If the amount of water in your body is reduced by 1%, you'll feel thirsty.

If the amount of water in your body is reduced by just 1%, you'll feel thirsty.

Next to air, water is the most essential element for our existence. Water is much of what we are, as the average human body is 60-70% water.   If you wait to drink until you are thirsty, you are already 1-3% dehydrated.   So drink before you get thirsty, especially in warmer climates. Thirst mechanisms are not totally reliable and are particularly impaired in the elderly.  Research suggests that at best 70-80% of us walk around in a  state of mild dehydration.  Imagine how much more at risk we are when out in the hot weather. Under ordinary circumstances, the average adult loses 10 cups of water daily by breathing, sweating, and eliminating.  This is magnified when there is excess heat, sweating, and activity.

Source