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.

Enhance energy expenditure Appetite suppressant effect Repartitioning effect

Does Fasted Cardio Have an Appetite Suppressant Effect?

The answer is no. In fact, this study found that feelings of appetite were lower with fed cardio. Also, ad libitum calorie intake was lower over 24 hours after fed cardio versus fasted cardio.

Thus, we can cross it off our list.

Enhance energy expenditure Appetite suppressant effect Repartitioning effect

Does Fasted Cardio Have A Repartitioning Effect (Enhanced Fat Loss and Less Lean Mass For A Given Deficit)?

The primary reason people choose to do fasted cardio is that you burn more fat during exercise compared to fed cardio. The problem with this line of thinking is that it's short-sighted. First, the fat you burn during exercise isn't important for fat loss. Otherwise, you would never lose body fat doing things like interval training, which rely primarily on carbohydrates for fuel. Yet, we know you can lose just as much body fat through carb-burning exercise like interval training as you can with more steady-state fat-burning exercise, as long as the energy deficit is the same. It's not the type of fuel used during exercise that's important; rather, it's the energy deficit that's created over a 24-hour period.

Second, the increased fat oxidation during fasted cardio doesn't tell you where the fat is coming from. Half of the fat you burn during steady state cardio comes from intramuscular triglycerides (fat droplets stored inside your muscle tissue), NOT body fat. Also, the more you train, the more your body will rely on intramuscular triglycerides. Finally, exercise increases the oxidation of dietary fat consumed after the exercise session is over. This is similar to how a low-carb, ketogenic diet enhances the burning of dietary fat for fuel, but doesn't enhance fat loss if the energy deficit is similar to a high-carb diet. Thus, there's no evidence that doing fasted cardio will enhance body fat loss, even if there's a temporary increase in fat oxidation. If anything, fasted cardio will simply enhance your utilization of intramuscular triglycerides and dietary fat for fuel.

Third, while fasted cardio may enhance 24-hour fat oxidation as shown in the following graph, it does so by sparing carbohydrate, NOT protein or lean mass.

Basically, what is happening is that, when you do fasted cardio, you reduce your muscle glycogen (carbohydrate stored in your muscle tissue) by around 18% (this amount will vary depending upon the duration and intensity of the cardio). Any carbs that you consume over the rest of the day will go to replenishing that muscle glycogen rather than being burned (that's why you burn less carbs over 24 hours as shown in the above graph). Since your body is storing carbs in this case, it's considered to be in a "positive carbohydrate balance". But it's important to remember that the carb stores in your body are limited. Over an extended period of time (days and weeks), your body cannot continue to store more and more carbohydrate; it can't be in a positive carbohydrate balance forever because there's a limit in how much it can store. Thus, your body will adjust over time by increasing the rate at which it burns carbs and decreasing the rate at which it burns fat. This is why we need to concern ourselves with what happens over an extended period of time; even what happens in the 24 hours after a session doesn't tell us the whole story.

When it comes to body fat loss over time, we don't care whether fasted cardio spares carbs; we care whether it spares protein. The research has shown that fasted cardio does not have a protein sparing effect, whether that's through examination of protein oxidation rates or through urinary nitrogen (if you lose body protein, it shows up as nitrogen in your urine).

In fact, if you continued to do fasted cardio over time, and your body didn't adjust its carb and fat burning rates, and you weren't consuming enough calories and carbs to replenish the muscle glycogen you lost during fasted cardio, then you would eventually reach low muscle glycogen stores over days and weeks. Once you reach around a 50% reduction in muscle glycogen, your body starts to burn more protein for fuel. This is not what you want if you're trying to lose body fat and maintain muscle.

Thus, the evidence doesn't support a tissue repartitioning effect, at least not the one that we'd prefer. Therefore, we can cross if off our list.

Enhance energy expenditure Appetite suppressant effect Repartitioning effect

The Numbers Don't Make It Worth It

Now, let's assume for a moment that there is some sort of tissue repartitioning effect of fasted cardio, and that the increase in fat burning actually comes from more body fat loss. If you look at the chart above, there was a 112 calorie difference in fat burning over 24 hours between fasted and fed cardio. Let's assume 50% comes from intramuscular triglycerides, and the other 50% comes from body fat. That's 56 extra calories from body fat, which is 6 grams of fat. If you did fasted cardio 5 days per week, that's 30 grams of body fat per week. To lose one extra pound of body fat (454 grams), you would need to do fasted cardio for 60 minutes per day, five days per week, and 15 straight weeks. That's hardly something I would consider a fat loss advantage, especially since it could easily be wiped out by an increase in appetite and calorie intake. You could achieve better results simply by increasing your daily energy expenditure with more training, and/or reducing your calorie intake further.

Real World Results

Of course, all of this mental masturbation doesn't matter as much as real life results.

The question is, what happens when we take groups of people, put them on fasted or non-fasted cardio programs, and look at fat loss? Fortunately, I collaborated on this with Brad Schoenfeld, and we published our study in 2014. We took 20 young females and had them do 1 hour of fasted or non-fasted cardio, three days per week for 4 weeks. We found no significant differences in fat loss between the groups.

Now, granted that our study was limited in sample size and duration (only 4 weeks). However, this is not the only study to show no difference in fat loss between fasted and fed cardio. Another 6-week study showed no differences in fat loss between fasted versus fed interval training.

Do What You Prefer

The fact is that there's no reason to do fasted cardio to try to enhance fat loss. There's no good evidence to support it mechanistically, and the numbers indicate you wouldn't get any meaningful fat loss advantage out of it even if an advantage existed. The current randomized controlled trials, while they have their limitations, also don't support its use. The bottom line is that you should simply do the type of cardio that you prefer.