Tamara Hew-Butler, PhD
Tamara Hew-Butler, PhD, is an associate professor of exercise physiology at Wayne State University, Detroit. Fluid balance during exercise is among her areas of focus. Education.Wayne.edu
How many ounces of water should you drink a day? You likely have heard that you should drink eight eight-ounce cups a day. Reality: That venerable hydration guidance doesn’t hold water even though it has been repeated for decades. There’s actually not a drop of science behind it. So what’s the real answer to the question, How much water should I drink a day?
The amount of water you should take in is dictated by the amount of water you lose. We lose some water through urination and respiration, but we lose most water through perspiration—and how much someone sweats varies dramatically depending on temperature, activity level and size. Example: A 300-pound offensive lineman enduring a tough summer football practice will sweat much more—and require many times more water—than a 95-pound white-collar worker sitting at a desk in an air-conditioned office.
Some people carry travel mugs everywhere to ensure that water is always within reach. Others use smartphone apps that remind them to drink…and some even monitor their urine for clues about how well hydrated they are. But one strategy stands far above the rest when it comes to avoiding dehydration—drink when you feel thirsty.
Our brains have sensors that monitor the salt levels in our blood and warn us when those salt levels indicate we require hydration. The thirst signals we receive from these sensors increase in intensity as our need for water becomes greater…and they are impossible to ignore well before dehydration reaches the point of danger. Fact: People almost never die from dehydration unless they’re stuck somewhere without access to water. If you haven’t experienced strong thirst signals, that’s not because you’re overlooking them—it’s because you’ve never been truly dehydrated.
Helpful: It isn’t just the water you drink that affects your body’s hydration levels—many foods have very high water content. Watermelon is around 92% water, for example—eating watermelon can provide all the hydration someone needs even on a hot day. And despite the persistent myth, coffee and tea do provide hydration despite their caffeine content.
Drinking too much water, on the other hand, can be lethal. Excessive hydration causes the cells to swell, and brain cells can swell only around 5% to 8% before the brain becomes too large for the skull, potentially causing coma or death. The best way to avoid this danger—if you’re not thirsty, don’t drink.
Helpful: You might have read that urine color can be used to gauge hydration. There is some truth to this—darker urine is correlated with dehydration. But urine color is not sufficiently precise or reliable to recommend using it as a guide to when to drink.
