Fasted cardio is a type of training people swear by for fat loss. Personal trainers will prescribe it, athletes embrace the feeling, while some bodybuilders see it as essential for competition prep. But is exercising on an empty stomach actually legitimate? We’ll take a look into the science to see…
What is fasted cardio?
As the name suggests, fasted cardio is all about exercising in a fasted state. You’ll often find people tend to do it early in the morning before they’ve eaten breakfast. But, as with anything, some athletes like to break stereotypes…
With intermittent fasting on the rise, fasted cardio can be done at any time. The only requirements are that whoever is training is in a fasted state.
The whole idea behind fasted cardio became popular in 1999. Bill Phillips, a bodybuilder, and author of the book Body for Life coined the term as a way of accelerating fat loss. Since then, it’s become as popular as popping pre-workouts among other things…
But, what made Bill thing blazing into a crisp morning afoot a pair of Asics was a good idea?
Well, the method behind his madness was that your glycogen stores ran low while you sleep. So, when you got up and headed out for a run, you’d grab all your energy from unwanted fat stores.
He also made a point of mentioning that fasted cardio lowers insulin levels. According to Phillips, this is a good thing as it increases fatty acid breakdown. We’ll get round to whether this is true or not in a short while.
Then you’ll be able to decide if you’re down for interval rows without a dose of PB on toast first.
Because the only way to find out if fasted cardio is worth the stomach rumble we’ll inspect… the science.
Fasted cardio theory
Now, fasted cardio isn’t exactly split into two hungry camps. However, there are two distinct theories about why you should do it.
Diminished glycogen theory
The first one we’ll look at is the idea of diminished glycogen stores. Essentially, the thought goes that after a period of fasting, especially sleep, the energy stored in your muscles is lost.
For those interested, this glycogen is generated and then shelved when glucose goes unused. Imagine eating a Snickers bar, only using half of the sugary calories and depositing the rest for later. Some might also end up as fatty acids and saved as adipose tissue.
All sounds good, right? Empty the tank and tap into the fat stores to fuel an air-dyne bike dash?
Sorry to disappoint, but it’s all false economy. Believe it or not, your glycogen stores don’t really alter much while you sleep. This is because although you’re not snacking, you’re still at rest. While you’re still and only occasionally turning you’re not burning much energy at all.
So, what does that mean for fasted cardio? Essentially, you’ll probably still have glycogen in your muscles to use. Therefore, this exact theory about having to use fat stores is untrue.
Fasted cardio prioritizes fat burning theory
The second theory is that fasted cardio encourages the body to burn fat over carbohydrates. This is due to insulin levels being lower in the morning, which means they won’t get in the way of lipolysis (fatty acid breakdown).
Insulin is secreted into the blood when we eat carbohydrates. It helps our bodies breakdown sugars and utilize them for energy. All good so far…
Issues with insulin arise in the way that it inhibits the process of fat oxidation. This is when your body breaks down triglycerides into fatty acids and glycogen. Therefore, too much insulin means a lack of fat burning activity.
Biologists have confirmed this, which makes the theory actually true.
One study has shown that people who ingested a glucose-milk drink mid-cardio session saw a shift to prioritising carbohydrates. The test saw two groups separated into fed and unfed during a four-hour, steady cardio session. Intensity wise, we’re talking no greater exertion than 30%. 90-minutes in the fed group were given 200 g of glucose… The results were certainly interesting.
What they showed was that the fed group did make a shift to a carbohydrate seeking metabolism. Therefore, their bodies were prioritising sugar as fuel, and not fat . The control group, however, showed signs of increased fat breakdown in comparison.
Plus, a more recent study from Australia confirmed this. According to the team behind it, performing cardio in a fasted state does facilitate better fat oxidation .
This proves that fasted cardio does prioritize fat burning for energy. When it all comes down to a single session, saying no to a bite to eat beforehand WILL help you shift to a higher state of fat burning.
But it’s important to understand that this isn’t the book closed for the no breakfast crew. In fact, we’re only just getting started… There’s a lot more to fat loss than one single bout of exercise.
Is Fasted Cardio better for fat burning?
It’s true, fasted cardio can help you burn more fat in a single session. Yet, why aren’t we so sure it’s the best method to get lean?
