“Drink before you’re thirsty—by that time it’s too late!”

“Weigh yourself before and after workouts and drink enough so that you never lose weight.” 

“Drink as much as possible while working out—you have to replace the water you lose in sweat.”

If you’ve ever played a sport, worked with a personal trainer, or followed a fitness “expert” on social media, you’ve probably heard exhortations like these.

Become even a little dehydrated, you’re told, and your performance sags, your health suffers, and you lay yourself open to heat stroke, headaches, and even death. 

Thus, you must protect yourself against the perils of dehydration by constantly drinking throughout the day and especially before, during, and after workouts.

Is this hullabaloo about hydration warranted, though? 

Is dehydration as damaging to your athletic performance and health as it’s often claimed? 

How much do you really need to drink while working out?

In this article, you’ll learn the answers to all of these questions and more. You’ll learn what science actually says about how much water you should drink to optimize your athletic performance and how to make sure you drink this amount. (Hint: it doesn’t involve lugging around a jug of water).


What Is Dehydration? 

This question is harder to unravel than it may seem at first blush. 

The most common definition of dehydration you’re likely to come across is something like “losing too much water from the body so that it can’t function normally,” but this is begging the question. 

What constitutes “too much” water loss? 

Is losing any amount of water from the body detrimental, or do you need to lose a certain amount before the negative effects kick in?

What constitutes “normal” bodily function, and how does this change when you become dehydrated? And by how much? 

Poke around online and you’ll struggle to find clear, science-based answers to any of these questions, which is one of the reasons there’s so much confusion and misinformation about the effects of dehydration. 

That said, most personal trainers, coaches, and journalists hew to the position of the American College of Sports Medicine on dehydration and performance: 

  • Any amount of fluid loss (usually expressed as the amount of body weight lost over several hours) is considered dehydration.
  • Any amount of dehydration reduces performance, but the more dehydrated you become, the more you suffer.
  • Mild dehydration (usually defined as 1-to-2% loss of body weight) is acceptable but still suboptimal, and losing more than 2% of your body weight significantly reduces your performance and increases your risk of heat stroke and other problems. 

What does science say, though? 

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Dehydration Doesn’t Reduce Performance

Raise your hand if you’ve heard this one before:

“For every 1% loss of body water, your performance declines 10%.” 

While the particulars vary, the message is always the same: even small amounts of dehydration are disastrous for athletic performance, and thus you have to continually drink while working out to perform at your best.

For example, the American College of Sports Medicine recommends that you “consume fluids at a rate sufficient to replace all the water lost through sweating.” 

On the face of it, this argument rings true. 

We all know that water is the most abundant molecule in the human body and that it’s required for myriad functions. We also lose a lot of it through sweat, and thus it stands to reason that this would degrade our athletic performance if left unchecked.

A reasonable hypothesis (and profitable, if you sell sports drinks), but research shows this is hogwash. 

Studies have repeatedly shown that mild dehydration—usually around 1-to-5%—doesn’t impair performance in runners, cyclists, and other athletes in balmy, hot, or humid conditions, and that forcing yourself to drink more than required to quench your thirst doesn’t improve performance and may even decrease it. 

In fact, the fastest athletes also tend to be the most dehydrated at the end of races. For example, the runner Haile Gebrselassie lost 9.8% of his body weight in the course of winning the 2009 Dubai Marathon in a time of 2:05:29. 

This is actually the norm among top athletes. The winners of marathons, triathlons, and other extreme endurance races usually lose about 5-to-10% of their body weight by the time they finish, with most of this weight loss coming from sweat.

For instance, a study conducted at National Yang-Ming University found that one out of four runners in a 24-hour ultramarathon lost 7% or more of their body weight by the end of the race, and the fastest runners tended to be the most dehydrated. On average, all of the race participants finished about 5% lighter than when they started.

Another striking example of this comes from a study published in the British Medical Journal that compared the finishing times and dehydration levels of 643 runners after the 2009 Mont Saint-Michel Marathon. They found a nearly perfect inverse correlation between hydration status and running speed—the fastest runners were consistently the most dehydrated. 

Specifically, runners who finished in less than three hours (very fast) were about 3% dehydrated, runners who finished in 3-to-4 hours were 2.5% dehydrated, and runners who took more than 4 hours to finish were 1.8% dehydrated. It’s worth quoting the researcher’s conclusion in full, as it’s a stark refutation of standard hydration dogma:

“These data are not compatible with laboratory-derived data suggesting that BW loss greater than 2% during exercise impairs athletic performance. They match an extensive body of evidence showing that the most successful athletes in marathon and ultra-marathon running and triathlon events are frequently those who lose substantially more than 3–4% BW during competition.”

Of course, some might argue that these athletes would perform even better if they stayed hydrated, but there are two reasons this reasoning doesn’t hold water (harhar): 

First, top athletes spend hundreds of hours each year testing various hydration and fueling strategies in their training, and many spend thousands of dollars on gear, coaching, supplements, and training plans to slice seconds off their race times. If simply drinking more water boosted their performance, they’d make damn sure to do it on race day. 

Second, randomized controlled trials (RCTs) have also directly proven that mild dehydration doesn’t decrease athletic performance. 

