Should you do your cardio on an empty stomach or should you eat first? There are a lot of divergent opinions on this topic. Here is what the research says and my opinion.
Cons of Fasted Cardio
Corisol, the fight or flight hormone, is at it’s highest in the morning. Morning fasted cardio can lead to a higher cortisol response than “fed” cardio. Since cortisol is highest in the morning, then doing fasted cardio can cause it to go even higher.
Why is this potentially bad? The primary function of cortisol is to mobilize stored energy (body fat). So, wouldn’t this be a good thing – to increase it with your cardio? Yes, but it will have a potentially more pronounced negative effect on muscle mass, and that makes things worse. Fasted cardio can directly lead to muscle breakdown, by a decrease in muscle protein synthesis. It can also decrease nutrient uptake by the muscles.
In addition to potentially impacting muscle mass, cortisol levels that stay elevated will lead to metabolic slowdown, and this is really bad. Rather than go into the science behind this, I’ll just mention briefly that the cause is by messing with the thyroid hormone and it’s natural conversions. When thyroid gets messed up, metabolism slows down, and you burn less overall calories. Not good for fat loss.
And to make matters worse, constantly high cortisol can also screw up your how your insulin works, and in some cases lead you down the path of insulin resistance. The more insulin resistant you are, the harder it is to lose fat.
The type of cardio you do plays a role in the equation. High intensity fasted cardio (HIIT) is not an optimal choice because this type of training inherently relies more on glucose for fuel and it will raise cortisol even more. A low intensity workout is a better option to keep cortisol in check.
Pros of Fasted Cardio
There are numerous studies comparing fasted vs. fed cardio. For the most part, there is very little difference shown in terms of fat burned. However, a recent study found something very interesting that was never really explored before. In this study published in the American Journal of Physiology (Yung-Chih Chen et al. 2017), the researchers found that when cardio is performed fasted, it increases the level of certain enzymes that are responsible for the mobilization of fat and its use for fuel. What does this mean? It means that while it doesn’t necessarily burn more calories for fat loss, it rather programs the body to rely more easily on fats for fuel. In other words, it sets up your metabolism to burn fat more easily.
This is particularly relevant for those who have a hard time losing fat despite training and eating clean. These folks tend to be quite efficient at using stored glycogen for fuel, and are thus inefficient at mobilizing stored body fat for energy. For these people, fasted cardio could very well be a great way to re-program the body to rely more easily on body fat for fuel.
What Kind of Fasted Cardio
Since the goal of fasted cardio is not necessarily to burn more calories, but rather to program the body to be more efficient at mobilizing fat for fuel, and thus to burn more overall fat over the course of a full day, then you should use a form of cardio that doesn’t rely on glucose. If you do anything too intensely, the body will be stimulated to rely on glucose. As such, the optimal choice is low intensity cardio.
The best way to describe this is basically a brisk walking pace. And as mentioned above, lower intensity means less cortisol release, which reduces the negative impacts of fasted cardio. Research suggests that anywhere from 25 minutes to 1 hour is good, depending on your personal goals. The key is to have the right frame of mind when you get started in the morning.
Fasted cardio is NOT a direct fat loss tool with the purpose of burning calories. Fasted cardio is an INVESTMENT in re-programming your body to make future fat loss more efficient.