A New Zealand study in the British Journal of Sport Medicine (Wall et al., 2013) casts doubt on the whole idea that dehydration impairs sports performance in the heat.
Ten Kiwi competitive cyclists volunteered to undergo deliberate dehydration to -3% by performing two hours of submaximal exercise (walking and cycling) in the heat (33C).
They were then reinfused with saline to replace 100%, 33%, or 0% of fluid losses, leaving them 0%, -2%, or -3% hypohydrated, respectively.
Participants then completed a 25 km time trial in the heat, during which their starting hydration status was maintained by infusing saline at a rate equal to their sweat rate.
No difference in performance was seen between the groups, who were blinded to their treatment.
Wall, B.A., Watson, G., Peiffer, J.J., Abbiss, C.R., Siegel, R. & Laursen, P.B. (2013) Current Hydration Guidelines are Erroneous: Dehydration Does Not Impair Exercise Performance in the Heat. British Journal of Sports Medicine. doi:10.1136/bjsports-2013-092417. [Epub ahead of print]