Professional opinions vary on intake recommendations for water, from drinking eight 8-oz. glasses a day to relying on thirst. Every cell in the human body needs water. It regulates temperature, digests and processes foods, distributes nutrients, lubricates joints, protects organs, prevents constipation and rids the body of waste products. Daily intake needs differ, depending on individual activities and constitutions.
Dehydration
Thirst is the first sign that you need water. Symptoms of dehydration include dry mouth and lips, lack of urine, headache, dizziness, confusion and fatigue. Certain conditions require more water than normal to prevent dehydration: high temperatures, illness including vomiting or diarrhea, bladder infections and pregnancy or breastfeeding. Even cooler weather can deplete your body's water supply, because indoor heat causes a moisture loss.
Exercise
Exercise is another condition requiring an increase in water supply. While exercising, your body uses water to regulate its temperature in the form of evaporation, or sweat. If your body cannot sweat, whether from lack of hydration or excess humidity, you put yourself at risk for heat-related illnesses. The recommended amount of water during light activity is an additional 1.5 to 2.5 cups. Adjust fluid intake depending on the intensity and length of the exercise and the amount you sweat. For higher-endurance events, such as cycling or running, sports drinks are preferable to water to avoid sodium loss.
Evidence
In an August 2002 press release, Heinz Valtin, M.D., of Dartmouth Medical School, states that evidence is lacking for the "8 x 8" daily recommendation. He says this amount is not needed for the average person, and part of the daily intake may come from food and other drinks. Furthermore Valtin states that, despite earlier admonitions that waiting until you are thirsty thirst is waiting too long, thirst begins when blood concentration changes, which signals us with thirst.
Conclusion
Based on the daily recommended activity level and diet, drinking a glass of water with and between meals and before, during and after exercise equates to about 8 glasses of water daily. You can evaluate your intake needs according to activity and other foods and beverages consumed.
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