The notion that “fasted cardio burns more fat” dates all the way back to the 80s, and it has become ingrained in gyms all over the world.

Unfortunately, it’s not true.

This belief flashes back into the news every few years, the reason for the latest resurgence coming when Jennifer Lopez’s personal trainer described it as “the secret behind her great body” (ignoring the fact that J-Lo only recently started doing it and has been in shape for 25 years!).

So this comprehensive breakdown aims to put this myth to bed once and for all.

Let’s get stuck into the science, so I can show you why (despite it’s popularity) the belief that fasted cardio burns more fat is just another fitness myth.

WHY DO WE THINK FASTED CARDIO IS BETTER FOR FAT LOSS?

The theory behind fasted cardio is ROCK SOLID.

Our body burns carbohydrates for fuel, so by training early in the morning when there are very few carbs in our system, it has no choice but to use fat for fuel instead.

It makes perfect sense – in the absence of carbs your body has got to give you something to work with.

This is why we drag ourselves out of bed at F**k O’ Clock in the name of fat loss and start pounding the treadmill like angry mum storming her way to the shops with a faulty iron.

WHAT DOES SCIENCE SAY ABOUT FASTED CARDIO AND FAT LOSS?

Unfortunately, the theory above is wrong.

It turns out the human body is way smarter than we think.

A 2000 study published in the Journal of Applied Physiology showed us that performing fasted cardio actually causes no improvements in overall fat loss. It caused quite a stir at the time, because fasted cardio was still widely believed to be optimal for burning fat. (1)

However, since then we’ve had a mountain of new research re-confirming these findings.

One of the key studies to do so was Dr. Brad Schoenfeld’s huge meta-analysis on on the topic, which was published in the Journal of Strength and Conditioning Research in 2011. (2)

A meta-analysis involves researchers compiling the results of every study ever performed on a particular subject (in this case the topic was fasted cardio), to see if we can see a general pattern with results. This extensive analysis (which included a monstrous 33 clinical studies!) doubled-down on the previous research showing that fasted cardio is NOT better for fat loss.

Crazy, right?

It seems that burning fat for fuel in the gym doesn’t necessarily mean better fat loss results.

It’s a conclusion the American Council of Exercise have since re-confirmed again, and more recent studies have since! (3, 4)

Case closed…

… but WHY isn’t it better?

SO WHY ISN’T FASTED CARDIO BETTER?

It all comes down to energy balance.

I discuss energy balance in more depth in this post, but the crux of the matter is that being in a caloric deficit is BY FAR the most important factor in achieving weight loss – not which macronutrient we use for fuel during training.

Interestingly, it appears the body is capable of second-guessing us and adapting to its environment.

So while we will definitely tap into our fat stores during a fasted cardio workout, that doesn’t mean we’ll burn more fat over the course of THE ENTIRE DAY. It seems the body will burn more carbohydrates later on in a bid to restore the balance. Meanwhile, a guy who performs fed cardio will burn more carbohydrates during training, but his body will burn more fat throughout the day to keep the balance.

That means you can choose whichever approach you personally prefer – as long as you train consistently and keep your calories in line, you will see great results.

Make sense?

Great!

Also, it’s worth knowing that when I speak about fasted cardio, I’m generally referring to steady state cardio (a.k.a. aerobic cardio).

I’d never, ever recommend performing HIIT (high intensity interval training) in a fasted state, because the idea with HIIT is to perform it as explosively as possible so that your body can tap into your carbohydrate reserves – doing this in a fasted state would lead to sub-maximal performance and negate many of the benefits it offers. (5, 6)

SO WHY DOES IT KEEP BECOMING POPULAR?

The media have a lot to answer for on this one.

In their quest to gain popularity they often hail things as “magic fixes” (see also; a glass of red wine is better than an hour of exercise and eating small frequent meals speeds up your metabolism!).

Another part of the problem is bad science.

For example, a headline in 2019 added fuel to the fasted cardio debate by suggesting that “skipping breakfast before exercise leads to more fat loss.” (7)

When we dig deeper, we see that this study was so full of holes it could’ve doubled as a cheese grater! The trainees who lost more weight simply ate one less meal than the other group of trainees (!). These calories were never replaced, so we had one group consuming 300-400 calories less than the other group.

It’s crazy that this study made it all the way onto the news before anyone even looked at that, but this is the world we live in.

In terms of celebrity advice, I recommend taking it all with a pinch of salt.

J-Lo enjoys fasted cardio. Mark Wahlberg insists on training at 3:40am. Gwyneth Paltrow likes to put expensive rock diamonds in her hoo-haa.

Nine times out of ten they’re just following the advice of those around them.

References:

1. Febbraio M. A., et al. Effects of carbohydrate ingestion before and during exercise on glucose kinetics and performance (2000).
2. Schoenfeld B. Does Cardio After an Overnight Fast Maximize Fat Loss? Str Cond J (2011).
3. Hackney K.J., et al. Timing Protein Intake Increases Energy Expenditure 24 H After Resistance Training. Med Sci Sports Exerc (2010).
4. Paoli A., et al. Exercising Fasting Or Fed To Enhance Fat Loss? Influence Of Food Intake On Respiratory ratio And Excess Post Exercise Oxygen Consumption After A Bout Of Endurance Training. Int J Sport Nutr Exerc Metab (2011).
5. Wright D. A., et al. Carbohydrate feedings before, during, or in combination improve cycling endurance performance. J Appl Physiol (1991).
6. Schabort E. J., et al. The effect of a preexercise meal on time to fatigue during prolonged cycling exercise. Med Sci Sports Exerc (1999).
7. Edinburgh R. M., et al. Skipping Breakfast Before Exercise Creates a More Negative 24-hour Energy Balance: A Randomized Controlled Trial in Healthy Physically Active Young Men. The Journal of Nutrition (2019).

Who Is Russ Howe PTI?

Russ has been a personal trainer in the UK since 2002, and provided both training advice and full programs on this website since 2011.

His work has been featured in Men’s Fitness magazine, and the content on this website led to him being voted one of the world’s top 50 fat loss coaches by HuffPost.

Russ spends his time coaching men and women inside the legendary Powerhouse Gym, South Shields, and writing training tips for the 114,301 members of his popular free training e-mail (join it below).