A lot of the papers extolling the need for proper hydration are funded by the bottled water industry. Turns out the often quoted “drink at least eight glasses of water a day” has little underpinning scientific evidence. Surprisingly, the recommendation is traced back to a 1921 paper where the author measured his own pee and sweat, and determined we lose about 3% of our body weight in water a day, which comes out to be about eight cups.
More recently studies have found evidence suggesting that not drinking enough water may be associated a range of heart conditions such as cataract formation but also with diseases such as heart and kidney disease, and cancers including bladder and colon cancer.
However, the issue with many of these studies is that low water intake is associated with several unhealthy behaviors, such as low fruit and vegetable intake, more fast food, and those who exercise a lot drink more water due to exercise so no wonder they have lower disease rates.
So are people sick because they drink less, or are they drinking less because they’re sick? Well a Harvard study of 48,000 men found that the risk of bladder cancer decreased by 7% for every extra daily cups of fluid one drinks. So a high intake of water—like eight cups a day, eight times seven, may reduce the risk of bladder cancer by about 50 percent, potentially saving thousands of lives.
An Adventist Health study of 20,000 men and women found that those drinking five or more glasses of water a day had about half the risk of dying from heart disease compared to those who drank two or fewer glasses a day. And like the Harvard study, this protection was after controlling for other factors such as diet and exercise. So they suggest it was the water itself–perhaps by lowering blood viscosity, meaning thickness.
So based on all the best evidence to date, the World Health Organization recommends between 2 to 2.7 liters of water a day for women. That’s 8 to 11 cups a day for women, and 10 to 15 cups a day for men. Now but that’s water from all sources–not just beverages–and we get about a liter from food and the water our body actually makes. So these translate into a recommendation for women to drink 4 to 7 cups of water a day, and men 6 to 11 cups, assuming only moderate physical activity at moderate ambient temperatures.
We can also get water from all the other drinks we consume, including caffeinated drinks, with the exception of stronger alcoholic drinks, like wines and spirits. Note, though, in the cancer and heart disease studies I mentioned, the benefits were found only with increased water consumption–not other beverages–so unless you have conditions like heart or kidney failure, women should drink 4 to 7 cups of water a day and men should drink 6 to 11.
