Fasted Cardio: Greatest Fat Loss Weapon Or Tool Of Body Destruction?

Christian Thibaudeau

Co-founder of Thibarmy, Trainer

Articles, Fat loss

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Fasted Cardio: Greatest Fat Loss Weapon Or Tool Of Body Destruction?

Fasted Cardio: Greatest Fat Loss Weapon Or Tool Of Body Destruction?


Before I go on with the article I want to mention two things:

  1. I used to be against fasted cardio. I even wrote a few articles explaining why it was not a good idea since it could lead to muscle loss and didn’t burn more fat or energy than non-fasted cardio.
  2. In my latest photoshoot prep, in which I reached what I feel is my best condition ever, I did fasted cardio daily during the last 6 weeks. I didn’t lose muscle and feel that it greatly helped me get leaner.

What gives? Am I a turncoat?

Well, I guess I am, since being a turncoat basically refers to someone who suddenly changes his mind on a topic he has been pretty vocal about in the past.  But that’s me: I’m always looking for better ways to do things.  And if something or someone proves to me that I was mistaken in one of my opinions, I have no problem changing it.

To get back to our original topic: is fasted cardio good or bad when it comes to body composition and health?

My answer is that in the training world, nothing is ever completely bad (within reason) or totally good. Whether the end result is more positive or negative is highly dependant on the person and the application of the method.  Fasted cardio is no different.

Let’s first examine the good and bad aspects of fasted cardio before I make recommendations.

Fasted Cardio: The Bad

I’ve mentioned it in the past, fasted cardio can potentially lead to muscle loss and hormonal issues by leading to a higher level of cortisol.

One of cortisol’s main functions during exercise is to mobilize stored energy (which can be seen as a positive since it can help mobilize, and thus lose, fat). So, it is indeed useful. The more energy you have to mobilize, the greater the cortisol response will be.

Morning fasted cardio can lead to a higher cortisol response than “fed” or “post-absorptive” cardio for one simple reason: there is less readily available fuel in a fasted state. If you ate a meal 2 hours or less before doing your cardio (even longer in the case of a longer digesting meal) you will still have plenty of nutrients to use for fuel readily available in the bloodstream, therefore less need to mobilize it and as such, less need to pump out cortisol.

Also consider this: cortisol spikes in the morning. In fact, if your hormonal system is well regulated and you wake up without the use of an alarm clock, what wakes you up is the cortisol spike. Since cortisol is high in the morning doing fasted cardio can increase it even higher.

Why is cortisol elevation bad? After all, I did mention that it helps mobilize fat and thus can help you get leaner.  This is true, but when it comes to muscle mass, cortisol can have several negative impacts:

– It can directly lead to muscle breakdown (breaking down muscle tissue into amino acids, which can be eventually used for fuel)

– It can decrease protein synthesis (muscle building) both directly and by inhibiting mTor. Decreasing protein synthesis obviously makes it harder to build muscle.

– It can increase myostatin expression. Myostatin is the “muscle mass potential regulator”. The more active myostatin is, the harder it is to build muscle.

– It can decrease nutrient uptake by the muscles since it puts the body in “mobilization”, not storage mode.

There are other negative impacts on body composition from having continually elevated cortisol levels:

– While a cortisol spike followed by normal levels throughout the day can increase fat loss, cortisol levels that are constantly elevated can make you fatter by slowing down the metabolic rate. It does so by decreasing the conversion of the T4 thyroid hormone into the T3 thyroid hormone while increasing the amount of thyroid-binding protein. T4 is mostly inactive when it comes to metabolic rate (how much energy you burn), it is a reserve to produce the active T3, which is a metabolic rate regulator. If you convert less T4 into T3, your metabolic rate risks going down, in which case you will burn less calories during the day and are more likely to gain fat. A good sign that this is happening is that your body temperature goes down. Cortisol can also increase the level of thyroid binding protein. Why is this bad? Because TBP can attach itself to T3, making it unable to do its job. So not only do you have less T3, you also inhibit part of what you have!

