How much water should you drink each day? (A science-based guide to hydration)

I’m asked all the time about water. People want to know how much they should drink to prevent dehydration.

Dogma holds that you should drink at least eight ounces of water eight times a day. The 8×8 method is simple to understand and easy to remember. Unfortunately, this homespun wisdom isn’t well-suited to the complexity of human life and drastically oversimplified. There is no one-size-fits-all for nutrition.

As with most molecules you consume, there’s a sweet spot for H2O. Drink too little and you’ll experience a low-water state called hypohydration. But drink too much and you’ll dilute blood sodium levels, known as hyponatremia, which is even worse.

Yes, low sodium levels—and the associated muscle cramps, fatigue, and neurological symptoms—is an underappreciated problem. Athletes, in particular, are urged (by one camp, at least) to guzzle water before, during, and after events. In many cases, these overwatered situations end poorly.

The other camp is the “drink to thirst” camp. I’m more or less in this camp. But that doesn’t mean I’ll leave you with this soundbite, sign off, and spend the rest of the day with my family.

There’s more to hydration than drink when thirsty. It’s good advice, but it doesn’t apply everywhere. Plus it says nothing of electrolytes, which work together with water to balance fluid levels in your body.

Here’s the game plan. You’ll spend about six minutes reading this post, absorb some practical knowledge, and then be better equipped to make smart hydration decisions. Let’s do it.

Water Is Life

When scanning the cold reaches of space, astronomers look for signs of water. Where there’s water, there’s life.

When you were a child, you were about 75% water by mass. Assuming you’re an adult now, that number is closer to 60%. All that H2O isn’t there for show. It supports nearly all bodily functions, including:

• Thermoregulation (body temperature control)
• Promoting blood flow in blood vessels
• Transporting nutrients through the gut and into the bloodstream
• Carrying waste out of the body
• Acting as a medium for most biochemical reactions (energy production, etc.) in the body

So yes, having adequate water in your body is important. Water is necessary for life.

Hydration and Thirst

Hydration can simply mean drinking water, but I like another definition better. Hydration, according to this definition, is “the quality or state of being hydrated (especially: the condition of having adequate fluid in the body tissues)”.

The operative term here is adequate fluid in the body tissues. This is your goldilocks level of hydration. Not too little H2O, but not much either. Just right.

Achieving your goldilocks hydration doesn’t usually require calculation. Most of the time, you can rely on a built-in mechanism: thirst.

Thirst is a tightly-regulated system. Controlling this ancient urge are a slew of physiological triggers outside your conscious control. These thirst-inducing triggers include:

• Rising plasma osmolality (more electrolytes per unit of blood)
• Elevated angiotensin (a hormone that increases blood pressure)
• Reduced blood volume (less water available, less blood volume)

As a general rule, thirst is a good indicator of hydration status, but there are a few possible exceptions. I’ll cover those later. First I want to address dehydration.

Is Dehydration Really That Dangerous?

While visiting a hospital in 2017, I was astonished at the nutrition team’s anti-obesity strategy. It wasn’t focused on food quality. It was focused on preventing dehydration.

To be fair, there’s test tube data suggesting chronic dehydration interferes with insulin signaling, which wouldn’t be good for a type 2 diabetic. Problem is, this hasn’t been shown in humans. I would rather see more established interventions for diabetes, like low-carb diets and intermittent fasting.

But dehydration isn’t just slammed for causing diabetes. It’s also said to negatively impact mood, mental performance, endurance, and heat tolerance. The thing is, the evidence for these negative effects is weak at best.

Take the oft-cited link between dehydration and heat stroke. About 700 Americans die every year from heat stroke, and it’s often claimed these tragedies were driven by hydration issues.

There are many problems with this assumption. First, those who die of heat stroke tend to be infants and senior citizens, both populations that can’t efficiently cool themselves with sweat. Drinking more water wouldn’t solve this issue.

Also, it’s not clear that dehydration raises core body temperature—a prerequisite for heat stroke. In one study, athletes stopped exercising at a lower core body temp in a dehydrated (vs. hydrated) state. This doesn’t align with the heat stroke hypothesis.

Finally, if any group should pay dearly for dehydration, it’s sweaty athletes. But according to a review on MDAlert.com: “There seems to not be a single case of death resulting from sports-related dehydration in the medical literature.” Not a single case.

