The Hydration Rule That's Worked for 400 Million Years

There's a reason trying to down a gallon of water a day can feel like torture, and both the athletic and wellness benefits of such practice remain firmly in doubt.

Despite popular belief, there's no widely agreed upon benchmark for daily water consumption.

That "drink eight 8-ounce glasses a day" thing likely derives from a 1945 recommendation by the U.S. Food and Nutrition Board. The recommendation stated people should ingest 1 milliliter of water for every calorie they consume.

The recommendation was apparently not based on any known research, and the sentence immediately following it, which was that "most of this quantity is contained in prepared foods," was soon forgotten.


Despite popular belief, there's no widely agreed upon benchmark for daily water consumption.

That "drink eight 8-ounce glasses a day" thing likely derives from a 1945 recommendation by the U.S. Food and Nutrition Board. The recommendation stated people should ingest 1 milliliter of water for every calorie they consume.

The recommendation was apparently not based on any known research, and the sentence immediately following it, which was that "most of this quantity is contained in prepared foods," was soon forgotten.

Somewhere along the line, the U.S. Food and Drug Administration began using 2,000 calories a day as an "average" reference point, which translates to 2,000 milliliters of water a day (or about 67 fluid ounces) per the 1945 recommendation. Ignoring the fact that food and beverages outside water were also originally intended to count toward this total, you'd need to drink about eight 8-ounce glasses to get that much water.

It's also a really easy figure to parrot, and over time, drinking eight 8-ounce glasses of water a day became viewed as essential to good health.

"The origins of that weren't based on any specific series (of studies) or one specific scientific study," says Dr. Tamara Hew-Butler, an associate professor of Exercise and Sports Science at Wayne State University whose expertise is in exercise-associated hyponatremia. "And that number actually included other beverages, not just water, and it also included the moisture content in food. But from those origins, it caught on."

In recent years, many people have begun taking the 8x8 rule as the absolute bare minimum of water they should consume each day. The belief that more water is always better has proliferated, leading to the wide-spread popularity of "gallon challenges," which involve downing a gallon (or 128 fluid ounces) of water a day.

Upping your water intake to a gallon or more a day is often said to be associated with weight loss, better skin, better energy, better mood, "flushing out toxins," a reduced risk of illness or chronic disease—if you can think of a positive health outcome (real or imagined), someone somewhere's probably said you could achieve it if you just drink more water.

Let's start with the facts: adequate hydration is essential to good health and high performance.

Water plays a crucial role in almost every important bodily process. Water transports nutrients and oxygen, supports proper muscle contraction, improves joint function, and fights fatigue. Being significantly dehydrated has many undesirable effects, and if you totally deprive a human of any fluids, they will soon die.

These things are all true, but most people do not need to down a gallon of water a day to stay hydrated, and the effects of mild dehydration (and the risk of becoming clinically dehydrated) are often exaggerated. Yes, people can and do become dehydrated, but many of the recent studies declaring huge swaths of the population to be walking around in significantly dehydrated states use contentiously low baselines for what qualifies as dehydration.

Rather than chasing an arbitrary number of fluid ounces, simply drinking when you're thirsty is the smartest, simplest option for the vast majority of people.

Hew-Butler refers to our level of thirst as a natural "app" hooked up to our brain designed to protect us from drinking too much or too little water.

"Everyone's looking for a perfect guideline that works for every individual in every situation. But thirst is governed in your brain. And it's not just the amount of water that's important to your body—your body doesn't really care about the absolute amount of water. The body cares about the balance between the water and sodium," Hew-Butler says. "There are osmoreceptors in our brain that actually, in realtime, every second, sample the combination between water and salt going through the circulation of our brain. So if the water content goes down and it increases the concentration of sodium, then that triggers thirst, so you can actually replace the water back to the amount that you need."

While athletes training for multiple hours a day or workers performing highly demanding physical labor can benefit from drinking before or beyond thirst (particularly in hot, humid climates), if they have consistent access to fluids and are allowed to drink as they please, their body will often guide them in the right direction.

"Drinking 8-10 cups of water right before (a workout), that's inappropriate. Because exercise in and of itself will stimulate anti-diuretic hormones so you don't pee it out. That's evolutionary—it's not good to be competing and (having to) pee a lot," says Hew-Butler. "For me, having a sports drink and water available for a sports team and allowing athletes to drink what they want and when they're thirsty is the safest way to do it…Drink to thirst is a catch-all. There's a lot of nuance to that, but it's the safest bet for all situations for most people."

The idea that if you're thirsty, you're already dehydrated is just another misguided (yet oft-repeated) hydration myth.

While serious side effects from consuming too much water are rare, it does happen. If you consume too much water, the balance of water to sodium inside your body can get thrown out of whack.

Hyponatremia, which is the most common type of electrolyte imbalance, occurs when the concentration of sodium in your blood falls below 135 milliequivalents per liter.

Mild hyponatremia can be asymptomatic, but symptoms can include headaches, nausea, vomiting, fatigue and confusion. In severe cases, hyponatremia leads to water intoxication, which causes cells throughout the body (including the brain) to swell with fluid. This can lead to coma or death.

Several team sport athletes have died from hyponatremia (usually due to being made to drink huge quantities of water quickly in response to cramping), but it's most common in endurance athletes.