– If it is constantly elevated, cortisol can also make you insulin resistant. The more insulin resistant you are, the harder it is to lose fat.

Why? Being “resistant” to a hormone means that your body doesn’t respond strongly to it; its receptors are desensitized.  So, to get the job done (in the case of insulin; getting the nutrients inside the muscles, liver, brain (in the case of carbs) and fat cells) you will need to produce more of that hormone. Here’s the thing: your body is really bad at doing two opposite things at once. Insulin puts you in “storage mode”. While you are in storage mode it is hard to mobilize stored energy.

This means that as long as insulin is elevated you will be really ineffective at taking fat out of the fat cells to use it for fuel. If you are insulin resistant, you produce more insulin when you eat, and when you produce more insulin, it takes a lot longer to come back down.  As a result, the period during which you are good at mobilizing fat is a lot shorter. In the end, you will use a lot less fat for fuel daily, making it harder to lose and easier to gain.

To get back to cortisol. Cortisol releases glycogen from the muscles (and liver) and turns it into blood glucose. This is great if you are training because you will be able to use that for fuel, but what if you are inactive? Blood sugar levels go up and stay up. Since your body doesn’t want elevated blood sugar, it will release insulin to lower it.  If cortisol is always elevated during the day, your blood sugar levels are constantly elevated and you will need to pump out insulin all the time to regulate it.  Then, if insulin is always elevated what happens? You get desensitized to it! In other words, you become insulin resistant!

Understand though that I’m talking about people who constantly have high cortisol levels; those who have a very stressful lifestyle, are undergoing severe personal issues or both.  Cortisol release in the proper amount at the right time is a good thing, whereas constantly elevated levels is not.

– Cortisol can decrease the level of sex hormones. See, testosterone, estrogen and cortisol are fabricated from the same “mother hormone”: pregnenolone. If you overproduce cortisol you have less raw material to produce these sex hormones. An easy sign of that happening is a marked drop in libido. Sex hormones are key for body composition. Testosterone is of course directly anabolic (increases muscle-building), but a healthy estrogen level also helps build muscle by elevating IGF-1 levels.

–  Finally, cortisol can lead to an increase in water retention due to its effect on elevating the anti-diuretic hormone (vasopressin) and aldosterone. Sudden bloating can be a good sign of elevated cortisol levels. For example, if a few hours after your workouts you look less defined and are holding water it is likely that you overproduced cortisol during your workout by doing too much.

When you look at it this way, fasted cardio sure looks evil, but not so fast! If you are someone with a very stressful lifestyle and are noticing symptoms of constantly elevated cortisol levels (lack of libido, water retention, soft/flat muscles) then yes, fasted cardio might make things worse. However, if your cortisol levels are normal it should not be a problem at all.

The type of cardio you use also plays a role. High intensity fasted cardio (intervals for example) might not be a good idea because it relies more on glucose for fuel and will raise cortisol more. A low-intensity workout is a better option. YES, you will burn less calories but as we will see that’s not what fasted cardio is for.

Fasted Cardio: The Good

If you look at the studies comparing fasted cardio and cardio done in a non-fasted/post-absorptive state you will see either no difference in the amount of fat burned, a non-significant (not likely to make a difference) advantage for fasted cardio or a non-significant (not likely to make a difference) disadvantage for fasted cardio.

In other words, if you look at the number of calories and fat grams burned during fasted vs. fed cardio there is likely not a significant difference.  But as I said earlier that’s not why I think fasted cardio can be useful.

A very recent study (Yung-Chih Chen et al. 2017) found that when cardio is performed fasted, it increases the level of several enzymes that are responsible for the mobilization of fat and it’s use for fuel.

