How to Stay Hydrated During Exercise

How important is staying hydrated during exercise? If you're lugging around a fanny pack water bladder or sports beverages, you may be doing it wrong.

Brock Armstrong
6-minute read
Episode #456

When I first started running semi-seriously, I diligently carried a water bottle on my belt, as instructed by all the running magazines and websites. Then, as my (ahem) thirst for the sport grew, I graduated to an embarrassing fanny-pack-style beverage bladder. But, oddly enough, by the time I achieved the level of having a shoe sponsorship and coaches, I wasn't carrying anything with me at all. A few sips at random water fountains or the on-course aid stations worked just fine.

There's a reason I tell that story. For many years, the rule-of-thumb for exercise hydration has been that if you sweat out more than two percent of your body mass, your performance will decline. That's based on military research meant to prepare U.S. soldiers for hot climate combat during the Second World War. In practical terms, the rule means that a man who weighs 80 kilograms (176 pounds) can only sweat out 1.75 litres (about 60 fluid ounces) of sweat before he will start to fall apart performance-wise.

Depending on how hard you exercise, and also how hot it is where you're exercising, even sipping from an electrolyte-laced water bottle may not stave off that two percent water loss.

The problem is that depending on how hard you exercise, and also how hot it is where you're exercising, you could potentially drip that much onto the gym floor in about an hour. And even sipping from an electrolyte-laced water bottle may not stave off that two percent loss. Studies have shown that in more jiggly types of activities (like running), athletes only manage to replace about half of their fluid. If they attempt more than that, they run the risk of getting a severe side stitch and having to assume the doubled-over, hand on a hip stance that many of us know so well.

A French researcher took 643 runners and weighed them before and after a marathon. The researcher found that only the slowest folks in the race managed to keep their fluid losses below two  percent. Many eyebrows raised in an "I told ya so" fashion.

Then another study found that the star Ethiopian runner, Haile Gebrselassie, often lost about 10 percent of his body weight during his record-setting marathons. That rule-of-thumb about sweating more than two percent of your body mass was officially retired. In the sports science field, anyway. The sports beverage industry, on the other hand, kept right on beating that drum.

Star Ethiopian runner, Haile Gebrselassie, often lost about 10 percent of his body weight during his record-setting marathons.

In his 2012 article for the Globe and Mail, Alex Hutchinson quoted Dr. Trent Stellingwerff, a physiologist at the Canadian Sport Institute in Victoria, saying "Anyone who has worked in the field with athletes has probably realized years ago that a strict two-percent dehydration cut-off just doesn't work." Stellingwerff went on to say that he usually aims for a three to six percent dehydration, depending on the conditions (how hot or cold it is) and individual tolerance. It is quite clear that some people handle hot weather better than others and some people, like me, simply sweat more.

So, how did the two percent rule go so wrong? The problem with the studies that lead to that rule is that the researchers had deliberately put the subjects into a state of dehydration using saunas and diuretics before asking them to go exercise. I am sure you can imagine that there's a big difference between being forcibly dehydrated through extreme means and simply being thirsty after a hard workout. I wouldn't perform very well either if I was abused like that and then asked to run a fast kilometer.

Drink to Thirst

Despite the best efforts of the sports drink industry, a few years ago a new measurement hit the community. And that was to simply "drink to thirst." Led in no small part by professor and author Tim Noakes. He stated way back in his 2010 paper that "Drinking ad libitum appears to optimize performance and safety during exercise in many situations. The presence of thirst, not of water loss, may be the biological signal that impairs exercise performance in those who drink less than their thirst dictates during exercise."

In what I would call a definitive 2018 study, researchers had athletes complete a 20K trail run and allowed them to either drink a specifically measured amount of liquid that would replace their (expected) sweat loss or just drink when they felt thirsty. At the end of the race, the finishing times of these two drinking strategies were identical despite the fact that the "drink to thirst" runners lost 2.6 percent of their body weight, on average.

Stay hydrated but don't worry

The lesson I take from all of this is to simply not freak out about staying hydrated. Sure, if you're in extreme conditions (hot, humid, and high intensity) staying on top of your hydration is prudent. Just make sure you stay aware of your internal cues and you will be fine.

The lesson that I take from all of this is to simply not freak out about staying hydrated.

