Replace Lost Fluids
In hot weather, your body self-regulates its core temperature through a variety of means. Heat is moved from muscles and released through the skin by convection. Heat also leaves the body in water vapor as a runner exhales and is dispersed via sweat, a process that cools the skin and releases heat during evaporation.
But each of those processes works better when you’re optimally hydrated. The fluids you ingest before and during a race—both in the form of a carbohydrate/electrolyte beverage and water—act as conductants that help transport heat. The more fluid or water you have in your body, the more heat you can remove. If you get dehydrated—and thus have less fluid in your body—you limit the body’s ability to remove heat and thus limit your performance.
The key to maintaining performance in hot conditions is sufficiently replacing the fluids and electrolytes you lose while you’re running. Although there are recommended guidelines for consumer liquids during a run or race—roughly 4-6 oz. of carbohydrate/electrolyte beverage every 20 minutes or so—the amount of fluid needed varies greatly among runners.
Knowing your sweat rate is one way of estimating how much fluid you’re losing at race pace under those temperature conditions. If you have a high sweat rate, you might need to replace electrolytes more often than you need to take in carbohydrates. Practicing your fluid replacement strategy ahead of time is important to maximizing performance in warm and hot conditions, especially because the body can more quickly absorb electrolytes if they are consumed without carbohydrates.