How true is it that I should drink eight glasses of water a day?

Leigh Vinocur, MD
Emergency Medicine
We hear it all the time: We should drink eight 8-ounce glasses of water per day. But is this fact or fiction?

There are no good scientific studies to support the eight-by-eight rule. The origins of this recommendation date back to 1945. The original guidelines actually came from a misinterpretation of a recommendation from the Food and Nutrition Board. The recommendation stated that a person should have 1 ml (about 1/5 of a teaspoon) of water for each calorie he or she consumes. The average diet at the time was approximately 1900 calories, meaning you needed about 64 ounces of water per day.

Now the Institute of Medicine sets general guidelines for total water intake. It recommends that women consume a total of 91 ounces (that's about 2.7 liters) per day -- from all food and beverages combined. For men, it's about 125 ounces a day (or 3.7 liters). Depending on your diet, about 25% of the water you consume comes from your food.

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Dr. Mehmet Oz, MD
Cardiology (Cardiovascular Disease)
This little ditty has been circulating since a 1945 guideline recommendation from the Nutrition Board of the National Research Council (NRC) read, "An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods."

For some reason the last sentence, which is really an important point, got cut from the story telling. Hence the 8, 8-ounce glasses of water a day rule persisted. What the board really meant was humans need 2.5 liters of fluid a day (assuming a 2500 calorie diet). But fluid comes from many sources, not just water alone, including other beverages such as coffee and from fruits, vegetables, yogurt and rice.

Healthy people at rest naturally maintain a water balance and what you don't use you excrete as urine. Still, if you consume too much water too quickly, you can get water toxicity, a potentially life-threatening situation where the kidneys just can't manage processing large quantities of fluid all at once.

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Continue Learning about Water


Plenty of water in your diet helps digestion, skin, and muscle tone. It can help prevent cramping during exercise and even relive stress. Many times we are low on water - dehydrated, and don't realize it. Being dehydrated can mani...

fest with symptoms like fatigue and depression. In fact, by the time we are feeling thirsty, we're already slightly dehydrated. That's one reason that we hear the recommendation to drink eight standard glasses of water daily. Adequate water helps you digest food an helps your body get rid of wastes. Your kidneys need to process water for you to eliminate waste by urination, and getting enough water ensures that these wastes are diluted. Not getting enough water can encourage the formation of painful kidney stones.

Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.