Dehydration and Performance: The Right Way To Rehydrate When Running

While it’s not quite feeling like spring yet in most of the country, one concern that will surely be on many runners’ minds once it does is hydration.

Over the winter, it’s easy to get lulled into drinking minimal amounts of water, and once the temperature heats up, your first few runs in the heat can entail a rough reminder that drinking enough water is important to your performance.

This week, we’ll take a look at what the scientific literature has to say about staying hydrated while running and the kind of performance deficits you can expect when your body has less-than-optimal water reserves.

Dehydration and running

It’s certainly clear that being dehydrated can hurt your ability to run.

This was proved rather bluntly in a 1985 study by Lawrence Armstrong, David Costill, and William Fink. Using Lasix drugs, powerful agents which artificially induce dehydration by increasing urine production, the researchers examined the effects of dehydration on the performance of eight male runners over 1500m, 5000m, and 10,000m. Not surprisingly, the performance of the runners degraded significantly.

The results indicate that runners slowed nearly 80 seconds during a 5k and lost 2 minutes and 40 seconds over a 10k due to dehydration.

For half marathoners and marathoners, this data also makes it clear that hydration becomes more important as the race distance increases.

While Armstrong, Costill, and Fink measured blood plasma volume and urine output, the best predictor of each individual subject’s performance drop was how much their body weight dropped.

The average amount of weight lost due to diuretic-induced dehydration was around 2% of the subjects’ total body weight, or three pounds for your average 150 lb runner.

The truth behind the math

But, while measuring body weight is an often-recommended strategy for calculating your own water losses during exercise, it can be misleading in the real world, as R. J. Maughan and S. M. Shirreffs point out in a 2010 paper.

As they describe, changes in body weight reflect not only water loss due to sweating and respiration, but also the loss of fuel—just like your car gets lighter as the gas tank empties, your body weight drops as you burn carbohydrates and fats while you are out running.

Additionally, some water is generated by burning calories during exercise when the oxygen in the air combines with the carbs or fats in your muscle cells. So not all the water and weight you lose comes from sweat!

While these fuel-related losses only represent about a pound or so of weight during a long run or workout, it can muck up body weight and water loss calculations, causing people to overestimate how much water they need to stay hydrated.

This is why it’s important to use a tool like our free sweat loss calculator for runners that takes these factors into account when prescribing how much you need to drink to prevent dehydration.

Replacing electrolytes

It’s also crucial to remember that you should rehydrate with electrolytes, especially sodium, rather than just plain water.

In our in-depth post on the science of electrolytes for runners we dig into the research about why electrolytes are important. But, the brief version is that electrolytes help maintain your body’s fluid balance and contribute to many essential functions such as the movement of fluids and nutrients across your cells’ membranes, thus allowing them to carry on their metabolic activities such as contacting muscles.

So, how to replace?

My recommendation is to add electrolytes to the water you’re consuming throughout the day and during your workouts to maintain optimal electrolyte balance.

Of course, you can always opt for electrolyte drinks like Gatorade and Powerade, but those often contain high amounts of simple sugar. When you’re replenishing electrolytes throughout the day, you don’t want to be consuming high amounts of simple and artificial sugars.

Moreover, according to researches like Noakes, most sports beverages don’t contain nearly enough sodium to replenish what a runner needs.

That is why we recommend using a product specifically designed for endurance athletes like LMNT Recharge.

Specialized products like LMNT recharge have higher amounts of sodium to specifically meet the demands of runners. Plus, they don’t contain sugar, making them much healthier overall.

If you want to give them a try, they have a special offer where you can get an 8 pack sample for free. Check it out here.

Final tips

Through experience, you can find the ideal rehydrating strategy for your workouts, but using our sweat loss calculator will give you a good head start on what numbers you should be looking at.

If you struggle with carrying bottles, doing a loop course or dropping off a water bottle in a car before you run will allow you to strategically time your rehydration so you can get through your training feeling good.

Finally, don’t forget to add electrolytes to your water to make sure you’re maintaining optimal balance.