He found that the energy used to absorb the fluid was more detrimental than beneficial at sprint distances. Not enough fluid could be lost in that time, at that temperature, to cause a performance drop off. But for longer events in warmer weather, fluid consumption is absolutely necessary. Sweating keeps our body temperature within a safe working range. And all types of metabolic and cellular functions depend on maintaining a certain level of fluid within our bodies.
Fluid levels determine the viscosity of our blood as well.
When we lose too much water our blood becomes thick and cannot be pumped as effectively. Our heart strains to do the necessary work. The amount of liquid we can absorb per unit of time can potentially limit our performance in the heat during long efforts. Pure water is absorbed fastest, but we also need calories to maintain a prolonged effort over long distances. Water with a dissolved sugar solution is absorbed into our system more slowly than pure water, so the stronger the solution, the slower it's absorbed.