Exercise causes a rise in core body temperature that is further exacerbated with dehydration and exercise in the heat.
A review of the existing literature comparing the rise in core temperature to changes in body mass loss during exercise in the heat shows that the increase in core temperature for every additional 1% change in body mass loss is 0.22°C (.4°F). An athlete who is 2% more dehydrated than their competitor will be competing at roughly 1°F higher which can have decrements in performance the higher the core temperature.
With evidence indicating that dehydration results in an increased core temperature at a given exercise intensity, staying hydrated could help reduce the risk of exertional heat stroke.
Proper hydration during activity will also assist in attenuating a rise in core temperature during physical activity.
Sports in which protective equipment is worn (eg, football) should ensure that frequent hydration breaks are implemented into a practice session since equipment has been shown to affect heat dissipation from the body during exercise.
Read the Korey Stringer Institute (KSI) study on "The Influence of Hydration on Core Temperature."