To eat or not to eat: isn’t that always the question? Well in the case of fasted cardio for fat loss, that is the question. There has been much debate as to whether or not you will lose more fat while doing cardio fasted or performing cardio after consuming a meal or two or three.
Fasted cardio to burn more fat is a myth. While doing cardio in a fasted state may prioritize energy from stored body fat, the overall calorie burn is minimal due to exercise being low intensity or not being able to maintain long durations or higher intensities.
Before we dive into the reasons why fasted cardio is a myth, we must define what fasted cardio is and where it came from.
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What Is Fasted Cardio?
The idea behind fasted cardio is performing cardio before eating first thing in the morning. Because you are in a fasted state, glycogen levels are low (the levels of stored glucose, or sugar in layman's terms) causing your body to use energy from stored fat instead of carbohydrates (sugars).
In theory, by using stored fat as an energy source, you'd burn more body fat in a fasted state than you would by doing cardio in a fed state.
So obviously, performing cardio after eating would be the flip side of the coin. Fasted cardio is usually done by performing aerobic exercise in the morning before consuming any calories. Originally, it was prescribed as 20 minutes of intense cardio in Bill Phillip's book "Body For Life." Since then, many physique athletes and weekend warriors have used fasted cardio in low-intensity form as well.
The type of cardio doesn't matter for it to be fasted cardio. While originally it was prescribed as intense cardio, many use both low or high-intensity cardio when fasted.
Fasted Cardio vs. Fed Cardio
Have you ever seen someone smile while on the treadmill? I haven't.
Turns out, the evidence doesn't support this theory of fasted cardio, and fed cardio may be a better option if fat loss is your goal. We must think long-term. Not just acute responses to exercise. Fat burned in one session doesn't equal fat lost over longer periods.
The general rule is the greater carbohydrate burned throughout a workout, the more fat you burn in the post-workout period (that means pancakes for breakfast right?) [1].
For example, those who perform high-intensity interval training (HIIT) have been shown greater rates of fat loss over time compared to those who exercise in the "fat-burning zone" of low-intensity cardio.
When comparing fasted vs. fed cardio in college students, we see greater post-exercise oxygen consumption (more calories burned) in the fed group compared to the fasted group [2].
When comparing rates of fat loss in young women who performed low-moderate intensity steady-state cardio for 1 hour, 3 days per week either fed or fasted in a calorie deficit, we see no differences between groups for body weight, BMI, body fat percentage, waist circumference, or body fat [3]. Meaning both fed and fasted cardio results in similar weight loss outcomes.
More recently, consuming protein before moderate-intensity cardio results in more calories being burned after exercise compared to consuming only carbohydrates or being fasted [5]. Specifically, post-exercise fat burning was enhanced in the protein group.
Even high-level endurance athletes have been studied with those eating a carbohydrate-rich breakfast having superior average power outputs by 3% and faster time trial times than the non-breakfast group [4].
This happened even though the non-breakfast group made up for the carbohydrates and calories at lunch. So, breakfast may be the most important meal of the day when it comes to performance. I would extrapolate this data to lifting weights as intense lifting also has high carbohydrate requirements.
Overall, fasted cardio won't help you lose weight faster or shed fat quicker. If you are trying to improve your cardiovascular endurance, an untrained individual could definitely progress using fasted cardio.
However, being that performance is the main goal, there is little point to perform cardio fasted as you will limit the intensity and duration you will be able to train hindering your cardiovascular gains.
Does Fasted Cardio Burn Muscle?
This is where things get interesting. If you lift weights (being on Lift Big Eat Big probably means you do), losing muscle mass is seen as being worse than a global pandemic. One interesting finding when performing fasted cardio is the loss of nitrogen (which indicates a loss of total body protein) when exercising even at very low intensities [6].
However, ingesting protein before cardio can mitigate this protein loss while also enhancing fat burning [7].
Is It Bad To Do Fasted Cardio?
It is not inherently bad to perform fasted cardio. Exercising with low glycogen levels (i.e. fasted), has been shown to increase muscle glycogen by 22% which could potentially lead to more muscle fullness and a leaner physique over time [8].
However, there is a trade-off with protein degradation so, to take advantage of this effect, you would need to keep cardio under an hour at very low intensity (i.e. walking). However, ingesting protein before cardio seems to be a much better option when building a lean physique is your goal.
Is Fasted Cardio Better?
Based on the evidence we have; fasted cardio does not seem to be better for fat loss compared to fed cardio. Fed cardio allows you to exercise at a higher intensity and for longer without burning muscle while maximizing the calorie and fat burn post-exercise.
References
1. Schoenfeld, B. (2011). Does cardio after an overnight fast maximize fat loss?. Strength & Conditioning Journal, 33(1), 23-25.
2. Lee, Y. S., Ha, M. S., & Lee, Y. J. (1999). The effects of various intensities and durations of excercise with and without glucose in milk ingestion on postexercise oxygen consumption. Journal of sports medicine and physical fitness, 39(4), 341.
3. Schoenfeld, B. J., Aragon, A. A., Wilborn, C. D., Krieger, J. W., & Sonmez, G. T. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 11(1), 1-7.
4. Metcalfe, R. S., Thomas, M., Lamb, C., & Chowdhury, E. A. (2021). Omission of a carbohydrate-rich breakfast impairs evening endurance exercise performance despite complete dietary compensation at lunch. European Journal of Sport Science, 21(7), 1013-1021.
5. Gieske, B. T., Stecker, R. A., Smith, C. R., Witherbee, K. E., Harty, P. S., Wildman, R., & Kerksick, C. M. (2018). Metabolic impact of protein feeding prior to moderate-intensity treadmill exercise in a fasted state: a pilot study. Journal of the International Society of Sports Nutrition, 15(1), 1-12.
6. Lemon, P. W., & Mullin, J. P. (1980). Effect of initial muscle glycogen levels on protein catabolism during exercise. Journal of Applied Physiology, 48(4), 624-629.
7. Escalante, G., & Barakat, C. (2020). Fasted versus nonfasted aerobic exercise on body composition: considerations for physique athletes. Strength & Conditioning Journal, 42(5), 71-78.
8. Van Proeyen, K., Szlufcik, K., Nielens, H., Ramaekers, M., & Hespel, P. (2011). Beneficial metabolic adaptations due to endurance exercise training in the fasted state. Journal of applied physiology, 110(1), 236-245.