Fasted cardio is a somewhat popular craze in the fitness world. It’s essentially working out on an empty stomach, typically in the morning. Proponents argue exercising after an overnight fast forces the body to rely on fat for energy. This results in increased fat losses. Unfortunately, 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 body uses three macronutrients for energy: fats, carbohydrates, and proteins. Generally, fat powers lower intensity, longer duration exercise, while carbohydrates power higher intensity, shorter duration work. Typically, more than one macronutrient is used to provide power for a single workout session. It’s hard, if not impossible to force the body to prioritize fats over carbs. There are numerous studies looking into the effects of fasted cardio on fat loss.
What does research say about fasted cardio?
Study 1: Effects of fasted/fed cardio on body composition²
The first study looked at changes in fat and muscle mass in fasted versus fed aerobic exercise. All subjects were in a calorie deficit. Twenty participants were split into two groups: fasted exercise and fed exercise (meal one hour prior to workout). Both groups performed one hour of cardio on a treadmill three days per week. The sessions lasted 50 minutes in addition to a five minute warm-up and five minute cool-down.
The fasted group averaged an intake of 1,236 calories while the fed group averaged 1,277 calories. The results did not show significant differences between the two groups on: body mass, body fat percentage, fat mass, waist circumference, or fat-free mass. Both groups saw reductions in all categories. The authors concluded both approach lead to similar results.
Study 2: Fat and carb metabolism in fasted/fed cardio³
The next article was a review of 27 studies. The authors examined the effects of fasted and fed exercise on fat and carbohydrate metabolism. They wanted to know what energy source the body was using when exercising on an empty stomach compared to after a meal. After reviewing studies of 273 participants, this review concluded there was a significant increase in fat oxidation during a fasted state.
Study 3: Effects of fasted/fed cardio on appetite and calorie intake⁴
The third study examined the effects of fasted exercise on appetite and overall calorie intake. The study included 10 participants that performed both fasted and fed exercise. Both fasted and fed exercise sessions were one hour treadmill runs. During fasted exercise, participants exercise for an hour before breakfast. They were given a standardized breakfast 30 minutes after finishing their workout. They then had an all you can eat buffet for lunch and dinner. During fed exercise, participants had the same breakfast and exercised a few hours later. They were offered the same all you can eat buffet for lunch and dinner. Appetite ratings and calorie intakes were taken throughout the trial.
Results showed fed exercise did a better job at suppressing appetite than fasted exercise, however, buffet calorie intake was similar between both trials. Both groups created a negative energy balance and neither was more effective. The study concluded there was no benefit in exercising either before or after breakfast in terms of overall energy balance.
Study 4: Fat loss in fasted and fed states⁵
The final study compared fat loss in both fasted and fed exercise states in eight participants. All subjects performed the same workout in both fasted and fed states (running 36 minutes on a treadmill). Participants ate the same total calories throughout the day in both states. Results showed when the subjects exercised in the fed state, their bodies continued burning more fat for energy 12 and 24 hours after completing the workout. The authors concluded exercising in a fasted state did not enhance fat loss. They recommended eating a light meal before engaging in physical activity.
Current research does not support fasted cardio as an effective way to burn fat. Study 2 did conclude fasted cardio let to greater utilization of fat for energy. Unfortunately, simply burning more fat during a workout isn’t necessarily as effective as it sounds. The body is a complicated system and continuously adjusts the amount of fat it’s using. It’s much more important to look at fat utilization over a longer period of time such days rather than minutes and hours. Generally, burning more fat during a workout causes the body to burn more carbohydrates in the post-workout period. The opposite is also [unfortunately] true¹.
Additionally, a fasted workout increases protein breakdown. This is a major issue for anyone looking to preserve muscle and strength during periods of weight loss. Finally, training on an empty stomach is difficult, even at moderate intensities. A pre-workout meal allows training at higher intensity levels. This type of exercise burns more total calories and results in greater fat loss¹.
The Bottom Line
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.
- Schoenfeld, B. (2011). Does Cardio After an Overnight Fast Maximize Fat Loss? Strength and Conditioning Journal, 33(1), 23–25. doi:10.1519/ssc.0b013e31820396ec
- Schoenfeld, B., & Aragon, A. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 11(1), 54. doi:10.1186/preaccept-1860458401143511
- Vieira, A. F., & Costa, R. R. (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(07), 1153–1164. doi:10.1017/s0007114516003160
- Deighton, K., & Zahra, J. C. (2012). Appetite, energy intake and resting metabolic responses to 60min treadmill running performed in a fasted versus a postprandial state. Appetite, 58(3), 946–954. doi:10.1016/j.appet.2012.02.041
- Paoli, A., & Marcolin, G. (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. doi:10.1123/ijsnem.21.1.48