Many cases exist, however, on the flip side: overhydration.

Overhydration

Back in the 1980s, the US military advised troops to consume up to 1.8 liters of water per hour. These guidelines were later revised downwards after a slew of overwatering cases occurred.

In one unfortunate case, an army trainee was believed to be dehydrated, but really had extremely low blood sodium levels—a condition called hyponatremia. The prescribed treatment for his condition (more water) worsened his hyponatremia, and eventually proved fatal.

Hyponatremia gets less press than dehydration (perhaps because sodium has been unfairly thrown under the proverbial health bus…not unlike saturated fat and sunlight…topics for another day), but it’s arguably more dangerous. I’m trying to change the conversation here. Symptoms of hyponatremia progress from muscle cramps to fatigue to brain damage—and severe cases can be lethal.

I’m especially keen on reaching active folks with this message. Many athletes follow a set schedule of fluid consumption—loading up on water in close proximity to exercise—rather than drinking to thirst.

This has proven to be a recipe for exercise-associated hyponatremia. About 15% of endurance athletes may suffer from it! The problem has become so widespread that the International Marathon Medical Directors Association now recommends that water stops during a race be spaced at least 1.6 km apart. Fewer water stops, lower chance of overwatering.

So should everyone just drink to thirst? Well, this strategy is usually sufficient, but the truth is: Active people training in hot climates—along with some other populations—may need a more nuanced hydration strategy.

When A Hydration Strategy Is Necessary

The human body is an imperfect machine. Signals can get crossed, and “listening to your body” isn’t always the best advice.

At high elevations, for instance, the thirst mechanism doesn’t work per usual. You might need more water than you think.

Hot climates are another example. So much sodium and water are lost through sweat that some prehydration (before thirst sets in) might be wise. Long distance swimmers in particular should have a hydration strategy. Less opportunities to take a swig.

As a final example, those on low-carb diets have increased hydration requirements. Why? Because low-carb diets keep the hormone insulin low, and low insulin tells the kidneys to excrete more fluid and sodium. Believe it or not, this is a primary cause of the dreaded keto flu.

Read more: What causes the keto flu? (And 6 home remedies)

Let’s get to the practical stuff now. A good hydration strategy isn’t just about drinking water when thirsty. It’s also about optimizing electrolytes, especially sodium.

Increasing Sodium To Stay Hydrated

Let me take you with me to my jiu-jitsu gym several years ago. I was training several hours per day, rolling in a warm room against guys decades my junior, and sweating like Niagara Falls. My idea of fun, in other words.

Still, my energy was off and I wasn’t recovering well. Even off the mat, I didn’t feel 100% sharp.

I didn’t suspect electrolyte issues. I was supplementing them, albeit in limited amounts. Finally, my former coaches helped me diagnose the problem: I was low on electrolytes. Specifically, I was low on sodium. When I used an electrolyte mix to bump my sodium intake to about 4–6 grams per day, the problem resolved.

Other trainers might have suggested I drink more water, but drinking more water would have made my problem worse. I would have continued lowering my sodium levels, possibly bridging into clinical hyponatremia.

Let’s go beyond my personal anecdote now and see how sodium helps athletes in the literature. The evidence here is pretty convincing. In a 2014 randomized controlled trial, researchers were able to reverse exercise-associated hyponatremia by giving runners a saline solution (salt water) after a long race.

So there’s your strategy. Replace water and salt after exercise, not just water.

How Much Water Should You Drink?

You’re supposed to drink eight glasses of water a day, thirst be damned. This recommendation is totally arbitrary and fails to appreciate the variability between each person, but it’s nonetheless fixed in the public consciousness.

Hydration isn’t that simple though. Here are some nuances we just covered:

• Hydration means maintaining “adequate fluid” in your body—super important for all aspects of physiology.
• Thirst is usually a good indicator of water needs, but there are exceptions. (Training in heat, for instance).
• Dehydration isn’t the health crisis it’s alleged to be. Heat stroke is caused by heat, not dehydration.
• The larger problem is overhydration, which can cause dangerously low sodium levels.
• Compared to the general population, low-carb and active folks have increased hydration needs.
• Staying hydrated means consuming adequate water and electrolytes, especially sodium.

To sum up: There’s no set amount of water you should drink a day. Experiment, drink to thirst, get enough sodium, and see how you perform. See you back here soon.