Q: I’ve heard “fasted cardio,” exercising on an empty stomach right when you wake up, results in more calorie burn. Is it true?
A: Exercising on an empty stomach is a topic not all experts agree on, and the science on fasted cardio is not totally clear. (Nothing is simple, I know!)
However, there’s a lot of evidence that you can use to determine if the approach is right for your body.
First, you’ve got to understand how food fuels sweat sessions. Carbs are the body’s preferred source of fuel for energy (while protein is used for muscle growth and repair). The body first turns carbs into glucose, which your body can use immediately as fuel. Then, it stores any excess as glycogen. During exercise, when no glucose is available (meaning you haven’t just eaten), it turns to the stored glycogen for fuel, first.
When the glycogen is gone, it turns to fat. (This isn’t a perfectly linear process. At lower workout intensities, it may use more fat for fuel, hence the famous “fat burning zone.” Using more fat for fuel doesn’t necessarily lead to more fat loss, though, since calorie burn is so much lower when you’re working out at that intensity.)
The Argument for Fasted Cardio
Proponents of fasted cardio argue that if you exercise right when you wake up, before eating, your glycogen supply will be low, since you haven’t eaten since dinner the night before (unless you had a midnight cupcake…whoops!). Without enough glycogen for fuel, they say, the body will start burning fat sooner, since it’ll need it to get you through the workout.
Some small studies support this, like this one from 2013 that showed exercisers who skipped breakfast burned 20 percent more body fat (that day) than those who ate before hopping on the treadmill.
However, a 2014 study in the Journal of the International Society of Sports Nutrition looked at the results of fasted cardio over a much longer period of time. After a month, researchers found no difference in body composition (in terms of weight, body fat percentage, waist measurements, etc.) between members of groups that ate before exercising and those that ran on empty.
Fasted cardio doubters also argue that there’s no way to know when your glycogen stores run out, so even if you run in the early morning without eating, you may have enough left from the night before, meaning you won’t burn more fat at all.
And depending on how long and intense your workouts are, you may find your performance suffers if you don’t fuel beforehand. If you’re running 12 miles to train for a race, for example, or lifting heavy weights, your body will likely need the fuel to keep you moving.
The bottom line
Like with many things, I recommend experimenting to see what feels good for your body. If you wake up and feel great doing 45 minutes of cardio on an empty stomach and are seeing healthy weight-loss results, keep it up. Just make sure you get some protein and carbs in soon after you’re finished. (Smoothie, anyone?)
If you run on empty and feel like totally depleted after 10 minutes, it’s probably not the approach for you. And in general, if your workouts are long, fast-paced, or include high-intensity intervals or heavy weights, it’s a good idea to eat before.
Check out this post for my recommendations on the best pre- and post-workout snacks, and you’ll be ready to move.