Dehydration Myths: How Much Water Should You Drink During Exercise?

Once, I was reading about a guy who was on selection for the Special Boat Service, or SBS, the special forces unit of the Royal Navy.

During the hills phase, he had a special routine he’d follow every night, which went something like this:

Before going to sleep, he’d drink a liter of water from a bottle he kept next to his bed.

About three hours later, he’d wake up to answer a call of nature. Then, on the way back from the toilets, he’d fill his bottle again and drink it before going back to bed.

“My goal was to be always pissing clear.” he says. “That would mean I was fully hydrated. We’d be marching with loaded bergens, but the weight they stipulated wouldn’t include food and water. My routine meant I’d be able to go out with less liquid, which meant gaining a small but perhaps decisive advantage.”

But just how important is it to drink so much water? How much of an impact does dehydration have on your endurance? Should you follow a hydration schedule, where you force yourself to drink a specific amount of water each day?

Let’s find out…

Thirst vs Dehydration

There’s plenty of research to show that if you dehydrate someone, either by putting them in a hot room or giving them diuretics, their endurance will suffer.

However, many of the early studies on dehydration and exercise didn’t distinguish between dehydration (the loss of fluid) and thirst (feeling like you want a drink).

Like hunger, thirst can affect motivation and perception of effort, which in turn can have an adverse effect on performance.

In dehydrated subjects, drinking small amounts of water during exercise has been shown to improve performance, even in the face of continued fluid losses [1].

Put differently, it’s not necessarily dehydration that’s causing the problem, but wanting to have a drink and not being able to.

What’s more, the way some studies were set up, dehydration and thirst weren’t the only things with the potential to affect performance.

In his book Endure: Mind, Body and The Curiously Elastic Limits of Human Performance, Alex Hutchinson describes one of the early studies linking dehydration with a drop in time to exhaustion:

Consider, for example, a 1966 US Army study which found that being dehydrated by 2 percent caused a 22 percent decrease in time to exhaustion. To achieve this level of dehydration, the subjects had first walked to exhaustion on a treadmill, then spent six hours confined to a room at 46 degrees to promote sweating – all before even beginning their exercise test. 

“Other studies have used diuretics to promote dehydration, and most forbid the subjects from drinking during the bout of exercise. It’s not remotely surprising that endurance is reduced under these conditions: in addition to being dehydrated, the subjects are tired, thirsty, and probably pretty annoyed by the whole process.”

In other words, there were lots of other things going on in that study contributing to the dip in performance. Dehydration certainly wasn’t the only factor.

How Weight Loss During Exercise Affects Endurance

Conventional wisdom has it that losing just two percent of your body weight during exercise is enough to impair performance.

Your heart is forced to work harder, your perception of effort goes up, and your body heat rises, increasing the risk of heat stroke.

But don’t wait until you’re thirsty before you start drinking. By that time it’s too late, and you’re already dehydrated.

Or so the theory goes, anyway.

However, in real-world racing conditions, such as marathon, ultramarathon, and ultra-triathlon races, exercise-induced weight loss in excess of two percent doesn’t seem to harm performance.

In fact, several field studies show that as exercise-induced weight loss increases, endurance performance gets better, rather than worse [2]. The fastest finishers in races are often the ones who lose the most weight.

When French researchers weighed 643 runners before and after a marathon, it was the fastest runners that lost the most weight [3]. Only the slower runners kept weight loss below two percent.

“These data are not compatible with laboratory-derived data suggesting that body weight loss greater than 2 percent during exercise impairs athletic performance,” say the authors. 

“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 percent body weight during competition.”

This doesn’t mean that drinking slows you down, or that you should go out of your way to dehydrate yourself.

However, it does suggest that exercise-induced weight loss of two percent or more isn’t necessarily going to harm your performance during endurance events.

Where Does the Weight Lost During Exercise Come From?

Weighing yourself before and after exercise is supposed to help you determine how much fluid you’ve lost, mainly via sweat. The idea is that a change in weight indicates a change in hydration status. The more weight you’ve lost, the more dehydrated you are.

However, not all of the weight lost during exercise comes from fluid. 

In one study, a group of South African Special Forces soldiers marched for 16 miles (25 kilometers) in temperatures reaching a high of 44.3 degrees Celsius. They did so while carrying their backpack, rifle and water supply, weighing a total of 57 pounds (26 kilograms) [4].

While the soldiers were allowed to drink as much water as they wanted, they still lost around four percent of their body weight.

However, not all of that lost weight came from water. Rather, a one kilogram loss in weight was associated with only a 200-gram loss in total body water.

So, where did the lost weight come from?

Some of the weight lost during prolonged endurance exercise comes from the fuel, mainly carbohydrate and fat, used to power that exercise.

When carbohydrate and fat are burned off, or oxidized, the sequence of chemical reactions generates something known as metabolic water, or water of oxidation [5].

The oxidation of one gram of carbohydrate produces around 0.6 milliliters of water, while the oxidation of one gram of fat generates around 1.13 milliliters of water [6]. This “new” water adds to the fluid available to your body, which helps to explain the disconnect between weight loss and fluid loss.

How Drinking to Thirst Affects Endurance

In a review published in the journal Sports Medicine, researchers wanted to find out if endurance athletes perform better when they follow a pre-planned drinking protocol designed to minimize the loss of fluid during exercise, or simply by drinking when they’re thirsty [7].

After analyzing the results of eight studies, the researchers found no major difference in exercise performance between the two drinking strategies.

Here’s how they sum up their findings:

“Despite drinking to thirst being associated with an hourly rate of fluid consumption half as much as programmed drinking, and resulting in a dehydration level considered sufficient to impair endurance performance, both strategies were found to similarly impact 1–2 hour cycling or running performances conducted at moderate to high intensity and under temperate to warm ambient conditions.”

This doesn’t mean drinking to thirst is the way to go for all people, all of the time, and there are situations where planning your drinking is a better alternative. 

Aging and cold exposure, for example, can attenuate your sensation of thirst [8, 9], which means you can’t necessarily rely on thirst to tell you when it’s time to drink.

If you’re training or racing with limited access to water, or there’s a big gap between water stations, and you’re not able to carry water between those stations, then some kind of a drinking plan is likely to be beneficial.

Final Thoughts

Ignoring thirst and not drinking anything is going to harm your performance sooner or later. However, most people don’t need to worry about hydration schedules, or force themselves to drink a specific amount of water each day. 

Unless you’re an elite athlete trying to push yourself beyond all known limits of human performance, or doing a lot of exercise in very hot or humid conditions, chances are you’ll do just fine by drinking when you’re thirsty.