A desperate dieter: “Hey, Egis (that would be me), I’ve heard that performing cardio in a fasted state burns more body fat than doing it in a fed state. Does this idea have any merit? Is this some nifty magic trick I could use?”

Me: “Ever heard about a zombie idea? A zombie idea is something that no matter how many times it’s killed, it just keeps rising up again. Fasted cardio is a zombie idea.”

You see, many desperate dieters believe that doing cardio first thing in the morning before morning “brekky” (that’s how my Australian client Claire calls breakfast) helps to burn more fat. But how does this idea holds up against the evidence?

Schoenfeld et al. have debunked this dumbass fitness claim in a 2014 study1 :

This is the picture of the first study that looks into fasted cardio versus fed state cardio for fat loss

They assigned 20 healthy young women to either a FASTED training group that performed exercise after an overnight fast or a FED training group that had a meal prior to exercise:

All females performed one hour of steady-state cardio three days per week to sweat enough to fill a lake and spoil all of their workout clothes. They were also prescribed a nutrition plan that put them in a 500 calorie deficit:

Since both groups were in a calorie deficit, both groups showed a significant loss of weight. However, over four weeks, no significant between-group differences in weight loss were noted. Researchers concluded that:

This is the picture of the first study's conclusion on fasted cardio versus fed state cardio for fat loss

A meta-analysis2 published in 2017 summed up the current research on the topic and concluded with:

This is the picture of the second study that looks into fasted cardio versus fed state cardio for fat loss

Researchers also added:

This is the picture of the second study that looks into fasted cardio versus fed state cardio for fat loss and its conclusion

2013 study3 also agrees with this by stating:

This is the picture of the third study that looks into fasted cardio versus fed state cardio for fat loss

So it appears that anyone telling you “do fasted cardio and ye shall achieve greater fat loss” is wrong. That’s because people misunderstand science.

Ever heard of the “fat oxidation” term? It’s often thrown around when people talk about fasted cardio.

Many people believe that doing cardio first thing in the morning before breakfast increases fat oxidation better than when you’re in the fed state. However, they’re confusing fat oxidation with fat loss just like they’re confusing nutrients with calories.

You see, fat loss is the process of fat mass decreasing whereas fat oxidation is the process of fat turning into energy.

For example, when you eat carbohydrates or protein, your body secretes insulin. As you may know, insulin inhibits fat oxidation which means less fat is available to be used as energy. However, this DOES NOT STOP fat loss from occurring because a negative energy balance reduces body fat irrespective of carbohydrate consumption4 and fat oxidation.

Another example would be a dumbass diet of extreme unsustainability called keto.

Keto zealots think they burn (oxidize) heaps of pounds of fat. Which they do. But what do you expect when you only eat fat and protein? Your body increases fat oxidation because it has all this DIETARY fat flushing in to be burnt off. But all this increased fat oxidation means jack shit for fat loss if you’re not in a calorie deficit5.

The same goes for fasted cardio. Not eating a meal before a cardio workout means that fat is the only energy source for your body to use. Many think this results in more fat loss but it’s just more fat oxidation6.

Not only that but there’s evidence7 showing that if fat oxidation is greater during any given time of the day (during cardio for instance), it’s compensated by greater carbohydrate usage later in the day:

This is the picture of the study that looks into fat oxidation perfoming fasted cardio versus fed state cardio

That is, even if you do burn more fat doing fasted training (not only cardio), your body can switch to burning more carbohydrates later in the day to cover its energy needs.

With that said, instead of worrying about how many calories and fat you burn doing cardio, start worrying about how many total calories you eat and burn every twenty-four hours. Fat loss is only governed by total energy in versus energy out over the course of the day/week/month.

The moral of the story is that if you’re doing cardio to lose fat, you may do so in a fasted or fed state depending on your personal preference. The evidence doesn’t support the claim that fasted cardio is better.

The time of day you work out and whether it’s done fasted or fed is secondary to how many total calories you burn. Schedule your workouts for the time that you’re most likely to enjoy it and stick with it.

Motivated to start losing fat and achieve the body you’ve always dreamed of? Then apply for my Online Coaching Program.

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