the energy put into exercise converted into heat and then lost. The extra heat has to be dissipated in order to keep the core body temperature within the safe limits of around 37-38°C and if fluids are not taken on, the blood will thicken and reduce the heart's efficiency, increasing the heart rate and raising the body temperature.