How Much Water Are You Supposed To Drink A Day?

Fountain of Youth

You’ve heard it numerous times; drink more water! You probably made it your New Year’s resolution. And why wouldn’t you? Staying hydrated promotes weight loss, detoxifies your body, minimizes headaches, aids digestion, and increases energy.

At its most fundamental, water hydrates. Those hydration levels are the measurements our body’s water reserves. Conversely, a lack of hydration is indicative of low water reserves.

Dehydration happens for several reasons, two of the more prominent being; lack of water, or a state of diuresis. The latter happens when you consume diuretic substances like coffee, tea, alcohol. Although not always in liquid form, diuretics cause more frequent urination, which leads to dehydration.

With so much to offer, there’s no reason to ignore water’s miracle like benefits. Just one issue though; how much water is enough for “proper” hydration?

Eight by Eight and Other Nonsense

There are many different recommendations to go by. But more isn’t always better. The suggestions range from eight eight-ounce cups to the overly simplistic; “until your urine isn’t yellow.” But no matter which number you arrive at, chances are it’s the wrong one.

Why? Because there’s no “magic number.” Here’s the simplest explanation; when’s the last time you came across a figure that explained how every human – accounting for idiosyncratic factors; age, genes, and physical environment, to name a few – should operate? There can’t be one recommendation for so many different people.

Luckily, research backs up the common sense reasoning. Specifically, a 2010 University of Lausanne study1 examined the validity of “new recommendations with regard to water requirements.” The key takeaway? We can’t set recommended targets, instead, we should try daily minimums.

The study noted that “the regulation of water balance is very precise,” and that “healthy adults regulate water balance with precision.” More importantly, the research suggests that setting a target number, or even a minimum number, may cause an adverse effect.

Recommending any “magic number” may lead to dehydration since you may need different fluid amounts day to day. Those variations depend on the amount of activity, weather, and other factors.

What can you take away from the study? Two things; your body is adept at regulating water, and that there are too many outside (and internal) variables to recommend a daily intake number.

The study concludes that no recommended average can be recommended for humans overall. However, even though a minimum is also unsatisfactory, the researchers set an absolute threshold of 1.5L of water a day.

You Know What You’re Doing

The University of Lausanne study made several noteworthy points. Specifically; your body is very adept when it comes to internal water balance regulation! In other words, you regulate your hydration well enough to not worry about chronic dehydration. The notion is mirrored in other research.

A 2005 US Army Research Institute of Environmental Medicine study2 found that “humans regulate daily water balance remarkably well across their lifespan despite changes in biological development.” The study noted that “day-to-day hydration is generally well maintained so long as food and fluid are readily available.

However, the researchers’ distinction – availability of fluid and food –  is worth noting. Specifically, it suggests that an “acute or chronic water deficit, whether from strenuous physical exercise or heat stress,” requires excess water intake. More so than usual.

How much? 3.7 liters for men, and 2.7 liters for women “meet the needs of the vast majority of persons.” Further, the water intake may include any source; drinking water, water in beverages, or water in food.

But there’s an even bigger takeaway from both studies; the first study suggests 1.5 liters, while the second suggests almost double. The inconsistency is troubling but explanatory.

Why There’s no Magic Number

First, it’s worth remembering that these studies aren’t conducted under a singular methodology. In fact, what’s considered as “strenuous physical exercise or heat stress” in the 2005 study, may not qualify as an “outside variable” under the 2010 study’s standards.

Second, recommendations differ because “despite its critical importance in health and nutrition, water intake – along with its rational recommendations for populations – is minimally researched compared to most other nutrients.”3

Hence, a 2005 Institute of Medicine of the National Academy of Science Report on water intake acknowledged4 a “limited database upon which to express Estimated Average Requirements for water for different population groups.”

Subsequently, “while it might appear useful to provide an EAR (Estimated Average Requirement) for water, given the extreme variability in water needs, the data does not allow for such recommendations.”

A clear picture of the problem emerges. Given the extremely variable profile of the human population – affected by different internal and external factors – there is no single level of water that would uniformly ensure adequate hydration.

In turn, any recommended numbers tend to greatly differ from one another as they test different sets of the population and use different methodologies. Ultimately, the search for a “number” is futile.

Dehydration and Exercise

One thing we do know for sure, however, dehydration plays a huge role when it comes to physical exercise. Unlike its relatively little-known effect on the general population, its effect on exercise is well documented.4 5 6

In fact, a 2007 study7 found that “during challenging athletic events, it is not uncommon for athletes to lose 6–10% of body weight in sweat loss, thus leading to dehydration if fluids have not been replenished.”

Further, a seminal University of Texas study8 showed that even under milder physical activity, individuals may still “experience decrements in performance to reduced endurance, increased fatigue, altered thermoregulatory capability, reduced motivation, and increased perceived effort.”

The findings were mirrored – to a minor effect – in a later study9 by the US Army Research Institute of Environmental Medicine. The research indicated that “dehydration alters cardiovascular, thermoregulatory, central nervous system, and metabolic functions. One or more of these alterations will degrade endurance exercise performance when dehydration exceeds 2% of body weight.”

You’re Thirsty

Luckily, dehydration during physical exercise isn’t totally debilitating. Frequent rehydration can reverse the aforementioned side effects “and also reduce oxidative stress.”10 But it’s important not to rely on thirst alone as a signal for more needed water.

As one study11 has found, “individuals may not hydrate adequately when allowed to drink according to thirst.” Hence, relying on voluntary fluid intake may not be enough during physical exercise.

So do you Have to Chug Water?

Does it have to be water exclusively? What about fluids from food and other beverages, do those count?

Unfortunately, most research relays “no information to allow us to assume that consumption of water alone or through beverages containing water affects hydration any differently.”12 13

Though there is some data that suggests that water might have different metabolic effects “when consumed as water alone rather than water contained in caffeinated or flavored or sweetened beverages.”14 15 However, the data is “inconclusive” and requires more research.

Did you get all that? Not only do we not know, or ever really can know how much you should drink, we don’t even know if all liquids hydrate you the same way. So don’t feel guilty as you reach for a sugary beverage.

How to Avoid Dehydration

The research may be inconclusive, but anecdotal evidence and common sense save the day. Here are some quick tips;

  • Drink plenty of water before, while, and after any physical activity. Increase your intake in hotter environments during more intense exercise. Don’t stress numbers! Just make sure to hydrate throughout the exercise. At least every 20 minutes.
  • If you’re on a high protein diet, increase your intake.
    • By the way, check out all of protein’s amazing benefits in our brief explainer!
  • Avoid diuretics like alcohol, tea, and coffee, which exacerbate dehydration!
  • Drink more water if your diet is high in sodium.


Leaving with more questions than you came with? Don’t. It’s simple, we don’t really know how much water you need. Nor should we, because there is no single “you.”

To issue one standard number, would assume that there is one standard human, with the same exact genes, idiosyncratic internal health variables, and identical outside living environments.

As you know too well, that’s not the case. So don’t aim for an exact figure, instead stick to a combination of listening to your body and following common sense guidelines!


1 Comment

  1. I personally value you discussing this here. I have been doing study on my own and I’m glad I’ve got this to add to my set.

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