| How much water should I really drink in a day? — Kelly, Atlanta You've surely heard the oft-repeated wisdom that you should drink somewhere around eight glasses of water a day (or 64 ounces). Personally, I've found that's not nearly enough. At my desk, I keep my trusty, 40-ounce Stanley tumbler, and easily drink two of those just during work hours. If I don't, I wind up with flagging energy and a splitting headache.
"There is no one amount of water that everyone should drink every day since we are all different sizes, have different levels of physical activity and we all have different diets, from where we get our water from foods and fluids," says Riana Pryor, a researcher at University at Buffalo who studies hydration.
The whole eight glasses of water idea seems to have probably stemmed from a 1945 recommendation from the US Food and Nutrition Board of the National Research Council. However, scientific evidence doesn't really back it up.
"It may work great for some people but can be too much or too little for others," says Pryor. "We generally need to drink more water when we spend time in hotter environments, which cause us to lose more water through sweating."
Basically, we need to replace the fluids we we are losing every day, says Judy Tung, an internal medicine physician at Weill Cornell Medical Center. We lose water from going to the bathroom and from breathing and perspiring. "Two-thirds of the human body is made up of water," Tung says. "Water in the circulatory system ensures adequate blood flow to vital organs. In the gastrointestinal and urinary tract it helps to flush pathogens and toxins out of the body. In the skin it contributes to skin integrity and elasticity, and in the joints lubricates movement. It also helps to maintains normal body temperature through sweating and respiration."
Water intake, Pryor adds, isn't only limited to glasses of H2O but also the water in other foods and drinks we consume.
"People who eat more foods like fruit and vegetables get much of their daily water needs from their diet, so they would need to consume a smaller percentage from fluids," she says. If you're now wondering whether you're adequately hydrated, Pryor says there are some clues. The first is the color of your urine; if it's light like lemonade you're hydrated, if it's more like apple juice you should drink more water. (If it's ever brown or red, she says, you should see a medical provider as you are likely severely dehydrated. Some supplements and medications can also affect the color of your urine.) How thirsty you are is also a good indicator. "If you are feeling thirsty, you are already somewhat dehydrated," she says. If you're going to be spending time in heat, you can also weigh yourself before and after, with the difference in weight being how much water you should drink. Your body, in other words, is a better guide for how much water you need than any old rule of thumb. — Kristen V. Brown | 
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