How Much Water Do You Really Need to Drink?

Recent scientific research is debunking that eight glasses rule.

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Humans have always needed water to survive, but our unquenchable thirst for optimal hydration is largely a 21st-century phenomenon. 

Water accounts for between 50 percent and 70 percent of our body weight and is necessary for every one of our bodily processes and functions. But starting with the bottled water boom in the 1990s, Madison Avenue advertising executives have done more to fuel Americans’ obsession with hydration than any public health guidance—they were, after all, the ones who convinced us to spend money on something most people can get for free. 

Drinking water isn’t simply touted as a way to maintain healthy functioning; rather, it’s positioned as a miraculous cure-all with the power to improve and invigorate every aspect of your health: boost your mood, moisturize your skin, increase your energy levels, and bolster your memory and ability to concentrate. “Drink water” is the answer to any wellness woe or query, and we’re often led to believe we’re nowhere near as hydrated as we should be. It’s no wonder we don’t leave home without our water bottles.

But for all the hydration hype, the scientific evidence supporting these claims is surprisingly scarce. Interestingly, the same is true of the long-standing conventional wisdom that everyone should be drinking eight glasses of water each day.  

So, are we drinking enough water? How much do we actually need? And why does it even matter? The answers aren’t so simple.

Why We Need Water

The Many Benefits of Hydration

Every cell, tissue, organ, and system in the human body requires water. Without it, we couldn’t survive. “Water is to the body what gasoline is to a car,” says Benedict Ifedi, M.D., a primary care physician at Memorial Hermann Medical Group in Katy, Texas, who is also board-certified in sports medicine. “Normal day-to-day life requires you to be well-hydrated—including any thinking, walking, or physical activity. Hydration is also important to maintain adequate function of your major organs, including your heart, kidneys, and liver.” 

Our body needs water to perform a variety of other vital, everyday functions too. “A few reasons we need to be adequately hydrated are for the lubrication of our joints, digestive tract, and mucous membranes; to transport nutrients throughout our bodies; and to assist with digestion, and the regulation of body temperature and blood pressure,” explains Hillary Ake, R.D., a sports dietitian with expertise in exercise science and hydration. 

Like the other organs in our body, our brain also relies on proper hydration to operate. And while there’s some scientific evidence that being even mildly dehydrated can influence concentration, critical thinking skills, memory, mood, and energy levels, more research is needed to fully understand this link. 

Along the same lines, scientists are only beginning to examine the connection between hydration and mental health. At this point, there’s limited evidence suggesting that drinking enough fluids may reduce the risk of depression in adults, and separately, that dehydration may increase levels of cortisol—widely known as the “stress hormone.”

Hydration's Link to Long-Term Health

Recent research from the National Institutes of Health (NIH) found that people who stay sufficiently hydrated appear to live longer, and develop fewer chronic illnesses—including heart failure, stroke, lung disease, diabetes, and dementia—than those who don’t get enough fluids. The study, published January 2, 2023 in the journal eBioMedicine, used data from 11,255 adults gathered over a 30-year period to reach this conclusion.

“For example, if you’re chronically dehydrated, it can contribute to injury to the kidneys, which, over time, can lead to kidney failure and end-stage renal disease, requiring dialysis,” says Dr. Ifedi, who was not involved in the study. “With end-stage renal disease, the body is unable to regulate the input and output of fluids, which can then cause problems with some of your other major organs, like your heart.”

It’s important to note, however, that while this research observed a correlation between hydration and chronic illness in the data—and which can certainly help inform medical practice and our own behavior—more studies are needed to definitively determine whether staying optimally hydrated can prevent certain diseases and prolong our lives.

The Role of Electrolytes

Whenever there’s discussion of hydration, it’s a safe bet that electrolytes will come up, too. Commercials for sports drinks can make them sound like some type of (legal) performance booster, available to everyone in supermarket beverage aisles. But the truth is, they’re something we all need, regardless of our athletic abilities.