An elegant example of this comes from a study conducted by scientists at Brock University, who found that cyclists performed just as well when 3% dehydrated as they did when fully hydrated during a 2-hour ride that ended in an all-out time trial in 95 ℉ heat. There was also no meaningful difference in core temperature, heart rate, or other physiological variables between cycling when fully hydrated or dehydrated.

Several other randomized controlled trials have found the same thing: dehydration doesn’t decrease performance.  

Keep this in mind the next time you hear someone sermonize about the importance of staying hydrated. If many studies on marathon runners, ironman triathletes, and cyclists have found that dehydration doesn’t reduce performance or negatively impact physiology despite exercising for hours in the heat, how much of a difference do you think it will make during a 1-hour workout in an air conditioned gym? Or when you’re sitting on your keister in an office?


At this point you may be wondering, if water is an essential nutrient, then dehydration must hurt your performance at some point, right? 

Yes, but it’s much harder to reach this threshold than most people realize. 

Although scientists aren’t sure exactly how dehydrated you have to be before your performance skids, this point probably varies from person-to-person and is likely higher than 5-to-10% of body weight. It also likely improves as you get fitter (as evidenced by the fact that faster athletes can tolerate greater levels of dehydration). 

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Overhydration Is More Dangerous than Dehydration

Here’s something most sports supplement companies don’t want you to know: drinking too much water is more likely to reduce your performance, damage your health, or even kill you than not drinking enough. 

Scientists have known since 1985 that guzzling excessive amounts of fluid quickly dilutes the sodium concentration of your blood, leading to a condition known as exercise-associated hyponatremia (EAH), aka water intoxication

While the full story of how this problem developed is beyond the scope of this article, the long story short is that supplement companies, in particular Gatorade (now owned by PepsiCo), began aggressively marketing the idea that dehydration was the athlete’s Achilles heel to goose sales.

Before this time, standard practice amongst high-level athletes was to drink when thirsty, and many marathon runners and other athletes subsisted on a few sips of water during competitions without issue. 

In a brilliant stroke of social engineering, Gatorade spent millions of dollars advertising the dangers of dehydration in magazines, on TV, at races and sports events across the world, and by funding research on dehydration and electrolytes (almost all of which erroneously extolled the benefits of sports drinks), and they still do today. 

For example, if you participated in a marathon in the 1980s, there was a good chance you’d be handed an “educational” pamphlet instructing you to drink at least 40 ounces (1.2 liters) of water per hour of exercise, which for many people would be over a gallon of water during the race.

It’s also no coincidence that the American College of Sports Medicine (ACSM), which instructed athletes to “drink as much as tolerable” during exercise in their 1996 position statement, is largely funded by Gatorade and their affiliated research organization, the Gatorade Institute of Sports Science. In fact, they’re still one of the ACSM’s primary “corporate partners.” 

After a decades-long menticide marketing campaign would make Edward Bernays proud, Gatorade is now synonymous with hydration, and tens of millions of athletes have become conditioned to believe they should drink as much as possible while working out to avoid the perils of dehydration.

And people have died as a result. 

Since the early 1980s there have been hundreds of confirmed cases of fatal water intoxication among soldiers, marathon runners, triathletes, and other athletes. Many others have developed severe symptoms such as cerebral edema (brain swelling), coma, nausea, and fatigue.

As far back as 1986, a study published in the Journal of the American Medical Association noted this disturbing trend, writing “the potential dangers of severe dehydration and the need to drink adequately during prolonged exercise have been so well publicized that, as a result, some individuals may choose to ingest dangerously large volumes during prolonged exercise.”

Not only does overhydration kill far more people than dehydration, it’s also harder to rectify once it sets in. 

The worst irony in all of this is that water intoxication is incredibly easy to avoid: Ignore the marketing babble from companies like Gatorade and misinformed fitness gurus. Just drink as much as you need to quench your thirst, and no more.

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The Best Way to Stay Hydrated

“If you don’t need to remind your body to breathe, you don’t need to remind it to drink.”

— Professor Ross Tucker

There is one good reason to drink before and during workouts: to avoid thirst.

While it’s not clear how dehydrated you have to become to see a dip in performance, the sensation of thirst can throw you off your stride long before you reach this point.

In other words, while dehydration doesn’t directly reduce performance to a meaningful degree, feeling thirsty does. 

Thus, many studies have shown that simply drinking when you’re thirsty is the best way to stay sufficiently hydrated to maximize your performance. Even if you become somewhat dehydrated, so long as you aren’t thirsty, you’ll perform at your best. (On a related note, some new research indicates that the benefits of eating a pre-workout meal are largely thanks to reducing hunger rather than just supplying calories.) 

Even if you become slightly (or very) dehydrated while working out, so long as you drink to thirst after your workout and throughout the rest of the day, you’ll give your body as much water as it needs to replenish your losses.

You don’t need to deliberately force yourself to drink before you’re thirsty, weigh yourself before and after workouts to calibrate your water intake, or follow some convoluted “fueling plan” that has you gulping fluid like a fish. 

Drink when you’re thirsty, don’t drink when you aren’t thirsty, and you’ll be fine.

+ Scientific References