As in everyday life, it's not an emergency to be temporarily dehydrated (no fannypack-style hydration belts necessary). If our human meat sacks were indeed that sensitive, and we were that weak of a species, we would never have survived long enough to leave Africa 315,000 years ago.

When to use sports drinks

Let me offer one caveat. Just because it's not an emergency to get a little dehydrated doesn't mean we should completely shun all the modern science that has gone into our understanding of fuelling during a race or hard workout. Let's look at what to use and when.

The term "tonicity" is used a lot concerning sports drinks. Tonicity is a measurement that is used to compare the thickness of one liquid to another. The tonicity of a sports drink can affect how your body absorbs it. In the sports hydration world, we are comparing the tonicity of a beverage to that of human blood, which has an osmolarity of ~285 to 295mOsm/kg.

Tonicity is a measurement that is used to compare the thickness of one liquid to another.

The point of this measurement is to determine if a drink is either:

  • hypertonic (higher concentration),
  • isotonic (the same concentration),
  • hypotonic (lower concentration than blood).

The difference between these three measurements tells us how quickly your body will absorb the beverage into your bloodstream. This is a determining factor in not only how well a beverage will hydrate you but also in how well it will replenish your energy stores.

Let's look at each one of these concentrations.


Hypertonic drinks contain a lot of carbohydrates, making them a higher concentration than blood. This means they're good for recovery and replenishing lost calories. Most recovery drinks fall into this category and often have protein added into the mix as well.

Hypertonic beverages are a good choice for energy recovery but aren't as great when dehydration is a focus. When you consume a hypertonic beverage, the fluid found in your intestine also becomes hypertonic. That means that your body needs to move water out of the bloodstream back into the intestine to allow for absorption of the nutrients through the gut wall. So it is best to stick to hypertonic drinks for quick calorie delivery, not rehydration.


Most sports drinks you find in the convenience store are isotonic, or similar to our blood. These are typically considered helpful during short-duration and high-intensity workouts or events. Isotonic beverages can contain six to eight percent carbohydrate, which means they can deliver energy into your body quite rapidly.

But, isotonic beverages can upset your stomach, especially when taken in large amounts in a short amount of time. This is especially true in hot conditions when your blood supply is focussed on keeping your skin cool and your muscles moving, not concentrated on your gut.


Hypotonic drinks tend to be absorbed into the bloodstream quickly because they have a lower concentration than blood. This is due in part to them usually only containing a six percent or lower solution of carbohydrate. That makes them ideal for longer endurance events and overall hydration.

Hypotonic beverages are often referred to as having a favorable osmotic gradient—the water in the beverage can flow easily through the gut wall and into blood vessels. If you think back to your grade school biology class, you may remember that osmosis is moving from an area of lower concentration (in this case our gut) to an area of higher concentration (our blood). See? Biology does come in handy in your grown-up life!

Which is best?

As you now know, hydration isn't an emergency. And if you have read previous articles, you know that chomping down on protein-rich food or popping amino acids during a workout or immediately after is also not a huge priority. Also, unless you are planning to crush a second workout later on the same day, refilling your carbohydrate stores can also wait until your next regular meal. For the most part, having some water when you feel thirsty and eating a healthy, regularly sceheduled meal is all you need to do.

Hydration isn't an emergency.

Just remember, that rule goes out the window during very long events, in hot conditions, or when you've skipped a meal (doing your workout in a fasted state).

Here's my advice. If you're just out for a regular training session or a short race (especially in temperate conditions), you probably don't have to pound a bunch of sugar, coloring, and additives to maximize your performance. Simply sipping from your bottle of water is fine. But if you're planning to go hard for several hours (especially in hot conditions) and you really want to perform at your absolute best, then choosing the appropriate beverage could be in your best hydration interest.


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All content here is for informational purposes only. This content does not replace the professional judgment of your own health provider. Please consult a licensed health professional for all individual questions and issues.

About the Author

Brock Armstrong Get-Fit Guy

Brock Armstrong was the host of the Get-Fit Guy podcast between 2017 and 2021. He is a certified AFLCA Group Fitness Leader with a designation in Portable Equipment, NCCP and CAC Triathlon Coach, and a TnT certified run coach. He is also on the board of advisors for the Primal Health Coach Institute and a guest faculty member of the Human Potential Institute.