Electrolytes are minerals in your blood and other body fluids that have an actual positive or negative electric charge,” Dr. Ifedi explains, noting that calcium, potassium, magnesium, sodium, chloride, and phosphorus are all common electrolytes. “They help your body regulate chemical reactions and maintain balance between fluid inside and outside of your body.” 

When we sweat, we naturally lose electrolytes. “During exercise or any activity in which a person gets warm, sweat is used as a method of thermoregulation—meaning that your body releases heat at the skin’s surface and the evaporation of sweat cools the skin,” Ake explains. “The hotter you are, the more you sweat in order to keep your body temperature at a normal level. Sweat is primarily made up of water from our blood, but also contains electrolytes—mainly sodium and chloride.” 

The good news, she says, is that most people get enough electrolytes from the food and beverages they consume during the day that they don’t need to reach for sports drinks or other specialized hydration products to replenish these minerals. (There are exceptions—more on this in a bit.)

We’ll need to drink more fluids on days when we’re active and sweating (or sick and losing fluids more easily); but even on less active days we need to stay sufficiently hydrated to keep our body up and running and to replenish the water it loses naturally through breathing, sweating, urine, and stool, Dr. Ifedi says.

We Need Water to Live, But It's Not a Magic Cure-All

Because water plays such a fundamental and ubiquitous role in overall health, the message seems to have moved past the basics—that staying properly, realistically hydrated is important and keeps us alive—and onto the sensationalist: that hydration is a panacea for any health, wellness, or beauty problem, and that more water is always better. We’ve come to believe that chugging water comes with special “benefits”—which makes these everyday functions seem like exclusive perks for those who are in the know, have access to the right and best products, or can drink the most water. 

Take the familiar claim that staying hydrated “boosts” concentration and memory. In reality, drinking water isn’t going to give you a superhuman cognitive advantage. What it will do is help you avoid dehydration, which may impact your focus. It’s also worth noting that some of the so-called “benefits” of staying hydrated (like the myth that drinking water leads to supple, moisturized skin), though relatively harmless, don’t have the science to back them up.

We’ve come to believe that chugging water comes with special “benefits”—exclusive perks for those who are in the know, have access to the right products, or can drink the most water. 

The Source of Water Intake Recommendations

If you know one thing about staying hydrated, it’s probably that you’re supposed to drink eight 8-ounce glasses of water, or roughly 2 liters, every day. The “8x8 rule,” as it’s also known, is thought to have originated in a three-sentence paragraph in a 1945 report from the Food and Nutrition Board of the National Research Council, which recommended that adults drink 1 milliliter of water for every calorie consumed—or around 2 liters for a 2,000-calorie diet. But for some reason, doctors and public health practitioners seemingly stopped reading there, ignoring the caveat in the last line clarifying that “most” of the water (and electrolytes) we need is contained in the food we eat.

“These recommendations, however, were not based on any robust research,” Dr. Ifedi says. In addition to lacking any scientific evidence, this long-standing hydration prescription has never taken into account factors like age, size, diet, or climate. In other words: That every human being is different. So how is it still the standard?

“The short answer is inertia,” he explains. “The details of water intake and hydration are not gone over in great detail in medical education, and once something as common as the ‘eight-glasses-of-water-per-day’ [recommendation] is learned, it’s difficult to break the momentum to unlearn it.”

The '8x8 Rule' Doesn't Quite Cut It

Part of why the “8x8 rule” caught on and stuck around for so long—despite the absence of research to back it up—is its simplicity. “At some point, saying eight glasses was easy from a public health perspective, but the science for individualized hydration regimens has been around for a long time,” Ake says. 

Realistically, though, not everyone is going to take the time to calculate how much water they actually need on a daily basis—or trust themselves to know when they need to replenish their fluids—so it’s not hard to see why so many healthcare professionals have opted to stick with this blanket rule of thumb for decades.

This universal advice may soon go the way of the United States Department of Agriculture’s original 1992 food pyramid and get an overdue update thanks to a new study on hydration. Published in the journal Science in November 2022, the research—which involved more than 5,600 participants from 26 countries—is considered the most rigorous on the subject to date. It found that most people don’t need to drink eight glasses a day in order to stay sufficiently hydrated; in fact, for some, that’s too much water.

