Fasted cardio has gotten a lot of attention as being a more efficient way to burn fat than doing cardio after having eaten that day. This is perpetuated by many celebrities and high profile fitness personalities claiming that fasted cardio was their secret weapon for burning fat to get shredded.
The logic behind this claim is that the body has to burn fat for energy when it no longer has food as fuel.
So, does this mean that if your goal is to burn fat as fast as possible that you should do fasted cardio? Let’s dig into the science.
The Science Behind Fasted Cardio
One of our body’s primary sources of energy is breaking down carbohydrates into glucose. It is then transported through the blood to supply our cells with the energy they need to function.
When we exercise, our muscle cells are using glucose as one of their primary sources of energy.
However, our bodies are typically only capable of storing about 1,500-2,000 calories worth of glycogen (the stored form of glucose) . If you were to not eat, you could end up depleting your glycogen stores. In this scenario, your body would need to look elsewhere for a source of energy. One of those places is your fat stores, which contain a plethora of stored energy (about 3,500 calories per pound of fat).
This is where the claim that fasted cardio is superior to fed cardio originates from.The claim is that if you were to do your cardio in a fasted state, your body would have to derive more energy from the fat stores. In theory, this makes perfect sense.
So, case closed right? Sadly, it’s not as simple as that.
Using Fat For Energy
The body’s other preferred form of energy is fat. Even when we have full glycogen stores, our body is still using fat for a portion of the energy it needs.
Furthermore, breaking down fat to be used for energy by the muscle cells is a slow process. As a result, the body is not able to keep up with its energy needs while doing moderate or high intensity cardio. The breakdown of fat is able to keep up with low intensity cardio, however, that still does not make it a key for fat loss.
Basically, depending on the intensity of our exercise, the proportions of overall energy needs are met by different percentages of glucose versus fat. The higher the intensity goes, the higher the percentage of glucose used is compared to fat.
This is not to say that high intensity cardio burns less fat than low intensity cardio, just that more glucose is needed for the higher intensity sessions.
To use an analogy, it can help to think of our body as a hybrid vehicle that runs on fuel, glucose, and also electricity, fat. When we are out for a casual Sunday drive, the vehicle can get most of its energy needs from the electricity. However, if we want to step on it and bring the vehicle up to highway speeds, it will have to start burning more and more fuel to feed the engine. At both speed levels, the body is using about the same amounts of electricity (fat) to function, but when you step on the gas, it has to use more and more fuel (glucose) to keep up with the pace.
Meanwhile, back in science-y terms, it is true that when we do cardio in a fasted state, our body breaks down more fat cells. However, just because the fat cells are broken down, does not mean they will be used for energy. If they are not used during the session, the fat will end up being stored again.
Moreover, even when fat is used more during a fasted cardio session as a result of glycogen stores being somewhat depleted, after the session has ended, the body will compensate and burn less fat during the day while using more glycogen once you have eaten.
This is due to this pesky thing called evolution that has made the body very good at surviving by conserving its energy stores (fat).
More Problems with Fasted Cardio
One thing that many people overlook when it comes to fasted cardio is that just because they are hungry, does not mean they have depleted their glycogen stores and have to burn just fat for energy. As was mentioned earlier, these glycogen stores are capable of holding about 1500-2000 calories. If we were to truly deplete our glycogen stores, we would have to fast for 2-4 days or exercise at a moderate to high intensity for several hours.
Marathon runners are well aware of this moment of depleting glycogen stores but it goes by a different term: hitting the wall. When these individuals have been running for 2-3 hours, they can have reached the point where they have completely depleted their glycogen stores.
Therefore, when these runners hit the wall, they notice a huge drop in energy as they can no longer maintain a moderate intensity level since the fat burning cannot keep up with it. They are then forced to run at a much lower intensity. This is why it is good practice for marathon runners to consume fast digesting carbohydrates during the race to replenish their glucose and keep their intensity up.
While you may not ever run a marathon, it could be advantageous for your cardio session if you are doing them in a fed state. This may allow you to perform better than if you were to have done it in a fasted state, burning more calories.
This is not to say that some people feel better when they do cardio in a fasted state. If you prefer fasted cardio because you feel lighter or more energized, then by all means, go for it. It is whatever works best for you. Just be aware that fasted cardio is not a secret to fat loss.
The only true way to lose fat is to consistently eat in a calorie deficit. It is not enough to just be in a calorie deficit while you are working out. This is because once you eat, your body will replenish your glycogen and fat stores.
To actually lose weight, you need to be in a long term calorie deficit. Even if you were to workout in that calorie deficit, if you eat more calories than you need during the day, those excess calories will be used to replenish the fat stores that were used during exercise. If you want more explanation of how to think about calorie deficits, check out my article I wrote on it HERE. It helps clear up a lot of confusion and misconceptions that people have.
So, even though fasted cardio does not offer any special benefits for fat loss, it can still be something that you like doing. For instance, if I go for a run before work in the morning, I will rarely eat anything beforehand. This is just because if I were to eat, I feel sluggish and cramp up since the food sits heavy in my stomach.
However, for most of my exercise sessions, I do prefer to eat beforehand because it personally gives me more energy to work harder in my sessions. This is advantageous for me because if I can work at higher intensities, I ultimately have better strength gains and burn more calories.
In the end, this all comes down to your training preference and I am not going to tell you what will work best for you. Experiment with what feels best for you. Plus, now you hopefully have the knowledge to make an informed decision without some of the BS that gets passed around by others.
Run fast, lift big.
 “The Body’s Fuel Sources,” Human Kinetics. [Online]. Available: https://us.humankinetics.com/blogs/excerpt/the-bodys-fuel-sources#:~:text=Provides%20energy%20in%20late%20stages,percent%20of%20the%20energy%20needed. [Accessed: 13-May-2021].