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Qualified Nutrition ©

Peter James Bell

BSc Nutritional Sciences (Hons)

MSc Sport & Exercise Nutrition

English Institute of Sport Performance Nutrition Intern

Competitive Bodybuilder

Fed vs Fasted Cardio

September 12, 2017


Will you burn more fat?


Will you lose more muscle?




We often hear these questions when discussing the wonderful topic that is fasted cardio!


First of all, what is fasted cardio? Well, the name explains everything! It’s doing a cardiovascular exercise in a fasted state, which is considered around 8 hours or more after your last meal. This is typically done upon waking, providing you didn’t sleep walk to the fridge!


In a fasted state, as we haven’t consumed carbohydrates for a long period of time, our glycogen stores are lowered, meaning there is less available to use as a source of energy. Therefore, in order to perform an exercise, our bodies oxidise fatty acids instead. This is often where people are misunderstood. Just because more fatty acid oxidation occurs during the exercise, it does not equate to a greater degree of total fat loss throughout the day.


Weight loss and weight gain is dependent on energy balance. In order to lose weight, you must expend more energy than you consume. Similarly, in order to gain weight, you must consume more than expend! Energy balance DOES NOT CARE if you expend 300 kcals fed or fasted!


To support this, Brad Schoenfeld et al. conducted a study in which they examined the body composition changes of 2 different groups. One group completed their cardio in a fasted state, the other group did theirs in fed state. Following the study, although both groups lost a significant amount of weight, no significant differences were found between the two groups. They concluded that:


“These findings indicate that body composition changes associated with aerobic exercise in conjuction with a hypocaloric diet are similar regardless whether or not an individual is fasted prior to training”


So it would appear that in terms of body composition, fasted cardio will provide you with no extra benefits. However, this does not mean that it is a complete waste of time. For instance, aerobically exercising in fasted state has been shown to improve our metabolic flexibility. In simple terms, this is the bodies ability to metabolise both carbohydrates and fats efficiently when required to do so. Our bodies preferred energy source is glycogen from carbohydrates, so by depleting this it must oxidise fatty acids instead, subsequently enhancing it’s ability to do so. 


The most commonly reported reason for doing fasted cardio is actually nothing to do with the science behind it, but practicality. It can sometimes be difficult finding the time to exercise, by setting your alarm just a little bit earlier and exercising first thing in the morning, the task becomes less daunting. Besides, exercise has been shown to release endorphins which will set you up psychologically for a positive day. There’s a reason everyone feels good after they exercise!





The answer to this is really dependant on the intensity and duration of the exercise performed, and of course the individual’s genetic makeup. High Intensity Interval Training (HIIT) is very demanding on the body, and with this comes a high cortisol stress response. Cortisol is an extremely catabolic hormone which if levels are elevated, puts your muscles at a much greater risk of being oxidised for energy, and we don’t want that! So by doing HIIT first thing in the morning on an empty stomach, the chances of you losing muscle are increased tremendously. 


By doing Low Intensity Steady State (LISS) cardio instead, the body is put under much less stress and cortisol levels are not elevated to the same extent. Try to limit your cardio to no more than 1 hour per session, as by this point your body will be desperate for some nutrients. Upon finishing, as you will probably be hungry, go prepare yourself a nutritious breakfast to refuel! To further protect your muscle, consuming BCAA’s could be beneficial to ensure there is a supply of amino acids readily available for absorption. 




In terms of body composition, there are no proven benefits in opting to do your cardio in fasted state over a fed one. But you may improve your metabolic flexibility and it can be convenient to get it out the way first thing in the morning! Just remember it’s probably best to avoid HIIT on an empty stomach and consider supplementing with BCAA’s!





Schoenfeld, B., Aragon, A., Wilborn, C., Krieger, J. and Sonmez, G. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition, 11(1).



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