Some people think that fasting cardio is better than doing it after a meal if you are looking to lose body fat, but it’s not really true and we’ll explain why.
A few years ago a study was published comparing two groups. One group did fasted cardio and the other group did cardio after a meal. The result was that the group that fasted, got a greater caloric deficit at the end of the day, which can lead to greater fat loss. This caused a lot of controversy, but it has a very simple explanation; the fasting group achieved a greater caloric deficit because they simply ate less.
The rationale behind fasted cardio is that if you are fasting, your insulin and liver glycogen levels will be lower, and this can cause you to burn more fat doing cardio, which would ultimately lead to more fat loss.
In any case, what has been seen in many studies, such as this one, is that fasted cardio does not lead to greater fat loss at the end of the day if the diet is the same in both groups. To understand why this happens, we have to know that although we oxidize fat, we will not necessarily lose body fat.
This loss of body fat will only occur if the fat we oxidize is greater than the fat we store. Regardless of whether we oxidize more or less fat, this amount of fat oxidized must necessarily be greater than the fat stored for the net balance to allow us to lose body fat.
For you to understand this we can also extrapolate it to ketogenic diets. In them fat oxidation increases because increasing the amount of fat we eat increases our enzymes in charge of metabolizing this fat, but you will not lose body fat on a ketogenic diet unless the calories you spend exceed the calories you eat, because this is what will ultimately allow the fat that is oxidized to be greater than the fat that is stored.
Fasting cardio does not necessarily result in greater fat loss at the end of the day even though we oxidize more fat by doing it than in a cardio session done after a meal. This is because it has been shown that if you burn more fat during fasted exercise, you will simply burn less fat during the rest of the day as a compensatory mechanism.
Similarly, if the cardio we do is not fasted, our fat oxidation during exercise will be lower, but our fat oxidation throughout the day will be higher, and thus we will get the same end result in terms of body composition. In other words, in order to improve body composition, it is not relevant whether we do the cardio on an empty stomach or after a meal.
Fasting cardio is not a better tool than cardio after a meal. The practical application of all this is that it is up to you to do it on an empty stomach or to do it at another time of the day.