Water is ESSENTIAL to life and to good body functioning and makes up about 60 percent of your body weight. Every system in your body depends on water. For example, water flushes toxins out of vital organs, carries nutrients to your cells and provides a moist environment for ear, nose and throat tissues.

While lack of water can lead to dehydration, and ultimately death, low water intake can produce mild dehydration which isn’t life threatening but can drain your energy, cause constipation, cause headaches, impair mental functioning and can make you tired. In order for your body to function properly, you must replenish your water supply by consuming beverages and foods that contain water.

Regarding the amount of water we should drink daily: the amount depends on many factors, including your health circumstances, how active you are, and where you live.

The average person living in a temperate climate can calculate their daily water requirements by this simple formula; divide your weight in pounds (lbs) by two and this will give you the daily ounce (oz) recommendation. In other words, if you are :

8 stone / 112 lbs ÷ 2 = 56oz / 1 litre 656 mls
9 stone / 126 lbs ÷ 2 = 63oz / 1 litre 863 mls
10 stone / 140 lbs ÷ 2 = 70oz / 2 litres 070 mls
11stone / 154 lbs ÷ 2 = 77oz / 2 litres 277 mls
12 stone / 168 lbs ÷ 2 = 84oz / 2 litres 484 mls
13 stone / 182 lbs ÷ 2 = 91oz / 2 litres 690mls
14 stone / 196 lbs ÷ 2 = 98oz / 2 litres 898 mls

Or you can go by the Institute of Medicine determination that an adequate intake of water for men is roughly 3 liters (about 13 cups) of total beverages a day and for women it is 2.2 liters (about 9 cups) of total beverages a day.

Just remember that you may need to modify your total fluid intake depending on how active you are, the climate you live in, your health status, and if you’re pregnant or breast-feeding. Exercising means you need to drink extra water to compensate for fluid loss through sweating. During long bouts of intense exercise (like a marathon) it is recommended to drink a sports drink containing sodium. You may need additional water in hot or humid weather, or during the winter in heated indoor environments (which may cause your skin to lose moisture). If you have a fever, vomiting or diarrhea you should use an oral rehydration solution. Pregnant or breastfeeding women need additional fluids to stay hydrated, especially when nursing. The Institute of Medicine recommends that pregnant women drink 2.3 liters (about 10 cups) of fluids daily and women who breast-feed consume 3.1 liters (about 13 cups) of fluids a day.

On average, food provides about 20 percent of total water intake. For example, many fruits and vegetables, such as watermelon and tomatoes, are 90 percent or more water by weight.
In addition, beverages such as milk and juice are composed mostly of water.

Beer, wine and caffeinated beverages — such as coffee, tea or soda — are not effective fluid replenishing beverages.

The general rule of thumb is that if you drink enough water or fluid so that you don’t normally feel thirsty, and produce 1.5 liters/6.3 cups or more of colorless or light yellow urine a day, your fluid intake is probably adequate. If you’re concerned about your fluid intake or have health issues, check with your doctor or a registered dietitian. They can help you determine the amount of water that’s right for you.
Reference: Mayo Clinic (Mayo Foundation for Medical Education and Research), Scottsdale, AZo