Fasted cardio has long been a popular method among physique athletes and fitness enthusiasts looking to shed body fat more efficiently. The theory suggests that performing cardio on an empty stomach, typically after an overnight fast, forces the body to use fat as its primary fuel source, thereby accelerating fat loss. But how does this approach compare to doing cardio after eating? In this article, we’ll break down the science behind fasted versus fed cardio and explore whether one method truly outperforms the other when it comes to burning fat.
The Theory Behind Fasted Cardio
The concept of fasted cardio is rooted in the idea that when you exercise on an empty stomach, your glycogen stores are depleted, and your body is more likely to burn fat for energy. Advocates of fasted cardio argue that this can lead to greater fat loss over time, particularly for those aiming to reduce stubborn fat in the last phases of a cutting cycle.
The Fed Cardio Perspective
On the other hand, fed cardio involves consuming a meal before exercise, ensuring that your body has readily available glycogen stores to fuel your workout. Supporters of fed cardio argue that having energy from food allows for more intense and sustained exercise, potentially leading to higher overall calorie expenditure and improved performance.
The Research: What Does the Science Say?
To put these theories to the test, a study led by Brad Schoenfeld, published in the Journal of the International Society of Sports Nutrition in 2014, aimed to determine whether fasted cardio indeed leads to greater fat loss compared to fed cardio.
In this study, 20 healthy young women were randomly assigned to one of two groups: the fasted training group (FASTED), which performed cardio after an overnight fast, and the post-prandial training group (FED), which did cardio after eating. Both groups engaged in the same volume of steady-state aerobic exercise—one hour, three days a week for four weeks—and followed a calorie-restricted diet to ensure a consistent calorie deficit.
Study Results
The results of the study were revealing. Both groups experienced significant weight loss and reductions in fat mass. However, there were no significant differences between the fasted and fed groups in any of the measured outcomes, including body mass, BMI, percent body fat, waist circumference, fat mass, and fat-free mass. This suggests that the timing of food intake relative to exercise—whether fasted or fed—does not significantly impact fat loss or changes in body composition when combined with a hypocaloric diet.
Understanding the Real Impact of Fasted Cardio
Here’s where it gets interesting: while fasted cardio might lead to burning a greater percentage of fat during your workout, your body will balance things out throughout the rest of the day. If you burn more fat during a fasted workout, your body will likely shift to burning more carbohydrates later. Over the entire day, the difference in fat loss between fasted and fed cardio is minimal, if any.
This means that fasted cardio doesn’t inherently lead to greater fat loss than fed cardio. The key takeaway is that the real driver of fat loss is maintaining a consistent calorie deficit, not the timing of your meals around your workouts.
Stubborn Fat and Fasted Cardio
Now, there’s some talk about fasted cardio potentially being more effective for targeting those last bits of stubborn fat—the kind that’s notoriously hard to lose. There might be something to this, especially for those who are already lean and looking to get that final shred. But for most people, focusing on overall fat loss through a sustained calorie deficit is going to be the most effective strategy.
Potential Downsides
Let’s not ignore the potential downsides of fasted cardio. Some people find they have less energy when they train on an empty stomach, which can lead to reduced performance. And if you’re already lean, you might be more at risk of losing muscle, especially if you’re not careful with your nutrition.
It’s all about finding what works best for you. If you feel great doing cardio before breakfast and it helps you stay consistent, go for it. If you perform better after a meal, that’s the way to go.
Conclusion
So, which is better: fasted cardio or fed cardio? The truth is, they’re both effective for fat loss when done consistently and paired with a calorie deficit. The best approach is the one that fits your lifestyle, keeps you energized, and allows you to stay consistent with your training.
Don’t get caught up in the details of when you should be doing cardio. Focus on what makes you feel the best and what you can stick with in the long run. That’s where the real results come from.
References:
Schoenfeld, B. J., Aragon, A. A., Wilborn, C. D., Urbina, S. L., Hayward, S. B., & Krieger, J. W. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 11(1), 54.
Vieira, A. F., Costa, R. R., Macedo, R. C., Coconcelli, L., & Kruel, L. F. M. (2016). Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: A systematic review and meta-analysis. British Journal of Nutrition, 116(7), 1153-1164.
Liu, Y., Wang, C., Li, W., & Li, J. (2023). Effects of fasted versus fed exercise on body composition: A systematic review and meta-analysis. Sports Medicine, 53(4), 691-702.
Paoli, A., Marcolin, G., Zonin, F., Neri, M., Sivieri, A., & Pacelli, Q. F. (2011). Exercising fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio and excess postexercise oxygen consumption after a bout of endurance training. International Journal of Sport Nutrition and Exercise Metabolism, 21(1), 48-54.
Kim, J. Y., Lee, J. H., & Kim, D. J. (2015). Effect of fasting on serum cortisol and insulin resistance in healthy men. Journal of Clinical Biochemistry and Nutrition, 57(2), 159-165.
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