One of the best classes I ever attended was at a National Karate and Jujitsu Union camp where they had a sports nutritionist from the University of North Carolina explain the cycle of specific elements built from nutrition. She explained the glycogen build up and decrease, and essentially went through everything we needed to know to "eat effectively" while we trained.
Dr. Susan Holliman was her name, and she exhorted us that the keys to peak performance were proper training and power eating. The best pre-workout foods were high carb, low fat with moderate protein, and lots of extra fluids.
The best post workout meals were high carb as well, and what we discovered was that the post workout meal was equally important as what you eat beforehand.
Sports drinks provided more appropriate salt and trace minerals that were expended by the body, and one of the main focuses of hydration is to keep the blood sugar levels correct.
Dehydration hurts performance more than any other mistake in training, and causes lots of bad effects, such as elevated body temperature, impaired performance (mental), heat exhaustion, hallucinations and heat stroke... even in a cool gym setting (if you're really hammering the training).
Changes in climate can be hazardous too, because the response of sweat glands is determined by previous exposure to heat, and you only acclimate to changes in climate approximately 80% over 7 days... so a trip to camp in the mountains can be harmful if you aren't careful (or to New Orleans, where the humidity drips through your clothes).
Some of the symptoms of Dehydration are dark urine, small volumes of urine, headache, and unusually elevated heart rate. Fluid intakes should begin several hours before working out (2-3 cups), 1-2 cups about 15 minutes before, and during the workout about every 15-30 minutes. Afterwards, keep drinking fluids until your urine is again pale.
While she recommended sports drinks, her overwhelming choice for normal consumption was water, water, water. Prevention of dehydration was paramount, and the old "train til you drop" will only have you getting off the floor... her recommendation was hydrate before, during, and after, and train as long as your muscles will allow.
All in all, she gave us a tremendous class, and I've kept the notes for over 20 years, and review them anytime I'm starting to go to a camp or seminar... or someone posts a question like this on the boards...