You’ve surely heard that you’re supposed to drink eight glasses of water each day. Doctors and nutritionists alike stress the importance of daily water intake. But where exactly did that recommendation come from? It’s actually from a single paper published back in 1921, and it’s based on just one subject. Here are the details.

Why 8 glasses a day?

Dr. Edward F. Adolph decided to figure out how much water humans lose every day by measuring the quantity of his own urine and perspiration. Adolph concluded that he lost 3 percent of his body weight in water every day. That works out to about 8 cups.

Physicians then took up Adolph’s eight cup result as their standard recommendation for how much water we should drink. But the obvious problem with Adolph’s experiment is that it only had one subject. We need many more data points to figure out how much water we actually need.

water, glasses of water, eight glasses


Does drinking more water really make us healthier?

Many studies have been done on water intake and health, and they have consistently found that people who drink more water have lower rates of diseases. But is better health the result of drinking more water, or the result of something else, like exercising, which also makes you drink more water?

This Harvard study on 48,000 men determined that drinking more water significantly reduces one’s risk of bladder cancer, independent of other risk factors. When the researchers compared study subjects in similar risk groups (comparing smokers to smokers, for example), they found that those who drank more water had a significantly lower risk of getting bladder cancer.

Another study on 20,000 men and women looked at water intake as a risk factor for heart disease. Here, the researchers found that people who drank five or more glasses of water a day had about half the risk of developing heart disease as people who drank two or fewer glasses a day. This benefit was independent of diet and exercise.


How much water do we need?

Based on studies like the ones mentioned above, the U.S. Institute of Medicine and the World Health Organization recommend consuming between 8 and 11 cups of water per day for women, and between 10 and 15 cups per day for men. This recommendation includes the water we get from foods, which is quite significant–about one liter per day.

If we get one liter of water from food each day, this means that women need to drink an additional 4 to 7 cups of water each day, and men need to drink 6 to 11. You will need to drink more than this if the weather is hot, or if you are doing rigorous exercise.

water, glasses of water, eight glasses


What about other beverages?

What about other liquids, like coffee and tea? This study found that black tea is just as hydrating as water, and this study found the same is true for coffee. The studies discussed in the previous sections, however, specifically looked at water consumption, not coffee or tea consumption, so we cannot be sure that coffee and tea have the same long-term health benefits. But they do appear to provide just as much hydration as water.

Edward F. Adolph’s eight glass per day recommendation may have been based on very little data, but it wasn’t far from the recommendations of the USIM and the WHO. If you’ve been drinking eight glasses a day, you might already be right on the money, but everyone’s body is different. As the summer heats up, the best guideline is to pack extra water, and drink whenever you’re thirsty.


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