Cool Off
Research shows that endurance athletes can go harder, longer by dropping their body's core temperature before or even during exercise.⁵ It makes sense: The cooler your core body temperature when you start a run, the more your temp will have to rise before you start to overheat.

One simple way to cool your body while hydrating, is to reach for cold beverages. Try making your pre-workout drink a cold one or putting ice in your water bottle or hydration bladder.

Bring Along Extra Fluids
With increased sweat rates, you likely need to drink more during your marathon training runs than you usually do. Expect to start sipping earlier and more frequently. Also, even if you can usually get through short or middle-distance runs with only water during and after them, you may need more than that now.

A scientifically designed rehydration beverage can help you keep your fluid and hydration levels up.

To accommodate your increased fluid and electrolyte needs, consider wearing a (larger) hydration pack.

Track All Things Heat and Hydration
Use your training app or diary to outline not just your runs, but also the heat index, how much (and what) you drank and how you felt during and after your run, Williams said. You can use that information to pinpoint the hydration strategy that's best for you.

FYI, the heat index is a combination of the air temperature and humidity. It's the "feels like" temperature listed in your weather app.

Weigh Yourself
Mild dehydration, consisting of losing at least 2% of your body weight in water, can negatively affect your run performance and result in symptoms of dehydration. So, if you weigh 160 pounds, your goal is to drink enough that you weigh no less than 156.8 pounds at the end of your run. (Every pound you lose during your workouts signals 16 ounces of water gone, Williams said.)

For the most accurate measurements, weigh yourself naked before and after your training runs. After runs, your clothes will be weighed down with sweat.

Download this infographic for hydration tips on conducting a "sweat test."

Go the "Extra Mile"
You don't stop losing sweat the second you cross the finish line. So, to fully replace any sweat losses, you need to drink more than you think within 90 minutes of your workout.

"It's recommended to drink 1.25 to 1.5 times the fluid volume that you lost," Williams said, adding, "For every pound (16 ounces) you lose in body weight, you should drink 20 to 24 fluid ounces of fluid."

Want to learn more electrolytes and hydration? Check out this story.

References
¹Baker LB and Jeukendrup AE. Optimal composition of fluid-replacement beverages. Compr Physiol 2014;4:575-620.
²Chapelle L. Pre-exercise hypohydration prevalence in soccer players: A quantitative systematic review. Eur J Sport Sci 2020;20:744-755.
³Arnaoutis G et al. Fluid balance during training in elite young athletes of different sports. J Strength Cond Res 2015;29:3447-3452.
⁴McDermott BP et al. National Athletic Trainer's Association Position Statement: Fluid replacement for the physically active. J Athl Train 2017;52:877-895.
⁵Sawka MN et al. Exercise and fluid replacement. American College of Sports Medicine Position Stand. Med Sci Sports Exerc 2007;377-390.