Avoiding Dehydration

Dehydration is cumulative, which means you can become dehydrated over successive days of training if you fail to hydrate adequately. Have you noticed that you feel sluggish, have a general sense of fatigue, suffer from headaches, a loss of appetite, feel excessively hot, feel light headed or nauseas - I know I have. These are all symptoms of dehydration. A good check to keep in mind is of your urine. It should be a pale yellow colour which indicates you are within 1% of optimal hydration.
Always start exercise well hydrated and during exercise drink according to thirst. Water is a suitable fluid replacement for low to moderate intensity exercise lasting less than 1 hour. For intense exercise lasting up to an hour a sports drink could be considered. Now we are entering an area that needs consideration - rapid fluid replacement is the main priority or fuel replacement is the main priority? 

Based on: The Complete Guide to Sports Nutrition, Anita Bean