Skipping breakfast before low-intensity runs or rides is smart because it forces you to use fat for fuel, making you metabolically flexible, says Javier Gonzalez, Ph.D., senior lecturer of human physiology at the University of Bath in England, who studies fasted exercise.
But you should never go into a strength or HIIT workout without eating beforehand, he notes. In an anaerobic state, it's harder for you to tap into fat stores. Instead, you need quick-burning carbs to have enough energy for heavy lifts or plyometrics. The same is true if you’re looking to PR in a race rather than heading out for steady-state miles.
The bottom line:
Before heavy lifts, HIIT sessions, or a race, Gonzalez recommends fueling with a small snack that’s low in fiber and fat, such as Greek yogurt or a banana.