Instead, the researchers say that our fluid intake requirements are tied to our “water turnover,” or the amount of water our bodies use and replace every day. And because a variety of variables determine our individual water turnover rate—like age, body size and composition, physical activity level, and the climate where we live—one-size-fits-all guidance doesn’t work. 

But without the cutting-edge hydration-monitoring technology used in this study to determine our own water turnover, how are we non-scientists supposed to figure out how much water we need to drink each day

Your Body Knows When It Needs More Water

For the average, relatively healthy person, most of it comes down to listening to—and trusting—their body. “When people have normal kidney and heart function, their body does an incredible job of managing its water and electrolyte content,” says Anthony P. Ardito, M.D., an internal medicine physician and vice president of the primary care service line at Catholic Health Service of Long Island in New York.

Even if we’re not closely monitoring our hydration levels, our body and brain certainly are. When we’re in need of more fluids, our natural thirst mechanism kicks in, and our brain sends us a signal, like a dry throat, to let us know that it’s time to drink. There are some exceptions to this—like older adults and people with certain conditions that decrease their thirst response—but for most people, the simplest and most effective way to stay sufficiently hydrated is to drink when they’re thirsty

This is the exact premise of recommendations given in the Institute of Medicine's (IOM) Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate, published in 2005, which notes that “[g]iven the extreme variability” in the amount of water people need based on multiple factors like metabolism, environment, and activity, “there is not a single level of water intake that would ensure adequate hydration and optimal health for half of all apparently healthy persons in all environmental conditions.” 

For most people, the simplest and most effective way to stay sufficiently hydrated is to drink when they're thirsty.

The paper goes on to affirm that, generally, the average healthy person really does end up drinking enough water everyday to keep their system hydrated: “on a day-to-day basis, fluid intake, driven by the combination of thirst and the consumption of beverages at meals, allows maintenance of hydration status and total body water at normal levels.”

Ake adds a few other variables to consider when it comes to your fluid intake. “The most important aspects of hydration are body size (mainly weight), environmental factors, medical conditions/medications, and physical activity,” she explains. “You'll need more water if you’re physically active, spend time outdoors in heat, humidity, or dry climates, if you live at a higher altitude, have certain medical conditions, or if you take certain medications.”

How to Stay Hydrated

Since hydration is so personal, learn to pay attention to your body’s specific signals and needs, and make it easier—both more convenient and more pleasant—to remember to sip throughout the day.

Drink when you’re thirsty.

Thirst is your body telling you that it’s time to drink some fluids. Instead of getting anxious about hitting arbitrary daily water intake goals, adopt the practice of intuitive hydration: Listen to your body letting you know it’s time to sip water.

Recognize other mental and physical cues.

“If you’re not sufficiently hydrated, you can have symptoms that include fatigue, dry mouth, thirstiness, fogginess, confusion, and fainting,” Dr. Ifedi says.

You can also learn a lot about your hydration status from your pee. “When you urinate, the color should be clear,” Dr. Ardito says. “If the color is dark yellow and appears concentrated, you probably aren’t drinking enough water.” He adds that if it remains dark despite drinking water, get in touch with your doctor, since this could be an indication of other medical issues.

Talk to your doctor.

Ideally, your doctor should bring up your water-drinking habits during annual checkups. If not, raise the subject yourself. This is your chance to ask any questions you may have on your water intake—something that’s especially important if you have a chronic illness or exercise a lot.

Don’t compare yourself to others.

The amount of water one person needs versus another depends on a variety of factors—height and weight, age, activity level, diet, and climate. This means that there’s a good chance that you have different hydration requirements than your partner, sibling, friend, and the person sitting next to you at work. If you see them guzzling 2 gallons of water a day, use it as a helpful reminder to check in with yourself—are you thirsty? Have you taken a break and a few sips of water in the last few hours?—but don’t feel pressure to keep up: Heed to your own needs.

