How Much Water Do You Really Need To Drink?

Staying hydrated is obviously important. But you may be surprised to find out how much water it takes.

Monica Reinagel, MS, LD/N, CNS
5-minute read
Episode #661
The Quick And Dirty
  • The amount of water required to stay hydrated varies greatly with environmental conditions and physical activity.
  • Claims that most of us are chronically dehydrated are not supported by the facts.
  • Fruits, vegetables, and other foods and beverages can supply the majority of your fluid needs.
  • Contrary to conventional wisdom, coffee, tea, and other caffeinated beverages are not dehydrating.

Cheryl writes:

"I’m hearing more and more about links between dehydration and reduced brain function. Does the news coverage of this topic amount to scaremongering, or do we all need to a concerted effort to drink more water?"

In the last ten years or so, a number of studies have looked at the effects of dehydration on various aspects of brain function, including concentration, memory, and processing speed—as well as more subjective things like mood and fatigue. And the research suggests that even relatively mild dehydration can have a measurable impact on how well your brain functions.

And that's not all. Dehydration can cause also headaches, constipation, and reduced athletic performance. 

Staying hydrated is obviously important. But you may be surprised to find out how much water it takes to do that.

Are you chronically dehydrated?

A while back, I got an email from a blogger asking me to contribute to a post he was doing on ways to drink more water.

“My reason for doing this post,” he wrote, “is the scary fact that 75% of Americans are chronically dehydrated. I'm hoping to create ‘dehydration awareness’ and provide inspiration for people to drink more water.”

(This blogger also happens to operate a website in which he sells lots of water-related products through affiliate links, but I’m sure that’s neither here nor there…)

I declined to participate in the post but I was curious about the claim that three-quarters of Americans are chronically dehydrated. A quick Google search shows this exact claim repeated all over the web (although not by any terribly reliable websites).

The original source for this “scary fact” appears to be a 2013 segment on a CBS affiliate station in Miami, in which they reported that “up to 75 percent of Americans may be functioning in a chronic state of dehydration, according to new research.”

No further details about this research are given, so there’s no way to check who did this research and what they actually found. And yet it seems to be in permanent circulation on the web as an established fact. Well, it may be established, but it is not a fact.

How much water does it take to stay hydrated?

The National Academy of Medicine is the institution responsible for setting our dietary reference intakes, or DRIs. But establishing a recommended daily water intake turns out to be really tricky, because there are so many variables, including your body size, the temperature and humidity of your environment, and your level of physical activity. These factors could potentially quadruple the amount of fluids you need to stay hydrated.  

The NAM settled on an Adequate Intake (AI) of 2.7 liters (about 11 cups per day) for adult women and 3.7 liters (or 15 cups) per day for adult men, with a disclaimer that "individuals engaged in activity at higher levels or in humid climates resulting in excessive sweat may need more."

Meanwhile, huge populations surveys, such as the National Health and Nutrition Examination Survey (NHANES), find that adults only report drinking about 4 cups of water on average per day.  And this may be at the root of this widespread belief that we're all chronically dehydrated. But there's a huge misunderstanding at work here.

Water is not your only source of water

The NAM's estimate of how much water it takes to be adequately hydrated isn't just talking about the plain water that you drink. It also includes water from foods and other beverages—and these latter two categories supply up to 70% of the typical person's water intake each day. In fact, when foods and other beverages are included, American take in an average of 13 cups of water each day... right in line with the NAM recommendation. 

Fruits and vegetables are up to 97% water by weight, so if you're doing a good job eating the recommended 5 servings of veggies and 2-4 servings of fruit, that's a lot of water right there. Milk, soup, yogurt, sauces, meat, and fish—they all contribute to fluid intake. Even coffee, tea, and soda help keep you hydrated.

Surprise! Drinking caffeinated beverages won’t dehydrate you

You've probably been told that caffeinated beverages like coffee and tea are dehydrating. Actually, they aren’t.

The chemical caffeine is a diuretic, meaning that it increases urine output. If you were consuming only caffeine, that would be dehydrating. But with a caffeinated beverage, you are also consuming lots of water—far more than you might lose from the diuretic effect of the caffeine

If you don’t drink caffeinated beverages regularly, drinking a cup of coffee ends up being the equivalent of drinking about 2/3 of a cup of water. In other words, drinking coffee will hydrate you—just not quite as efficiently as water will..

But if you regularly drink caffeinated beverages, the diuretic effects are almost negligible. In other words, if you drink coffee every day, your body retains the same amount of fluid from a cup of coffee as it does from a cup of water.

Drinking water is a good habit

Even if the risks of dehydration have been overstated, drinking plenty of water is still a good habit. In fact, drinking more than you might need just to stay hydrated can have several benefits:

  • Drinking more water—especially right before meals—can help you lose weight by filling up your stomach and helping you eat less.
  • It can also help prevent kidney stones in those susceptible to them.
  • Drinking more can ease constipation and—ironically—also alleviate water retention.
  • And because even mild dehydration can result in minor changes in cognitive function, a glass or two of water before a mentally challenging task certainly can't hurt.

The body has a fairly efficient mechanism for getting rid of excess water. Under normal circumstances, it’s hard to get yourself into trouble by drinking too much water. But most people can stay perfectly hydrated on significantly less than eight glasses of water a day.

Who is at risk of dehydration?     

There are a few situations where dehydration is more of a concern.

The thirst reflex declines with age and the elderly are at elevated risk of dehydration. Small children can more easily become dehydrated, especially during extended periods of fever, vomiting, or diarrhea

Those involved in sustained, strenuous exercise or spending extended periods of time in very hot or dry conditions need a lot more fluids to stay adequately hydrated. When you’re sweating a lot, you need to replace sodium and potassium as well as fluids to prevent a potentially serious condition called hyponatremia.

And finally, excessive thirst and urination can be warning signs of diabetes.

The bottom line on hydration

But barring ill health, extreme conditions, or intense physical activity, most people will stay well hydrated by eating a reasonably healthy diet and drinking water or other non-alcoholic beverages when they are thirsty. As a rule of thumb, if you are peeing several times a day and your urine is pale in color, you are doing fine.

About the Author

Monica Reinagel, MS, LD/N, CNS

Monica Reinagel is a board-certified licensed nutritionist, author, and the creator of one of iTunes' most highly ranked health and fitness podcasts. Her advice is regularly featured on the TODAY show, Dr. Oz, NPR, and in the nation's leading newspapers, magazines, and websites. Do you have a nutrition question? Call the Nutrition Diva listener line at 443-961-6206. Your question could be featured on the show.