One reason that many people advocate drinking six to eight glasses of water a day is because they fear that thirst kicks in too late. They are afraid that a person is already dehydrated by the time they start to feel thirsty. If you believe that you don’t start feeling thirsty until it’s too late, then it makes sense to want to ensure you are getting enough water whether you are thirsty or not.
This is just as much of a myth as the idea that you should drink six to eight glasses of pure water a day. In our review of the medical literature and in reviews conducted by experts in physiology and water balance, we could find no scientific studies claiming that thirst begins after a person is already dehydrated. In contrast, many studies conclude that thirst sets in when the concentration of the body’s fluids (technically known as the plasma osmolality) rises a very small amount. For you to feel thirsty, the concentration of the body’s fluids does not need to rise into the range that defines dehydration. Even a rise of less than 2 per cent in this concentration can lead to thirst, whereas most experts define dehydration as a rise in plasma osmolality of at least 5 per cent. Thirst sets in when the concentration is still within the normal range.
Your body is incredibly sensitive to changes in how much water it has and how much it needs. Your body releases a hormone called vasopressin when it needs to hold in more water or prevent your kidneys from releasing water (in other words, when it doesn’t want you to pee quite as much). The body releases vasopressin in response to even smaller changes in the concentration of your body fluids than the changes that make you feel thirsty. This results in instant, moment-to-moment regulation of your water balance and how much urine you pee out.
Trust your body! A healthy body is wonderful at regulating how much liquid you need, how much vasopressin it releases and when you feel thirsty. A healthy body responds quickly and accurately to the need for more fluids, and so you don’t have to force down fluids when you are not thirsty and you don’t anticipate any extra fluid needs.