Exercising can be done effectively without prior food intake, as the body utilizes liver glycogen, undigested food, and body fat for energy.

“We now know that during most all forms of exercise unless you're really focused on optimal performance like you've got to hit key lifts or you have to sprint at your maximum speed. And maybe even then that you can exercise faster just fine, because you're mainly relying on sources like glycogen from the liver, some undigested food sometimes, as gross as that may seem it's true, as well as body fat if the exercise about is extremely long.”