This is a excerpt from one of Lyle McDonald's blogs. http://www.bodyrecomposition.com/mus...et-part-1.html
While it should be a no-brainer, water intake is another place where trainees make basic mistakes (I am guilty of this myself). The effects of dehydration range from minimal (at 2% dehydration, strength and performance decrease) to painful (can anybody say kidney stones) to worse (at 10% dehydration, death can occur).
While there are many generalized water intake equations (such as 8 glasses per day), these may not be correct for everyone. To poach another guideline from my mentor, a good rule of thumb is 5 clear urinations per day, and 2 of those should come after your workout. Yes, that means looking in the toilet when you pee.
This gives trainees a way of individualizing water intake. Obviously someone who lives in a hot, humid environment (or trains in a non-air conditioned gym) will need more water than someone who lives in moderate temperatures and trains in a posh gym.
I’d note that, despite more dogmatic rhetoric to the contrary, all fluids contribute to hydration state (as do many high-water foods such as fruits and vegetables). Yes, even caffeinated ones; research clearly shows that the small amount of fluid lost from the caffeine is still much less than the amount gained by drinking the drink.
It’s worth nothing that recent research has found that plain water is actually the worst drink for rehydration following exercise. Milk was actually shown to be superior to either plain water or Powerade/Gatorade type drinks, most likely due to the potassium and sodium content. You can read more about this in the artilce Milk as an Effective Post-Exercise Rehydration Drink.
Finally, thirst is a poor indicator of hydration state. By the time you’re thirsty, you’re already a bit dehydrated."