Fitness myths die hard. In fact, fitness myths are like zombies. They just keep coming back to life no matter how much you kill them. For example, despite the overwhelming evidence against the carb/insulin hypothesis of fat gain, and despite how the evidence against it continues to accumulate, people still keep raising this hypothesis from the dead.

The use of fasted cardio to enhance fat loss is also a common belief in the fitness world where evidence has been accumulating against it. While the evidence against it may not be as overwhelming as the carb/insulin hypothesis, there really isn’t any evidence for it to justify its use.

Still, a lot of people swear by fasted cardio. The thing is, not only is there no evidence to support it, it doesn’t even make sense to use it when you examine it much more closely. What I’m going to illustrate to you is that there is no evidence-based mechanism behind how fasted cardio could plausibly enhance fat loss. While there’s other articles on the web that argue against the use of fasted cardio, many don’t go into the mechanistic reasons why it won’t work.

How We Lose Body Fat

To illustrate what I’m talking about, we first have to talk about how we lose body fat in the first place. We lose body fat when we create an energy deficit, i.e. we expend more energy than we take in via food. Since you’re not supplying enough food energy to the body to meet your daily energy needs, it needs to pull the energy from somewhere else. In this case, it pulls it from your body fat and protein stores. Now, ideally, we want it to pull everything from our fat stores, and maintain our protein stores (like muscle). However, for illustration’s sake, let’s assume that 90% comes from fat and 10% comes from lean mass like muscle. Now, let’s say I create an energy deficit of 500 calories per day. Of those 500 calories, 450 are coming from body fat and the other 50 are coming from lean mass. If I continue with this deficit on a daily basis, I will lose body fat over time (and a little bit of lean mass as well).

For Fasted Cardio to Enhance Fat Loss, It Has To….

Now, let’s take the scenario I just described. You’ve got this 500 calorie per day deficit (450 coming from body fat), and let’s say you’ve been doing it through a combination of diet and fed cardio. You decide you want to do fasted cardio instead of fed cardio because you think it will enhance your fat loss. Well, if fasted cardio is going to boost your fat loss, it has to do it through at least one of these three mechanisms:

Enhance energy expenditure. In this case, you boost the size of your deficit by increasing your energy expenditure while eating the same amount of food. Let’s say your deficit now goes from 500 calories per day to 600 calories per day. Now, you have 540 calories coming from fat (90%) and 60 calories coming from lean mass (10%).
Decrease energy intake. In this case, you boost the size of your deficit by decreasing your food intake, but keeping energy expenditure the same. In other words, the fasted cardio somehow suppresses your appetite. You now eat 100 calories less than you normally would, increasing the size of your deficit from 500 calories per day to 600 calories per day. Now, like #1, you have 540 calories coming from fat and 60 calories coming from lean mass.
Enhance fat loss and preserve lean mass for the same energy deficit (a tissue repartitioning effect). In this case, your deficit remains at 500 calories. However, more comes from fat and less comes from lean mass. Let’s say it’s now 95% and 5% (475 calories from fat and 25 calories from lean mass).
These are the only three hypothetical scenarios where fasted cardio could enhance fat loss over fed cardio. The question now becomes as to if any one of those 3 things actually happens when you do fasted cardio. Fortunately, we have scientific research to tell us.

Does Fasted Cardio Enhance Energy Expenditure?

The answer on this is no. When researchers compared the 24-hour energy expenditures of 60 minutes of fasted versus fed cardio, there was no difference.

The chart above shows the 24-hour energy expenditure for the two conditions in this study. Fasted cardio did not enhance energy expenditure, and thus this cannot be a mechanism behind how fasted cardio could enhance fat loss. Thus, we can cross it off our list.

  1. Enhance energy expenditure
  2. Appetite suppressant effect
  3. Repartitioning effect

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