Drinking water helps during exercise.

Water is essential to the body during exercise: It dissolves electrolytes—minerals that include sodium, potassium, and magnesium—and distributes them throughout the body, where their electrical energy triggers muscle contractions required for movement, Jampolis explains. An electrolyte imbalance can lead to cramping, but that's not the only side effect of drinking too little.

"When muscle cells are dehydrated, they break down protein (aka muscle) more quickly and build muscle more slowly, so your workouts are much less effective," she says.

What's more, the body loses fluids more quickly during exercise because it generates heat that's shunted to the skin's surface, where perspiration and subsequent evaporation (a cooling process) help with temperature regulation.

Staying properly hydrated also helps maintain your blood's volume, so you can optimize the expansion of blood vessels at the skin's surface to release heat, Jampolis says.

"If your body can't dump excess heat via sweating, you're setting yourself up for heat exhaustion or worse," she says. "Being adequately hydrated can improve your workouts by decreasing fatigue, which can allow you to work out longer and burn more calories." That's why it's so important to hydrate before and throughout your workout, not just when you start to feel thirsty.  source Johns Hopkins University