As the main chemical component of the human body, water makes up about 60 percent of your body weight. Your body relies on water to carry out many different functions, including bringing nutrients to your cells and flushing toxins out of your organs. Failing to consume enough water can lead to dehydration, a potentially serious condition that occurs when your body's systems don’t get the water they need to function normally.
Stages of dehydration. Dehydration varies from mild to severe, with severe dehydration considered a medical emergency. Mild dehydration, while not life-threatening, is more common and can cause you to be tired and lacking in energy. Symptoms of dehydration vary significantly, but dry mouth and skin, thirst and decreased urine output are all potential signs that your body needs water.
The replacement approach. When it comes to staying hydrated, there is no universal formula for calculating how much water you should drink, because water needs can depend on a number of factors, including the climate you live in and how active you are. However, doctors at the Mayo Clinic typically recommend eight or nine cups of water per day for the average adult living in a temperate climate. This is based on the replacement approach, which means you are drinking enough water to replace the fluids you naturally lose through daily breathing, sweating, urination and bowel movements. The widely recommended "eight 8-ounce glasses per day" rule isn't actually based in any concrete scientific evidence, but it is on par with the replacement approach and easy to remember.
Listen to your body. The Institute of Medicine offers a slightly different recommendation based on gender. It recommends three liters (roughly 13 cups) for men and 2.2 liters (approximately nine cups) for women. However, if measuring your water intake doesn't sound like your cup of tea, there is a lot to be said for learning to listen to your body. If your urine is generally colorless or only slightly yellow and you rarely feel thirsty, you are likely staying hydrated enough. Urine that is darker in color (dark yellow to amber) is generally a sign of dehydration, unless it is first thing in the morning or you're taking certain vitamins known to change your urine color.
Other beverages can be counted toward intake. So far, we've focused on water intake, because drinking plenty of water is a great way to stay hydrated. Not only is it readily available and inexpensive, but it's also free of calories. With that said, it's important to remember that other beverages, as well as the foods that you eat, also help replenish your daily fluids. For example, milk and juice are mostly made up of water, as are plenty of fruits and veggies, such as tomatoes and watermelon. Drinks that contain calories, such as tea, soda, coffee, wine and beer, aren't the best choice for staying hydrated, but when consumed in moderation, they can still be counted toward your daily fluid requirements.