Fasted or non-fasted cardio – research into this question has recently been receiving a lot of attention. People like to perform cardio to burn fat, but what actually happens during cardio from a physiological standpoint?
Should I perform fasted cardio to lose weight, to tone up, on my off days from training?
These are all questions that research has been trying to answer.
In this article, we will review the differences between fasted and non-fasted cardio, looking into which option may be the best for you and suggest practical recommendations for whichever style you choose.
What is Fasted and Non-Fasted Cardio
Fasted cardio is simple: wake up, roll out of bed, and head straight to the gym. Non-fasted cardio is the exact opposite – eat before you train.
Fasted cardio is a common tool or exercise used to lose fat by a lot of people within the bodybuilding and fitness industry. The “theory” is that when in a starved state after an overnight fast, glycogen levels (glucose in the blood/muscle) are low enough to cause your body to shift its energy source and burn more fat during exercise. [1,2,3].
Supporting this, multiple studies have shown that the ingestion of carbohydrates, (e.g. breakfast), before cardio reduces the delivery of fats into the mitochondria (our cells’ engine), which is where we burn them and obtain new energy .
As you can see above, training fasted is normally performed at a moderate intensity and allows you to maximize the amount of fat you can burn.
In contrast to this, fed cardio (i.e. you consume food before) lowers the amount of fat we burn, which is why people are picking the fasted cardio over fed cardio. Sadly, although it sounds great and clear cut, there is one problem with this basic concept.
Research has shown that it is important to consider that fat loss occurs not only in the exercise session, but is an on-going process throughout the day. Therefore, if you want to fully grasp how the body uses certain types of fuel for energy, then you should consider the entire days’ energy use, not just what is expended on an hourly basis.
In general, if you are FED and burn more carbohydrates during the exercise session, more fat will be utilized and burnt post-exercise. In contrast for FASTED cardio, you will burn more fat in the workout, but, you will burn less later in the day. As you can see, the end result is basically the same, you either burn more fat in the workout (if fasted) or you burn more fat later that day (if fed) .
Research on Fasted vs Fed Cardio.
Several research studies have tested these 2 protocols to try to establish whether Fasted cardio yields greater fat loss.
In one study they investigated the differences between fasted and non-fasted cardio on the breakdown and utilization of stored fat. The participants performed a low-intensity and moderate-intensity aerobic workout in both a fasted state and a non-fasted state 
The results found that the low-intensity fasted group saw a greater rate of fat oxidation after 90 minutes of cycling. The moderate-intensity condition showed no differences at any point in time. Therefore, while the low-intensity cardio did help the participants burn fat when fasted, these results did not extend to when the participants performed at higher intensity. 
While that study did show greater fat loss within the session, remember, we discussed how fat burn and fat loss over a 24 hour period is actually what counts.
A long-term study tested this and found when performing an aerobic exercise program for six weeks, there is no significant difference in total fast loss when comparing fasted and non-fasted groups 
The thermogenic effect of food is also important to consider when trying to understand which fuel source our body is using after we train.
For example, the consumption of carbs or milk before aerobic exercise significantly increases the amount of calories participants burnt post-exercise when compared to when in a fasted state. This means that when consuming food before training, oxygen expenditure and metabolism is higher, which further supports that just looking at the amount of calories or fat we lose when training doesn’t really paint a full picture [1,3]
Is Fasted or Fed Cardio Better? What’s best for me?
Whether your fitness goal is to lose fat or not, the possible effects that fasted cardio has on muscle tissue needs to be taken into consideration over the long term.
Although you certainly won’t lose noticeable muscle loss from the odd fasted session per year, doing it several times a week on a regular basis, especially when in a calorie deficit, may have a significant impact on muscle loss.
Based on research, nitrogen loss could be more than doubled when you train in a glycogen-depleted state or fasted. This nitrogen is directly pulled from muscle, which probably means a decrease in muscle mass over the long term 
So, if there isn’t any benefit for fat loss, which we’ve just seen above (remember, you may burn more fat in the workout but you burn less after the workout), why would you risk extra muscle loss for little or no rewards?
That’s the key question – there are few benefits, but there’s risk of muscle loss, so why risk it?
Conclusions and practical recommendations
There is some evidence that performing cardio in a fasted state may result in a greater fat loss during the session but this doesn’t seem to provide a significant impact on your goal or physique over the long term 
For healthy individuals who are just trying to lose weight, studies have shown you burn the same amount of fat over the long term when performing cardio in a fed state.
As emphasized throughout, fasted cardio does burn more fat within that 30-60-minute window, but it causes your body to burn less later that day. As discussed, you also increase the chances of burning muscle which is clearly not good.
If you do love to wake up and get going, it is recommended you have at least 1 – 2 scoops of whey or, as a bare minimum, 10g of BCAA before you hit the morning cardio. If you are trying to lose fat, the extra calorie burn will help, but it doesn’t really matter if you are fasted or fed. Instead, focus on your workout, diet, staying consistent and other advanced training methods such as HIIT, metabolic resistance training etc.
Helms, E. R., Fitschen, P. J., Aragon, A. A., Cronin, J., & Schoenfeld, B. J. (2015). Recommendations for natural bodybuilding contest preparation: resistance and cardiovascular training. J Sports Med Phys Fitness, 55(3), 164-78.
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), 54.
Schoenfeld, B. (2011). Does cardio after an overnight fast maximize fat loss? Strength & Conditioning Journal, 33(1), 23-25.