In the last decade, a theory has sprung which has convinced a good sum of fitness individuals into believing that performing cardio on an empty stomach is the secret to maximum fat loss. According to this idea, when the body has not consumed any food for long hours, for example from evening till morning, the glycogen levels start to decline.
Cardio is an exercise known to utilize glycogen stores in the body as its main energy source. However, with the depletion of the stored carbohydrate, the body will turn to the stored fats to fuel cardio. At the same time, since insulin has decreased with the fall of the blood sugar levels, the body becomes more stimulated to break down fats, transforming the fatty acids into energy source for the workout.
With the rationality of the idea, there is no wonder then that it
has become one of the strategies used by people who wanted to get lean and fit. And that is why, you would see a lot of people who would be running on the treadmill or going to the gym early in the morning, who haven’t had anything prior to their workout.
However, our body is not a vacuum where one process just happens while the rest of the system is at rest. In reality, our system will always adjust between the use of fats and carbohydrates as its main sources of energy. Of course, the more your body burns carbs, the more efficient it will be in burning more fats, not just during the workout, but even hours after it.
While the theory is right in saying that cardio on an empty stomach can increase the use of stored fat in the body, this only happens when you are training at a low intensity. For high-intensity and even medium-intensity training, your body will be breaking down more fat that is transformed into fatty acids.
However, the increased breakdown rate is higher than the body’s capacity to utilize these fatty acids. This means that, your body will have fatty acids which your muscles can’t even use. Once you are done with your workout and the physical activity has winded down, the fatty acids will be transformed into triglycerides which will then go back as stored fats. In the end, there’s really not much progress that took place when it comes to burning those fats.
“But what if I just brisk walk or do low-intensity cardio?” True, you can do that. However, fat utilization does not immediately take place when you are doing low-intensity cardio. At the first part of your cardio session, your body will get its energy source from your glycogen stores. Study showed that in the first 90 minutes of low-intensity cardio, fat utilization has not increased.
The shift only takes place after the first one and a half hours. If you can commit to more than two hours, then you can benefit from the perceived effect of fasted cardio. Otherwise, you will just be using a lot of your energy without getting the results that you are aiming for.
Cardio is known to provide the body with excess post-exercise oxygen consumption or EPOC. It is also called as the afterburn effect, since the body will still be burning fats up to 48 hours after the workout. Now, EPOC is represented through the calories which the body will continue to expend and eating prior to your cardio session can substantially impact this effect.
And majority of the calories utilized and expended by the body during EPOC come from fats. This means that your body needs to consume significant amount of calories to stimulate the afterburn effect.
Another factor that connects EPOC and fed cardio is the training intensity. HIIT is associated with faster fat loss compared to low-intensity and steady state cardio. However, when you are doing fasted HIIT, your body won’t have enough energy to keep you going. When your body has a depleted level of glycogen stores, there is no way you can increase your training intensity. With lower intensity, you will get lower burned calories, a lower afterburn and lower fat loss.
On top of that, fed cardio puts the body in an anabolic state compared to fasted cardio where it has catabolic effects on the body. When you are working out, it is your best interest to put your body in an anabolic state, to prevent protein breakdown. If you are a bodybuilder, fasted cardio will have detrimental effects not only to fat loss but also to your muscle-building goals.
It is clear that cardio on an empty stomach can lead to muscle loss and minimal fat loss. But what should you eat prior to your workout? When it comes to determining what pre-cardio nutrition is best for you, you have to take into account the intensity and duration of your training, as well as the meals taken during the day, as well as your body composition.
If you are doing HIIT, which uses glycogen as energy source, you will need to consume carbohydrates sufficient enough to fuel your exercise. Otherwise, your energy will be limited to sustain the kind of workout that you are aiming for. Another thing that you need to consider is that your muscle mass should be preserved in your cardio session. This means consuming protein, aside from carbohydrates.
A good practice would be to consume a mixed diet composed of ¼ grams of carbohydrates and 1/8 grams of protein per pound of your ideal body weight. With this, you will be able to fuel your workout and prevent your body from taking the energy from your muscles. If you are doing cardio in the morning, start your day with a shake made of natural fruit juices and whey protein.
Consuming carbohydrates and protein an hour or half an hour prior to your workout is beneficial for you, since workout helps in partitioning the nutrients that you have taken in. Regardless of what time you will do your cardio workout, keep in mind that the best way to approach it is with a fed stomach.