Written by Garett Reid | NSCA, CSCS, CISSN, M.S.E.S.S
Fact checked by Kirsten Yovino, CPT Brookbush Institute
FACT CHECKED
Fasted cardio is one of the more recent magic bullets for weight loss to hit social media similar to intermittent fasting. Basically, fasted cardio is when you run while fasted (aka HUNGRY or on an empty stomach), which makes you burn more fat due to lack of available glucose. If this were accurate, it would be pretty awesome. However, things are rarely as good as social media makes them out to be. It’s not always a total lie either. So, where does fasted cardio lay on the spectrum of science-backed and backed by bullshit?
In this article, we’re going to give a fair analysis of what research shows concerning fasted cardio. In this article, you’ll learn:
- What is fasted cardio?
- How fasted cardio works
- Benefits of fasted cardio
- Fast cardio explained
- Does fasted cardio burn fat?
- What type of cardio should you use for fasted cardio?
Let’s get right into this. Just how good is fasted cardio?
What Is Fasted Cardio?
Fasted cardio is a method of training, generally for fat loss, where you perform cardio after an extended period of fasting. Now, this doesn’t mean you just wait a few hours after you eat lunch, but rather a minimum of at least 8 hours is recommended; perhaps even longer. However, being in a fasted state has more to it than just not eating as it actually relies on physiological processes to have taken place, mainly two:
- Your body will shift further towards fat for energy due to a lack of available glucose
- Insulin will drop in the body, which will also encourage fat to be used for energy. High glucose levels encourage your body to hold onto fat making it more difficult to burn.
The theory is that during these fasted states, your body will utilize fat for fuel due to lower insulin levels and a decrease in glucose availability which has been shown in studies to occur1. As far as this is concerned, the theory definitely has science to make this claim appropriate. However, it’s not as simple as that (we’ll get more into this later.)
With this in mind, fasted cardio is often prescribed by fitness influencers and personal trainers to clients for faster weight loss. While they mean well, and the science does seem to back this claim, there’s more to meaningful fat loss than just looking at one hour of the day.
Exposing Fasted Cardio
In this day and age, getting information from social media is playing with fire. On the one hand, there is quite a bit of solid information on Youtube and Instagram. However, there’s a lot more bad information and they are relentless with their crap-throwing. To be fair, some of these accounts are truly just ignorant and misinformed (if this is the case, they should know they aren’t informed enough to have a channel, but I digress). However, a large portion knows they aren’t being totally honest, whether that’s just telling a bald-faced lie or bending the truth. This is just dishonesty so you need to be careful with who you follow.
As stated above, there is evidence to make the claim that your body will utilize more fat for fuel when exercising in a fasted state. However, fat loss doesn’t happen from that one hour session. In order to lose weight, you must look at what occurs during the whole day. And this is where the entire fasted cardio for weight loss starts to fall apart.
What you need to understand is that our bodies are always using both fat and carbs for fuel at all times. In fact, when you are resting, your body actually already burns more fat for fuel than glucose (carbs). However, when we start to increase the intensity of our exercise, our bodies begin to increase their energy needs. Therefore, our bodies will start to use more carbs for fuel as they’re easier to process. Basically, when we place a higher demand on our body for fuel, we will start to use the easier source, which is glucose.
All this being said, while your body may use more fat for fuel during a fasted state, it will compensate and burn more carbs later in the day (see science selection below). Or if you only eat fat, it will just burn fat. Or, if you only eat protein, your body will use protein. Understand that if your body needs energy, it will get energy from somewhere and your body using fat for fuel is not a unique experience only seen in fasted workouts.
Burning Fat For Fuel Vs Burning Fat For Weight Loss
The point being is that even if you only ate fat and burned fat (like keto), if you’re eating a ton of calories, you’re going to gain weight. One of the problems is that people seem to confuse “burning fat” with losing weight as they don’t fully understand that fat is actually already our body’s primary source of fuel for low-intensity activity. You need to distinguish between two different processes as they are not necessarily the same:
- Burning fat for fuel which occurs in a fasted state or after prolonged exercise
- Burning excess fat by eating less calories than you intake.
