Your hydration status isn't the only thing you should be concerned about when competing in a hot environment. A study published in the British Journal of Sports Medicine shows the highly individual adaptations to soccer competition in 70 to 109 degree heat.
Researchers had 19 semi-pro soccer players complete a match in 70 degree heat, then attempt to get acclimated to a hot, dry climate over a period of 6 days. The temperatures ranged from 100 to 109 degrees. After that, they took the field for a second game in 109 degree heat.
A heat response test undertaken at the beginning and end of the 6-day acclimation period showed an average increased sweat rate of 34% and an 18% reduction in sweat sodium concentrations. Individual differences varied by as much as 30%, with some players adapting very well to the heat and others experiencing real difficulty. Always pay attention to your individual performance and keep up with your calorie, water and mineral needs.