Do you drag yourself out of bed first thing every morning to perform cardio before breakfast in hopes of jacking up your fat burning capacity? If so, go back to sleep!
The fact is, performing cardio on an empty stomach does little to increase fat burning over exercising in the fed state. I recently wrote an article for the NSCA Strength and Conditioning Journal titled, Does Cardio After an Overnight Fast Maximize Fat Loss? that examined the research on the topic. Here is a synopsis:
1) When exercising at moderate to high intensity, the body breaks down substantially more fat than it can burn for fuel. This effect is magnified in experienced exercisers. Ultimately, the fatty acids get “trapped” due to exercise-induced ischemia and are unable to reach the muscle for oxidation. Thus, while there is an increased breakdown of fat when you don’t eat prior to performing cardio, the additional fatty acids don’t get burned and simply end up getting stored again in fat tissue.
2) Studies show a significantly greater amount of calories burned post-workout when you eat prior to training compared to if you don’t (i.e. the “thermic effect of exercise). The upshot is that performing cardio while fasting blunts fat burning following the exercise bout, which potentially can add up to a substantial amount depending on exercise intensity.
3) As much as 50% of the fat burned during exercise comes from intramuscular triglycerides (fat stored within muscle). This fat serves as a reserve energy source and has nothing to do with physical appearance. Thus, even if any additional fat was burned during the bout by not eating, the effects on body composition would be far less than simply looking at the total amount of fat burned.
4) Last, and perhaps most importantly, fat burning must be considered over the course of days—not on an hour-to-hour basis—to get a meaningful perspective on its impact on body composition. The human body is constantly adjusting its use of carbs versus fat. As a general rule, if you burn more carbs during a workout, you inevitably burn more fat in the postexercise period and vice versa. It all evens out in the end.
Bottom line: When you do your cardio is really not important. Like the Nike commercial says: Just do it!