Make hydration as easy as possible for yourself.

Have you ever felt thirsty, but opted to finish your episode or cleaning out your inbox before filling your glass? Don’t make yourself choose: Make an effort to keep a bottle or glass of water visible and within reach, Ake recommends, so it's at the ready when you feel thirsty, dry, or foggy. “When you’re bored in meetings or sitting in traffic, you might feel thirsty, and may be more likely to pick it up and grab a swig,” she says.

Eat your fluids (and electrolytes), too. 

With such an emphasis on drinking water to stay hydrated, we often overlook the fact that approximately 20 percent of our daily fluid intake can—and should—come from the foods we eat. The most hydrating foods are fruits and vegetables with a high water content, Dr. Ardito says, including cucumbers, celery, tomatoes, squash, lettuce, strawberries, melons, oranges, peaches, and blueberries.

Similarly, Ake says that the following foods can help replenish your electrolytes: olives, pickles, table salt, salty snacks like pretzels, tomato-based foods, soup, potatoes, bananas, yogurt, avocados, milk and other dairy foods, almonds, spinach, fish, pumpkin seeds, and dark chocolate.

Don’t forget that the other beverages you drink throughout the day also contribute to overall hydration.

Customize your water.

“Some people prefer water over other beverages and drink enough of it to stay sufficiently hydrated without even thinking about it,” says Ake. “For others, it’s more of a struggle.”

If you fall into that category, find a way to make water more palatable—whether that’s drinking it ice-cold, at room temperature, or with a flavor enhancer. “Whatever works for you, lean into it and make it a habit,” Ake says.

Don’t forget that the other beverages you drink throughout the day—like sparkling water, herbal tea, juice, and even milk (both dairy and non-dairy)—also contribute to your overall hydration. And yes, this includes coffee. “Even though caffeine has a mild diuretic effect, drinking coffee generally does not lead to dehydration,” Dr. Ardito says. 

When possible, avoid high-sugar sports drinks—and any other beverages high in sugar. While these types of electrolyte-enhanced sips are marketed as optimal hydrators, they’re not really necessary unless you’re exercising at very high intensity or sick and losing lots of fluids.

The Future of Hydration

Will advancements in medicine and technology change the way we hydrate every day? Here’s what’s on the horizon for hydration—and what the experts have to say about it. 

Intuitive hydration will become the norm.

The concept of drinking when you feel thirsty isn’t exactly mind-blowing, but moving forward, the messaging on hydration will likely move away from the one-size-fits-all “8x8 rule” and toward more intuitive hydration. As self-advocacy and patient-centered healthcare has become increasingly prominent, so too has an emphasis on listening to and understanding your own body—something that will continue into the future.

We'll have more access to more personalized data.

Part of the appeal of boutique or concierge medical practices is the personalized approach to healthcare. Doctors typically spend more time with their patients, including discussing test results and vital signs, and then use these data points to customize treatment, prevention, or long-term wellness plans. 

Along the same lines, we’ll likely have our own health data at our fingertips—whether it’s from our electronic medical chart, concierge medicine, or a wearable health-monitoring device—which will, in turn, enable us to make more informed decisions when it comes to staying sufficiently hydrated. In addition to drinking fluids when we’re thirsty, this information could help us adjust our water intake based on factors like physical activity, and whether we’ve traveled to a hot, dry climate.

Meanwhile, athletes will take a similarly data-driven approach, likely using even more sophisticated technology, Ake predicts: “The future of hydration in athletes is definitely more precise hydration measuring and tracking, and individualized hydration strategies,” she says.

The functional beverage and hydration supplement markets will continue to boom (for better or worse).

Ever since the mass marketing of Gatorade began in 1983, drinks formulated for athletes have been sold to average American consumers, who, for the most part, don't actually need any of the performance-enhancing benefits they advertise. It’s hard to believe we still buy into the commercials 40 years later, but we do. 

