Whether you’re looking to lose weight by running or looking to become more efficient at fuel consumption during a workout, fasted cardio is one way to train your body to tap into stored fat reserves. “Fasted cardio means you work out on an empty stomach—for example, first thing in the morning or after not eating for several hours, typically five hours or longer,” says John Heiss, PhD, who specializes in sports nutrition and as the senior director of product marketing at Herbalife. “The general thinking is that by not eating food, your body tends to burn stored fat for energy preferentially over the food you just ate.”
Although the concept of fasted cardio has been around for some time with much debate on its effectiveness, it’s recently becoming a popular fuel approach in endurance sports. Heiss says it could be especially beneficial for the average runner looking to lose weight and improve performance. However, much of how fasted cardio works depends on what your training goals are, what types of workouts you’re doing—i.e. high intensity interval training versus slower long runs—and how your body responds to it.
First, it’s important to look at fasted cardio and fat burning in a long-term context of weekly calories in and out. “Looking at total calories burned, or the types of calories burned only during a workout window is over-simplifying the equilibrium of body composition,” Heiss explains. “A good example of this is high intensity interval training (HIIT), where athletes might do 10 maximal efforts lasting 20 seconds each with only 30 to 60 seconds rest in between. The total number of calories burning during this session is quite low, but long-term increase in metabolism is quite significant.”
“Mix It Up”
Not eating before a run depends on what kind of workout you plan on doing. Heiss stresses that having food before a workout isn’t going to undo the fat-burning effects and not eating beforehand won’t burn away fat either. “My advice is to mix it up. Eat before some workouts, and start out on an empty stomach for others,” he says. “There is a near-infinite number of biological differences that occur during exercise between the fasted and fed-states, and training both these metabolic pathways causes different desirable adaptations.”
For a low-intensity workout such as a 1-hour tempo run, eating less beforehand and shifting those calories to a post-workout shake or bar, enhances enzymes that are activated to burn fat efficiently.
But that doesn’t necessarily mean there aren’t advantages to eating before a run. “You can exercise at a higher intensity for longer, resulting in greater fitness and health gains in the long run. What matters more is an overall healthy eating pattern, and exercising regularly,” says Heiss, who has completed several ultras.
Burning Carbohydrates vs Burning Fat
Most runners consume carbohydrates pre-run or race for a substantial source of energy. Think pasta dinners the night before a marathon. However in a fasted cardio approach the emphasis is placed on burning fat. “From a performance angle, if you’re running a part of the course that isn’t super hard, then you can be burning fat and save your carbohydrate stores for when you need them at higher intensity,” Heiss explains. “This essentially gives your body a larger fuel tank of energy at its disposal.”
For actual race day performance though, you need to train your body to be efficient at burning both types of fuels: carbs and fats.
As you progress in training leading up to a race or race season, your training will typically shift from long, slow distance runs, to more specific, higher-intensity workouts. Your nutrition strategy should also be periodized in this manner, progressing from general to specific. Heiss suggests doing some early season slow runs fasted to allow your body to develop efficient fat-burning, and then adjust to fed training and carbohydrate intake during high-intensity sessions for maximal performance gains and recovery.
“For races marathon-distance and shorter, carbohydrates will be the primary energy your body uses,” Heiss says. “So it’s important to not completely neglect training in a fed state. For those doing ultra-events, this is where the performance angle of training both fasted and fed can have positive impacts.”
When it comes to the elite-level or sub-3-hour marathoner, though, Heiss explains, “Those guys want to burn entirely carbohydrates during a race and not fat because they can’t generate enough energy from fat fast enough to keep up with their power demands. The other complexity is that to burn fat it requires more oxygen then it does to burn carbohydrates and someone that’s running a 2:10 marathon, their biggest concern is oxygen supply.”
When teaching your body to adapt to a fasted cardio method, Heiss advises 1. Work up to running on an empty stomach by gradually increasing duration and to keep the intensity low, 2. Don’t start so hungry that you binge eat after a workout and consume more calories than burned, and 3. Have a good quality protein shake afterward.
*This information was recently presented at Herbalife’s first-ever Satellite Symposium that took place before the American Society of Nutrition’s annual conference in San Diego.