I fell upon this problem literally as a young boy. In the 7th grade I went out for football. The two a day practices that took place before school started were always the worst. 90 degrees and often very high humidity. They always kept forcing water down us so we wouldn't dehydrate. During a blocking drill one afternoon I hit the young man across from me and he buckled like a sack of potatoes or something. It took a few seconds to realize that I had not done that great of a job blocking but something was wrong with him. He was just laying there in a pile. Well to make a long story short, his doctor found that due to all the water he was drinking he had low sodium concentrations in his blood. He was sick for quite a while after that and never did return to play football.