Still, it doesn’t really matter how much “fat you burn” if you’re eating a ton of calories. While the phrase “calories in, calories out” is a bit of an oversimplification, the gist is still accurate and explains why fasted cardio won’t just magically melt fat off your body. Your body will use fat, glucose (carbs), and even amino acids (protein, muscle) for energy.
Therefore, as long as you’re providing a surplus, you will gain weight. If you’re in a deficit, you’ll lose weight. It doesn’t matter what source your body is using for fuel!
As of now, assuming calories are equated for (more on this below), studies show that fasted workouts do not provide any extra benefit for fat loss compared to working out in a fed state. However, if one diet can help you get into a caloric deficit easier, you could say it’s “better” for you.
What Does Science Say About Fasted Cardio?
So above should give you a better idea of why fasted cardio for fat loss isn’t as legit as many would have you believe. Now, we can further back these statements and look at what studies say.
Let’s first explore the idea of your body using fat and carbs evenly throughout the day. In fact, this process was shown in a study from 20112. In a quick summary, two groups either ate before a run or they ran while fasted. They found that while the fasted group did burn more fat during exercise, they ended up burning more carbs throughout the remainder of the day. The opposite was seen in the fed state. This demonstrates that assuming your macro intake is the same, your body’s choice for fuel will even itself out throughout the day.
This makes sense as, again, while the body will use fat for fuel, it also uses carbs in the form of glucose. Therefore, if you don’t eat carbs before a cardio workout and save it for after, you’re just providing an increased amount of glucose later in the day, which will lower the amount of fat you use. It really is a balancing act.
A study in 2014 had two groups of female volunteers train in either a fasted state or fed state 3 days/week. At the end of the study, they found that while both groups of women lost a significant amount of weight, neither group did better than the other. It’s important to note that both groups had a similar loss of fat3.
Interestingly, there aren't actually a lot of studies that examine fasted training and fat loss even though it has become so popular. However, in 2017 a meta-analysis was performed, which resulted in five studies that examined fasted training and fat loss. They concluded that training in a fasted state did not produce superior results when compared to training in a fed state. They sum up their findings with the following quote.
“These findings support the notion that weight loss and fat loss from exercise is more likely to be enhanced through creating a meaningful caloric deficit over a period of time, rather than exercising in fasted or fed states.”4
And that’s that.
Benefits Of Fasted Cardio:
We know we just told you that fasted cardio doesn’t quite work the way some would have you believe, but that doesn’t mean it’s totally useless. In fact, it’s not useless at all. Here are some awesome reasons you may still want to include fasted cardio in your training.
1) May Create A Caloric Deficit
You’re probably thinking, “what the H-E-Double Hockey Stick!” as we literally just got done saying that fasted cardio doesn’t burn more fat than cardio in a fed state. However, as alluded to above, weight loss depends on what happens throughout the day rather than just a segment.
This study from 2019 found that when a group of men skipped breakfast before exercise (an overnight fast), they ate 400 fewer calories in a day when compared to when they did eat breakfast before cardio5.
Still, this group also ate on average, 150-200 fewer calories a day when they didn’t exercise! What this suggests is that while fasting cardio may not help you with burning body fat by the way it’s advertised, it can still be a useful method due to creating a caloric deficit.
2) Saves Time
You may think this is reaching but once you are able to go get a successful cardio session in without eating, you’ll know what we’re talking about. While this may not be a huge issue for some, we find it to definitely be convenient.
3) May Increase Body Fat Utilization Over Time
One of the supposed benefits of fasted training is that it can improve your body's ability to utilize fat stores for fuel, which is great if you want to maintain muscle mass as you burn off stored fat.
However, studies actually show this to not be the case. That being said, we have experienced this ourselves as well as seen clients and other athletes do this same thing. Basically, the claim is that over time, a person will be able to increase exercise duration and intensity while fasted.
We want to be clear, studies show this to not be the case BUT we have seen too much anecdotal evidence to say it doesn’t happen. That being said, take this with a grain of salt. (We told you that we will always be honest). But, if this were to be true, this could help improve performance, decrease cravings, and mitigate protein breakdown.
That being said, this likely requires the gradual increase of intensity, distance, or duration to improve.
On top of the above benefits, you have the regular fat burning benefits of cardio, such as burning calories to lose fat, improving body composition, and decreasing body fat percentage.
