Science 101

How Much Water Do You Really Need to Drink?

You’ve heard how much you need to stay hydrated. But do you really know why? Or how much? Here’s everything you need to know about drinking water— and what it means for your health.

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They say you are what you eat. More accurate: You are what you drink — especially when you’re talking about water. Up to 60 percent of our bodies are made up of water. This includes most of your major organs, including your brain, heart, and lungs. Even around 30 percent of your bones is actually H20. 

While it may seem odd to think about the majority of your body sloshing around in your skin, being more liquid than solid is important. Water helps us maintain our body temperature, helps shuttle oxygen and nutrients through the body, flushes out waste, lubricates joints and even serves as a protective shock absorber for the brain and spinal cord. 

Still, “researchers are really only just starting to understand hydration and water intake,” says Lewis James, PhD, a hydration researcher at Loughborough University in the UK. What they do know, however, is worth taking note. 

Everyday, you lose water through basic bodily functions like sweating, breathing, and going to the bathroom. You probably don't notice this fluid loss because our bodies have a complex system to regulate the amount of water going out compared to what we're taking in. When there's too little water in our blood, the pituitary gland secretes vasopressin, an anti-diuretic hormone that causes the kidneys to remove less water from the blood. The result? We pee less. At the same time, nerves in the brain trigger thirst. 

When you don’t drink enough water, your health can suffer. When you’re a little short on water, you may feel dizzy, tired, and excessively thirsty. This kind of mild dehydration (a loss of 1–3% of body weight) has been associated with low moods, reduced endurance, and impaired cognitive function. “Mild dehydration has been linked to cardiovascular disease, poor kidney function, and certain types of cancer, though the research on cancer isn’t definitive,” says James. 

If you're mildly dehydrated, you might have trouble concentrating on a simple task, like driving for example. Lewis says there's not yet a known link for being continually dehydrated and early cognitive decline when it comes to aging; it’s something researchers are looking into. 

Fail to replenish your fluids when you’re mildly dehydrated and you’ll reach what exerts call moderate dehydration (a loss of about 5% of body weight). Symptoms like fatigue and thirst become more pronounced at this stage. Researchers who used fluid restriction to study dehydration note that you’re unlikely to reach this level of dehydration accidentally because the desire to drink becomes strong once you’ve lost around 3 percent of your body weight.  Severe dehydration (loss of more than 10 percent of the body’s weight) is marked by confusion, a rapid heartbeat and breathing, and loss of consciousness. This state of dryness usually requires IV fluids to replace lost water. Untreated, severe dehydration  can lead to permanent brain damage, seizures, or even death.

As the building blocks of your organs and body, your cells need water to function, of course. However, according to James, there’s scant data on the effects of cellular dehydration. 

Scientists disagree exactly how much water you need to drink every day to support these healthy processes and avoid the negative ones. The long-standing rule of thumb to drink 8 glasses (64 ounces) a day was found to have zero scientific evidence, according to a 2002 review in the American Journal of Physiology. “The current guidelines are a bit higher, suggesting 3.7 liters [15.6 cups or 125 ounces]  per day for men and 2.7 liters a day [8.4 cups or 91 ounces] for women,” says James. (A higher percentage of women’s body weight comes from fat, which doesn’t have as much water as lean tissue, meaning women need more water per day.)

How Do You Know If You’re Drinking Enough Water?

Most of us fall short of the above numbers. According to the CDC, the average adult drinks less than 40 ounces of water per day (only about 30 percent of the recommended intake for men and 44 percent for women). Water intake needs vary from person to person (a young, healthy, physical laborer in a hot, dry climate is going to need more water than a sedentary person sitting in the AC) and even day to day, says James. That said, there are some pretty clear signs that you’re among the parched population:

  1. Your urine is dark
    Seeing dark yellow in the bowl is a sign you need to be drinking more water, says James. The exception: first thing in the morning when your urine is likely more concentrated. By mid-afternoon, your urine should be the color of pale straw if you’re well-hydrated. If the hue of your urine suggests you should drink more, spread your intake out over the course of the day, says James.

  2. You're not visiting the bathroom often enough
    Different people have different size bladders, but in general you should be peeing at least 5 times a day, says James, with more than just a trickle coming out. If you’re making fewer bathroom trips, you may need to drink more.

  3. You're parched
    When you’re dehydrated, your kidneys, which regulate urine and blood volume, send signals to the hypothalamus, triggering the thirst response. It’s typically a good guide to drink up. That said, don’t rely on thirst alone to tell you when to drink. “The sensations of thirst aren’t always related to dehydration,” says James, who notes he may be thirsty after giving a lecture because his mouth is dry. And some people, particularly older adults, may not feel thirst until dehydration has already set in.


It’s tempting to start guzzling by the gallon to avoid these affects, but James cautions that more isn’t always better. Overhydration—called hyponatremia—a dangerous drop in blood sodium levels that happens when you’re taking in more water than your body can pee out (generally your body can excrete about a liter [33 ounces or 4 cups] an hour), says James. If your body becomes waterlogged, your organs swell, including your brain. But unlike other organs, it can’t expand because of the skull, notes James. Hyponatremia leads to fatigue, vomiting, confusion, headaches, and sometimes death. It’s rare, but may be most common among elite athletes.

Water, Water, Everywhere

There’s another reason not to stress too much about counting how many glasses of H20 you swallow daily: Water is everywhere in the things we consume,” says James. All foods contain water. In fact, the average American gets about 20 percent of their daily water intake through food, according to a report from the National Academies of Sciences. Watermelons, for example, are more than 90 percent water by weight; eggs are about 75 percent water; a cooked chicken breast is about 61 percent.

Other drinks, like juice and milk also count towards hydration, says James. Contrary to popular belief,

so do coffee and tea. “The diuretic effect of caffeine is virtually negligible if you only drink a few cups of either a day. Your body retains a similar amount of water when you drink coffee or tea as it does when you drink straight water. Alcohol also counts, says James, though he doesn’t recommend drinking it to stay hydrated for obvious reasons. Still, water is probably your best source of well, water. “It’s calorie-free and cheap.” 

Plus, water is calorie-free, unlike sugar-sweetened beverages and juice. In fact, drinking more water might even help you lose weight. “There’s correlation data that those who drink more water take in less calories, but the research is mixed,” says James. Some studies show drinking water approximately 30 minutes before a meal helps you consume fewer calories. Plus, says James, being better hydrated improves exercise performance. “If you’re better hydrated you might be more motivated to exercise and increase calorie expenditure that way.”

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