• Introduction
  • The basics of fasted cardio
  • What does the science say?
  • Final thoughts
  • Sources
  • Fasting: reducing food intake for a set amount of time.
  • Fat oxidation: the process of breaking down fatty acids for fuel.
  • Glucose: blood sugar. It is the body's main source of energy, and derived from the food you eat.
  • Insulin: a hormone regulating the amount of glucose in the blood.
  • Muscle glycogen: an essential fuel store located in the muscles.


Fasted cardio has long been a hot trend in the fitness industry. It has even been described as a magical way of boosting fat burn while you exercise. Despite the lofty promises from trainers and online influencers, the scientific community is not as convinced of its effectiveness.

So, is fasted cardio beneficial for weight loss? Can it improve your body composition? Let’s take a closer look what the science behind it tells us.

The basics of fasted cardio

Fasted cardio refers to performing cardiovascular endurance exercises in a fasted state. This means that your body has had enough time to digest and metabolize the nutrients from the food you have consumed. According to sports science, a fasted state begins approximately 8 to 12 hours after your last meal. Fasted cardio exercises are usually performed first thing in the morning on an empty stomach.  

Studies have shown that fasting reduces the amount of glucose (blood sugar) and insulin in the blood, while also lowering the muscle glycogen (glucose stored in the muscle) storages in the body. In most situations, your body relies on blood glucose and muscle glycogen for fuel. Both of which are derived from your diet, especially foods rich in carbohydrates.

The theory behind fasted cardio is that the less readily available energy you have, the more your body is forced to use alternative sources to satisfy energy needs. Therefore, low levels of blood sugar and insulin push the body to shift from using carbohydrates to burning fat as a source of energy.

Although fasted cardio is widely used within the fitness industry, the science behind it is not entirely conclusive. Let’s take a look at what recent studies have shown.

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Fasted Cardio

Exercising on an empty stomach (8-12h of fasting)Can improve fat burn during exerciseCan increase protein breakdown during exerciseLower energy often reduces training intensity & volumeHas little effect on overall weight loss or body composition

What does the science say?

Although several studies have found fasting cardio to slightly increase fat oxidation (lipolysis) during exercise, the scientific community is yet to fully support the effectiveness of the theory. There are several reasons behind this. First, fasted cardio has proven to have little impact on weight loss or overall body composition. This is due to the fact that exercise accounts for only 15-30% of your daily energy expenditure. Therefore, it has a relatively small impact on weight loss when compared to your diet.

Second, having more energy for an exercise results in longer workouts, which leads to an increased calorie expenditure. Because fasted cardio specifically aims to reduce the amount of energy available, it often leads to lower training intensity and volume. Both of which can significantly reduce the endurance and health benefits the exercise would normally have.

Third, fasting does not mean you automatically switch from glucose to burning fat. Research shows that your body may start breaking down protein instead, leading to further loss of muscle mass. For example, one study showed that running in a fasted state yielded twice as much protein breakdown in muscles than in a non-fasted state.

Resistance training in a fasted state has also shown similar effects of lower performance and increased breakdown of protein. This becomes especially apparent the more intense the exercise becomes.



Burns slightly more calories during exercise.

No impact on overall weight loss or body composition.

May curb appetite throughout the day.

Decreased training volume & intensity
(reduced cardiovascular benefits).

May help avoid discomfort, indigestion, and potential nausea when training.

Increases protein breakdown
(hinders strength development).

Fasted cardio has little effect on weight loss or body composition.

Final thoughts

Although there is relatively little research about fasted cardio, it is important to remember that any exercise, whether in a fasted state or not, offers significant health benefits.

The theory behind fasted cardio theory claims that exercising on an empty stomach may have greater fat-burning effects – which it does. However, weight loss and reduction of body mass is likely a result of an overall calorie deficit, regardless of whether exercise is completed on a full stomach or not. Studies have shown that the most effective way to improve body composition is combining a healthy diet with a consistent exercise program.

Another thing to remember is that energy systems in your body work at all times, whether you are resting or not. Only their proportions may change depending on the intensity and duration of the exercise. So, strictly focusing on burning fat during the exercise may not lead to the results you are looking for. 

Most importantly, working out should not be just about counting calories or focusing on weight loss. There are tremendous physical, social, and mental benefits that exercising can provide you with. Perhaps the best advice to give is to find what you enjoy and try to do it consistently. 

Did you learn anything new about fasted cardio? Let us know in the comments. 


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  • Bak AM, Vendelbo MH, Christensen B, Viggers R, Bibby BM, Rungby J, Jørgensen JOL, Møller N, Jessen N. Prolonged fasting-induced metabolic signatures in human skeletal muscle of lean and obese men. PLoS One. 2018 Sep 5;13(9):e0200817. doi: 10.1371/journal.pone.0200817. PMID: 30183740; PMCID: PMC6124727.
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  • Ivy JL. Regulation of muscle glycogen repletion, muscle protein synthesis and repair following exercise. J Sports Sci Med. 2004 Sep 1;3(3):131-8. PMID: 24482590; PMCID: PMC3905295.
  • Poehlman ET. A review: exercise and its influence on resting energy metabolism in man. Med Sci Sports Exerc. 1989 Oct;21(5):515-25. PMID: 2691813.
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  • Schoenfeld BJ, Aragon AA, Wilborn CD, Krieger JW, Sonmez GT. Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr. 2014 Nov 18;11(1):54. doi: 10.1186/s12970-014-0054-7. PMID: 25429252; PMCID: PMC4242477.
  • Schoenfeld, Brad MS, CSCS Does Cardio After an Overnight Fast Maximize Fat Loss?, Strength and Conditioning Journal: February 2011 - Volume 33 - Issue 1 - p 23-25 doi: 10.1519/SSC.0b013e31820396ec 
  • Terada T, Toghi Eshghi SR, Liubaoerjijin Y, Kennedy M, Myette-Côté É, Fletcher K, Boulé NG. Overnight fasting compromises exercise intensity and volume during sprint interval training but improves high-intensity aerobic endurance. J Sports Med Phys Fitness. 2019 Mar;59(3):357-365. doi: 10.23736/S0022-4707.18.08281-6. Epub 2018 Apr 4. PMID: 29619796.
  • Vieira, A., Costa, R., Macedo, R., Coconcelli, L., & Kruel, L. (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. doi:10.1017/S0007114516003160

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