It turns out that there are two sides to this story. Experts say that fat loss shouldn’t be looked at on a scale of a single session. Instead, it’s a long term endeavor carried out over days, weeks, and usually months.
This is where things get interesting. Because according to opposing research, it seems like the benefits of fasted cardio might become balanced out.
How do we lose body fat?
There’s scientifically only one way you can burn fat from your frame. It isn’t a magic pill, remedy, formula, or workout… but energy! Simply put, you must work through more calories than you eat.
We call this a calorie deficit, or if you want to be particular, a negative imbalance. See if like going into a store and buying a new pair of gym shoes. You’ve only got a fifty in your wallet but boy do you want those MetCons.
So, what do you do? Dip into your savings to fund the rest. It’s the same with calories, if you only took in that fifty today, you’ll have to take the remainder from fat stores. There is literally no other natural way of burning through unwanted body fat.
As you can now imagine, healthy fat burning doesn’t happen overnight. It’s a long game and success isn’t always linear. If all it took was one hard go at a set of burpees after skipping breakfast, we’d all be lean…
Fat loss from deficit, not just fasting
According to leading researcher, Brad Schoenfeld, it’s the deficit that does the deed. In a paper published during 2014, he showcased his results from his own study on fasted cardio for fat loss.
To assess the popular technique he took twenty healthy people. He then had them undergo a hypo-calorific diet to achieve a negative energy balance. As we just mentioned, this part was vitally important for any subjects to achieve a degree of fat loss.
Before they exercised, one group was given a meal replacement shake. The other, however, we left to exercise fasted. Note that throughout the whole process both groups are in a calorie deficit.
Schoenfeld’s results showed that both groups showed significant changes in body composition. Both the unfed and fed subjects experienced fat loss, which was the aim of the operation.
However, he also saw that there was no significant difference between the group’s results. This lead him to believe that whether or not you do perform cardio fasted is not important. Instead, ensuring you’re exercising your way into a calorie deficit is. Simply put, calories going in need to be less than those going out .
Fat burning higher after fed cardio
Another interesting factor to consider is this. Further studies into fasted cardio show that although it increases fat oxidation during a session, it slows later in the day. Whereas on the other hand, fed cardio can work in reverse.
In one study from 2011, researchers decided to inspect RER after both fasted and fed cardio. This is your respiratory exchange rate, which is essentially how much oxygen is used by your metabolism. If your RER is around .70 it’s a sign you’re using fat as your main fuel source.
What the scientists discovered that those who performed fed cardio over fasted had a lower RER later in the day. This indicated that they were, in fact, using more fat for fuel when compared to the fasted exercisers .
Note that both test groups featured fit and healthy young men.
So, should you do fasted cardio for fat loss?
So, what does this mean for you? In the simplest terms, it means the in-session benefits of fasted cardio might be balanced out. While yes you increase fat oxidation intra-workout with a fasted state, you boost it after in a fed state.
Long story short there’s pros and cons to both. It’s all down to your personal preference and what you like to do. Some people say fasted cardio gives them a clear mind and feels good. Others, well, let’s just say it has them chomping on their training shoes and stomach screaming like a hungry grindcore band.
If you’re concerned about exercise performance, we advise going in there fed. You’ll have more immediate energy on tap and you’re at a better likelihood of hitting a new PR. But if we’re talking solely about fat loss – why not just try it? See how you feel and if you like it, awesome. If not, you’re not missing out on anything superior. There’s science for athletes on both sides of the street.
Good luck on your journey. Keep that muscle mindset going strong.
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 Aird, T., Davies, R., & Carson, B. (2018). Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. Scandinavian Journal Of Medicine & Science In Sports, 28(5), 1476-1493. doi: 10.1111/sms.13054
 Schoenfeld BJ, Aragon AA, Wilborn CD, Krieger JW, Sonmez GT. Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr. 2014;11(1):54. Published 2014 Nov 18. doi:10.1186/s12970-014-0054-7
 Paoli, A., Marcolin, G., Zonin, F., Neri, M., Sivieri, A., & Pacelli, Q. (2011). Exercising Fasting or Fed to Enhance Fat Loss? Influence of Food Intake on Respiratory Ratio and Excess Postexercise Oxygen Consumption after a Bout of Endurance Training. International Journal Of Sport Nutrition And Exercise Metabolism, 21(1), 48-54. doi: 10.1123/ijsnem.21.1.48