There is a lot of information out there around fasted cardio and fat burning. I’ve personally received a number of questions about whether or not fasted cardio increases fat burning, and I wanted to review the existing literature as well as add my thoughts.
Understanding Articles and Studies
I recently came by an article on Medium that reviewed recent studies about fasted cardio and concluded that fasted exercise does not reliably increase fat burning. Paradoxically, they did include a review of 27 studies that concluded “there was a significant increase in fat oxidation during a fasted state.” However, all the other studies reviewed concluded the opposite.
Their conclusion? “Fasted cardio does not work. At best it delivers results comparable to fed state exercise. At worst, it decreases intensity and performance, lowering total energy expenditure, and resulting in less overall fat loss. Fasted cardio seems to be more effort than it’s worth. It includes a lot of extra work without any added benefits.”
Is that true? Should we forget about exercising in a fasted state since it has no benefit?
No! The key is understanding who was investigated in those studies and if the results apply to you.
To their credit, they start the article by saying, “the body is a complex and dynamic system working on many variables. Simply exercising on an empty stomach may not be enough to increase fat loss.”
The Respiratory Quotient
That statement I can agree with. The body is complex and dynamic. In general, the body will use available glucose and glycogen first for energy. If those are in short supply, only then will the body change to fat burning. We can measure this by the respiratory quotient or RQ (see this video I did with Ken Nicodemus to learn more about cardiorespiratory testing and RQ).
If someone is eating a high carbohydrate diet, they are a requisite carbohydrate burner. Simply fasting for 12 hours is inadequate to turn them into a fat burner for a 60-minute exercise.
If, however, someone is following a healthy low carb lifestyle and has adapted to becoming a fat burner, then an overnight fast can greatly enhance the fat burning capacity of exercise.
The aforementioned studies, therefore, need to control for baseline diet and RQ, as that will greatly affect one’s ability to burn fat.
Duration of Exercise in a Fasted State
Also, duration of exercise matters. Longer exercise durations that deplete glycogen stores can help you transition to fat burning. This can be difficult, however if you have not cycled in and out of fat burning mode before as there is an adjustment period.
What’s the take home?
Be a fat burner at baseline. The better you burn fat at rest, the better you will burn it with exercise. If you follow a low carb lifestyle with time restricted eating and/or intermittent fasting, then you will train your body to burn fat. Exercising in a fasted state will enhance this capacity.
Hello fitness, good bye body fat.
Do you exercise in a fasted state? Let us know your experience and how it works for you!
Thanks for reading,
Bret Scher, MD FACC