Hydration ensures that body chemistry works optimally. A lot of people don’t realize that their fluid intake has a big impact on energy levels and workout performance. Also, the sensation of thirst is the last indication your body is dehydrated. By the time you feel thirsty, it’s often too late.

Years ago, well-meaning sports scientists advocated that urine color was a means of determining dehydration, and that "clear" urine means adequate fluid intake. However, this notion is false and it has been debunked by well-controlled research. Urine color does not provide an accurate indicator of hydration status or fluid requirement.

For optimum performance, the idea is to get your clients to arrive at training fully hydrated and to replace fluids as soon as they are lost.  The fact is, its difficult to determine if a person is drinking enough water to meet their requirements. A good rule of thumb is to drink 240 ml every fifteen minutes of exercise, and to take in more if the climate is hot or humid.

J Sports Med Phys Fitness 2007.