For example. fasted cardio increases PDK4 more than fed cardio. This enzyme increases fat oxidation (fat use for fuel) and decreases glycolysis (burning glycogen for fuel). So, it not only increases fat loss but it is also glycogen-sparing.

It has also a great impact on short-form hormone-sensitive lipase (HSL) and ATGL which increase the breakdown of stored fat so that it can be liberated from the fat cells (first step toward using fat for fuel).

It also affects how well muscle can uptake fatty acids (to use them for fuel) via the CD36 protein.

It is thus my belief that fasted cardio’s main benefit (over fed cardio)is not an increase in fat loss but rather lies in its capacity to program the body to rely more easily on fat for fuel.

This is especially interesting for those who have a hard time losing fat despite training and eating well; they are likely very efficient at using stored glycogen for fuel and inefficient at mobilizing and burning fatty acids instead. For these people, fasted cardio might be a great way to re-program their body to rely more easily on fat for fuel.

What Can Happen To Inefficient Fat Mobilizers?

If a lousy-mobilizer (someone with an important fat accumulation problem who has a very hard time losing it even when dieting and exercising) starts training for fat loss using the “cool methods of the day”: Crossfit, loaded carries, sprints, prowler pushing… intense stuff that works great for many people, they are likely to run into problems.

Their fat loss will stall really quickly because they are inefficient at mobilizing fat and they are undertaking a type of training (high intensity) that relies mostly on glucose for fuel, so they never improve their capacity to mobilize fat.

They do lose fat at first because the activity represents an important caloric expenditure, but they crash very quickly (especially if they are in a caloric deficit). They can’t focus, they have huge carbs cravings, they are lethargic, moody, and depressed. Eventually, they fall off the wagon and pig out, often never to come back to their plan.

Why? Because they:

  1. Are lousy at using fat for fuel
  2. Need to rely more on glucose
  3. Have reduced their carbs intake (thus reducing glucose availability)
  4. Have selected a way of training that relies on glucose

As a result, they run out of glucose and since the brain can only function on glucose or ketones, they mentally crash.

Someone who is good at mobilizing fat can avoid that problem. Why? Because by being good at using fat for fuel allows you to spare more muscle glycogen for the brain and even if you run out, being good at mobilizing fat allows you to more easily produce ketones for your brain. The poor fat mobilizers might not be able to do this which is why fasted cardio is a great tool for these people.  While it may not directly lead to more fat loss, it’s a great way to reprogram their body to be more efficient at using fat for fuel, which will make all their future efforts more effective and easier mentally.

What Type Of Fasted Cardio?

As discussed, the goal of fasted cardio is not to directly burn more calories. Rather, it is a programming tool to make your body more efficient at mobilizing fat and using it for fuel. The more efficient you are in that regard the more fat you will burn over a 24h period.

As such, we also want to employ a type of cardio that will not stimulate your body to rely on glucose for fuel. The higher the intensity, the more reliant on glucose your body will be, especially if you are inefficient at using fat. So, doing high-intensity cardio is not the right method.

What you should be doing is low-intensity work (as low as 100 and as high as 120-125 bpm), basically a brisk walking pace. So what if you aren’t burning lots of calories? That’s not the goal!

Furthermore, lower intensity also means less cortisol release which greatly reduces the potential downside of fasted cardio.

I find 30 to 60 minutes to work best. For me, that is either the treadmill on a slight incline or walking my dogs for an hour.

Again, don’t see it as a direct fat loss tool. See it as an investment that will make all your future fat loss efforts much more effective.


Yung-Chih Chen, Rebecca L. Travers, Jean-Philippe Walhin, Javier T. Gonzalez, Francoise Koumanov, James A. Betts, Dylan Thompson. “Feeding Influences Adipose Tissue Responses to Exercise in Overweight Men,” American Journal of Physiology – Endocrinology and Metabolism Published 14 March 2017 Vol. no. , DOI: 10.1152/ajpendo.00006.2017

– CT