Water is good for you. If you're thirsty, it's usually the best choice. But do you need 8 glasses a day? Does it count when you drink tea or juice or coffee? Do you need to force down another glass even if you aren't thirsty?
According to Heinz Valtin, a retired professor of physiology from Dartmouth Medical School who specialized in kidney research and spent 45 years studying the biological system that keeps the water in our bodies in balance, the answer is no. If you have kidney stones or if you get urinary tract infections easily, then drinking lots of water can be beneficial.
But there is no scientific evidence to support the notion that healthy people need to drink 64 ounces of water a day.
Valtin found that the 8 x 8 guideline may have originated from a misunderstanding. In 1945 the Food and Nutrition Board, now part of the National Academy of Sciences's Institute of Medicine, suggested that a person consume one milliliter of water (about one fifth of a teaspoon) for each calorie of food. The math is pretty simple: A daily diet of around 1,900 calories would dictate the consumption of 1,900 milliliters of water, an amount remarkably close to 64 ounces. But many dieticians and other people failed to notice a critical point: namely, that much of the daily need for water could be met by the water content found in food.
The Board revisited the question of water consumption in 2004. Its panel on "dietary preference intakes for electrolytes and water" noted that women who appear adequately hydrated consume about 91 ounces (2.7 liters) of water a day and men about 125 ounces (3.7 liters). These seemingly large quantities come from a variety of sources—including coffee, tea, milk, soda, juice, fruits, vegetables and other foods. Instead of recommending how much extra water a person should drink to maintain health, the panel simply concluded that "the vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide."
I'm sure you've heard that most people are so chronically dehydrated that they can't even tell when they're thirsty, and often mistake thirst signals for hunger. Barbara Rolls, professor of nutrition sciences at the Pennsylvania State University, disagrees. Her studies, she says, "found no evidence that people are chronically dehydrated." Although some drugs can cause problems with thirst regulation and the elderly may not experience thirst as intensely as younger people, Rolls maintains that most healthy people are adequately hydrated.
What about weight loss? Doesn't drinking water suppress your appetite?
Rolls disagrees here too, arguing that "drinking water and waiting for pounds to melt away does not work. We all wish it were that simple." She explains that "hunger and thirst are controlled by separate systems in the body. People are unlikely to mistake thirst for hunger." Furthermore, she reports that her studies "never found that drinking water with or before a meal affected appetite." Nevertheless, there are some elements of truth in the misperception. Rolls did find that water-rich foods—as opposed to stand-alone water—tended to help people consume fewer calories. And, she says, "there is a way that water can help with weight loss—if you use water as a substitute for a caloric beverage."
The body needs water to function properly, and dehydration hurts the body. But, "Water requirements depend so much on outside temperature, activity levels and other factors that there isn't one rule that fits everybody," Rolls says. And Valtin reminds us that drinking huge amounts of water can even be dangerous or fatal.
So how much water should you drink? Here's their advice: If you have specific medical concerns, talk to your doctor. But if you are healthy, Rolls recommends that you "have a beverage with meals and drink when you are thirsty." In other words, heed your thirst signals, enjoy that watermelon, and stop feeling guilty for not guzzling those extra glasses.
Thanks Scientific American