In fact, the electrolyte hydration drinks market is predicted to experience “overwhelming growth” through the end of the decade, driven by North American consumers who want to strengthen their immune system and live an active, healthy lifestyle, according to a November 2022 market research report. These products include ready-to-drink sports drinks and electrolyte beverages, just-add-to-water powder packets, concentrated liquids, hydration supplements, and functional water beverages, like alkaline, ketone, and birch water.

But simply having more hydration options to choose from doesn’t make them more beneficial. “The average healthy person in a non-tropical and food-secure environment doesn't need a marketed hydration product,” Ake says, noting that an electrolyte-replenishing product may, however, be useful in situations when a person is sick (think: fever, sweating, diarrhea, or vomiting), traveling somewhere with a different climate, or engaging in significant physical activity. 

“People who participate in high-intensity training lasting more than one hour; who exercise in a very hot environment—especially if they’re not acclimated to that environment; and who exercise twice a day, likely need to focus on electrolyte replacement,” she explains. 

Dr. Ardito has a similar take on hydration products that come in pill form. “Supplements designed to replenish electrolytes and vitamins are becoming more popular. However, unless you have an extreme exercise program or you’re exercising in extreme heat, drinking water and maintaining a healthy diet is probably all you need to do,” he confirms. 

More research will examine the pros and cons of IV drip therapy.

If someone is hospitalized and experiencing severe dehydration, they’ll likely receive intravenous (IV) fluids administered directly into their vein through a small tube. But over the past few years, IV hydration therapy has become increasingly available outside of healthcare facilities, popping up as trendy, on-demand wellness treatments in places like spas and dedicated “drip bars'' and IV lounges. Here customers choose an IV “cocktail” from a menu of different formulations of vitamins and nutrients and customize a drip protocol to support their particular wellness goals, whether it’s optimal hydration, more energy, glowing skin, or hangover or food poisoning recovery. Whether for health or cosmetic goals, these types of personalized infusion packages can range in price from $100 to $400. 

The underlying concept of these customizable drip protocols is the very same as hospital IV treatments: to deliver fluids and other vitamins and minerals directly into the bloodstream, bypassing the digestive system to hit your system with hydration ASAP. And while this type of treatment does hydrate faster than drinking water or other beverages, this type of hydration efficiency is really only useful in a life-or-death situation—not if you’re attempting to overcome jet lag or look well-rested for a big event. 

Ake, however, “would caution against using IVs outside of a medical context.” In fact, there’s no scientific evidence yet to suggest that heading to a voluntary IV drip bar is beneficial in any way (even for athletes). And though often administered by medical professionals, such as nurse practitioners and physician’s assistants, on-demand IV treatment aren’t regulated at the state or federal level, and do come with their fair share of risks, including the potential for infection at the injection site, drug interactions, allergic reactions, and fluid overload. 

We'll see big advancements in hydration tech and trackers.

Our current hydration fixation has coincided with the emergence and growth of health tracking technology, including wearables and smart devices. These products have been around for over a decade (the original Fitbit Tracker debuted in 2009), and their popularity shows no signs of waning

At this point, there’s an abundance of devices, apps, and gadgets available to us to help prompt and monitor hydration—and we can expect to see more as technology advances. For example, most smartwatches currently allow users to track their fluid intake manually by logging it in an app, but future models could include a built-in sensor within the wearable itself that would automatically detect and record hydration relevant data. Currently, there are a few dedicated hydration wearables on the market, but Ake predicts their technology will likely improve in the future and be of particular value among serious athletes. “I think wearables—everything from a patch to a wearable across the chest—that monitor sweat losses will become more precise and accurate, and used more widely in sports,” she says. 

Similarly, sales of reusable smart water bottles are expected to spike over the next few years, especially with the addition of features like UV-C light bulbs, which purify the water and sanitize the container. 

Ultimately, however, it’s unlikely any smart tech will truly surpass our own bodies’ abilities to keep track of hydration. “Even though new technologies are being developed to support the hydration process, our bodies have had millions of years of evolution [and are] designed to manage our overall hydration,” Dr. Ardito says. “As long as you give your body what it needs, it should be able to take care of the rest.”

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