I run six to nine kilometers every morning, usually on just a cup of coffee. Friends are always surprised that I'd put by body through that, but this pattern — known as a fasted workout or fasted cardio — has never hindered my workout, and may actually be beneficial.
"When insulin levels are low, your body has no choice but to burn fat. Naturally this happens faster when you haven't eaten," said ACE-certified personal trainer Lisa Niren, head instructor at Studio and a founding instructor at Peloton in New York City. "I do fasted cardio daily, and I've seen first hand that my body uses fat stores for energy rather than the glucose in my system, which is what happens if you eat prior to the workout session."
In order to be in a truly fasted state, you'd have to go without food for eight hours, which is why morning workouts are so effective.
To make the most of your workout, Niren suggests high-intensity interval training. "HIIT is the most efficient type of cardiovascular exercise to accompany fasting, which will yield the best results in the shortest amount of time," she said. "This type of cardio uses your body's twitch muscles, which have tremendous metabolism and hormonal benefits, meaning you will lose weight faster."
You should know, though, that fasted cardio is not for everyone. Both Niren and Kellilyn Fierras, MS, a registered dietitian, NASM-certified personal trainer, and instructor at EverybodyFights in Boston, said you should listen to your body.
"Some people may feel fine on fasted cardio and some (myself included) may feel super-tired, which will likely make their workout suffer," Fierras told POPSUGAR. "Eating before exercise is meant to increase one's performance and endurance, keep blood sugars stable, stimulate mental clarity, and delay fatigue. However, if you find you can work out and function properly on an empty stomach, it's essential that you properly refuel post-workout."