A study of 488 finishers in the 2002 Boston Marathon found that 13 percent had hyponatremia, while 0.6 percent had critical hyponatremia, which is categorized by a serum sodium concentration equal to or under 120 milliequivalent per liter.

The reason hyponatremia is particularly prevalent in endurance athletes is two-fold. One, many of them have had it pounded into their heads that they need to drink a massive amount of fluid both before and during the race to perform at a high level and avoid dehydration, and two, exercise causes us to secrete anti-diuretic hormone, or ADH, meaning we hold onto excess fluid instead of peeing it out.

The extended length of many endurance events exacerbates the issue. Simply ingesting sodium supplements in an attempt to counter-act extreme water intake does not appear to be an effective method of avoiding hyponatremia.

"If you drink a lot and then accumulate anti-diuretic hormone, you actually lose that capacity to pee out the water. Then it builds up and those are the people who get very sick and die," Hew-Butler says. "Things like exercise, anti-depressants, NSAIDs, stress—all those things can kick in the anti-diuretic hormone, and that's when people can run into a problem."

As for the idea you must down amounts of water well beyond your thirst to succeed in endurance sports, the data is far from convincing.

A 2019 meta-analysis of eight experiments published in the journal Sports Medicine found that ad libitum drinking (drinking as thirst dictates) and programmed drinking (following a pre-determined hydration program) "similarly impacted" 1-2 hour cycling or running performances at moderate to high intensity in temperate to warm conditions despite the fact the ad libitum drinkers' hourly rate of fluid consumption was about half that of programmed drinkers.

The ad libitum drinkers did lose more body weight during the trials, but their average performance was actually slightly better than that of the programmed drinkers (though the effect was small enough to be deemed "likely trivial" by the authors).

Of course, you may not always have access to water during a race, preventing you from necessarily drinking only when thirst dictates, but the point remains.

Beyond athletic performance, claims that drinking a gallon of water a day or beyond is some sort of panacea for health are largely unproven. Studies that have found a positive impact from high fluid consumption mainly centered on people with specific health conditions.

For example, if you have a history of kidney stones, drinking more than two liters of water a day may reduce the recurrence rate. Same goes for those with a history of urinary tract infections. But for stuff like lessening your risk of a cold, decreasing your risk of chronic disease, clearing up constipation, improving digestion, enhancing energy levels or beautifying your skin?

There's currently little evidence guzzling extreme amounts of H20 does the trick. Going from consuming very little fluid to consuming adequate amounts of fluid would likely help with such things, as would replacing high-sugar beverages with water, which is likely where much of the anecdotal evidence comes from.

But as long as you're not drinking so much water your life becomes threatened, are there any significant drawbacks to downing significantly more water than you need?

For the most part, no.

The human body, provided it is not in a state where anti-diuretic hormone has been released, is really good about simply flushing out excess water as urine (though the kidneys can only excrete about 1 liter per hour, so if you drink too much, too fast, it is possible to overload them).

If you're urinating more than 2.5 liters per day, you're experiencing polyuria, or excessive urination volume.

While polyuria is a symptom of certain diseases, it's often caused by people simply ingesting a high volume of fluids. When you drink a lot, you pee a lot.

Perhaps the biggest drawback of regularly drinking excess volumes of water is the inconvenience of polyuria, and more specifically, nocturia.

Nocturia, which is defined as "night-time urination one or more times, causing arousal from sleep", is a clinically significant sleep disturbing factor associated with negative impacts on sleep quality and health-related quality of life along with increased levels of daytime sleepiness.

A 2012 article published in the journal European Urology states that "one nocturnal void is often considered within normal limits, particularly in the elderly, but even one nocturnal void can be experienced as bothersome by some. At least two nocturnal voids are, in general, considered as definitely lowering quality of life."

If you're an otherwise healthy person who's getting up two or more times a night to relieve yourself, that could be an obvious sign that the amount of water you're consuming is excessive (though it's worth noting that nocturia does become more common as we age).

Of course, the fix is simple—just cutting back on your fluid intake is usually enough to make a difference.

A 2014 study published in the International Journal of Urology found that "guidance on water intake" and a subsequent decrease in average 24-hour drinking volume from 1693ml to 1286ml improved older men's nocturia and decreased their average 24-hour urine production from 2,187ml to 1,575ml.

While we're busting myths, drinks like milk, tea, coconut water and moderate amounts of coffee have been found to hydrate just as well as water, and while urine color can be a useful measure of hydration, it can be influenced by a variety of factors and is by no means a gold standard. While consistently dark, brownish urine is a sign something's most likely off, your urine doesn't have to be crystal clear or only slightly yellow for you to be hydrated, despite what charts like this may lead you to believe.

There's a reason our body is wired so that drinking well beyond thirst becomes unpleasant, and those cues are an ancient part of our biology. As the authors in this 2014 study published in the journal PNAS write, "the instinct of thirst was a cardinal element in the successful colonization by vertebrates of the dry land of the planet, which began in the Ordovician period about 400 million (years) ago."

Water is essential to life, but more water is not necessarily always better.

In situations where hyponatremia is not a concern, there's not a huge downside to drinking more water than you need, but if you find yourself constantly drinking beyond the point of thirst, getting up at night to use the bathroom, and never having anything less than totally transparent urine, don't be afraid to cut back a bit and see how you feel.

Perhaps you can then redirect some of that willpower into something that will make a bigger impact on your health than guzzling down an ocean's worth of water, such as eating more vegetables and less added sugar.

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