Fasted Cardio: Our Analysis
While we agree that fasted training is not the magic bullet that many claim, we still don’t find it completely useless, as others say. This is due to the several studies that do show benefits along with the countless number of personal friends and clients who have been successful on it when traditional dieting has failed. Further, we ourselves have found it to be an effective tool.
On the other hand, we realize that studies don’t show this to be the case, but we feel that one of the reasons is due to dieting in a lab is not the same as dieting by yourself in the real world. Regardless, this is one of those areas where studies don’t seem to completely line up with what we have experienced. Take that for what it is.
It’s important to know that while these studies didn’t show any extra benefits, fasted cardiovascular exercise was just as successful and equal in improvements as training in a fed state.
Basically, the whole “doesn’t hurt to try” scenario. Still, we’d like to see more studies done in real-world situations where the participants are left to their own vices and have to choose what and how much to eat. Again, the one study that did this showed that when one engages in a fasted morning workout (no breakfast), they tend to eat substantially less (400 calories).
While not the most exact answer, we feel this is really the honest take without being biased towards any one side. In a nutshell, if fasted cardio works for you, then definitely do it. Further, if you have never tried fasted cardio, we would guess you would likely be successful with it, whether that’s due to you getting into a caloric deficit easier or some other magical mechanism we have not discovered yet.
One thing to note: Fasted cardio is probably best for those following intermittent fasting. If you're doing a 3-day fast, you may want to take it easy on cardio and perform walks instead.
How To Maximize Fasted Training to Lose Weight:
Wherever you stand on the fast cardio debate, there are several things you can do to improve the effectiveness of fast training.
1) Use Caffeine With Fasted Training
Caffeine is not just part of your favorite AM beverage (actually, we drink coffee whenever now). It also happens to be one of the most studied and effective ergogenic aids (or supplements that can provide performance benefits). One of these benefits is that caffeine has been found to increase the body’s ability to oxidize fat (break down fat cells).
A Meta from 2020 reviewed 19 studies that examined the effect of caffeine and fat oxidation. They concluded that when fasted, taking caffeine before exercise can cause an acute increase in fat oxidation compared with what is normally seen6. However, there are a few caveats to improving its effectiveness.
- Doses of at least 3.0g/kg of body weight must be taken
- Caffeine must be pure with no calories, such as black coffee
- The fasting period should be at least 5 hours. However, this will need to be extended based on the size of the meal
- The caffeine dose should be taken 30-75mins before training.
- Training should consist of low-intensity aerobic exercise
- Caffeine prior to aerobic exercise produces greater results in an untrained population.
One of our personal favorite methods is to wake up first thing in the morning and pop some caffeine pills.
2) Perform Fasted Workout With Low-Intensity Aerobic Exercise
When you do choose to perform a fasted workout, you should only use low-intensity aerobic exercise. In fact, even things like HIIT would produce suboptimal results.
The reason being is that when you perform high-intensity exercise, your body requires glucose in order to supply ATP quickly. Without this, you won’t be able to produce an adequate amount of energy to support an effective workout. Therefore, not only will you not get optimal benefits from the training, your training will be of less intensity which would result in fewer calories burned which defeats the point.
Therefore, the best use of fasted cardio would be to perform low-intensity cardio, especially at first. We’re talking about a brisk walk or incline treadmill work. As you become acclimated, you could increase the intensity to about 70-80%HR max.
Cardio On an Empty Stomach Wrap-Up: A Mix Of Science & Anecdotal Experience
Like many magic bullets spread around the internet, fasted cardio is anything but. That being said, it seems to have fallen victim to a common trend in the science-backed fitness community. That is, there can sometimes be an extreme backlash against the aforementioned concept, in this case, fasted workout, because it’s not as effective as previously thought or because it doesn’t seem to be as effective in a lab setting.
At SET FOR SET, we definitely lean towards research but believe there’s also room for the anecdotal experience to be considered, especially when that experience is personal and numerous. Therefore, while we do recommend fasted cardio workouts for individuals looking for a (possible) effective fat-loss method, we also want to be clear in expectations and what’s likely actually occurring. We believe it can be a useful tool to use if you understand the correct mechanisms, that is it seems to work by restricting a person’s total caloric intake.
At the end of the day, compliance is going to be the most effective tool. Meaningful fat loss doesn’t happen overnight and requires months of dedication. With this in mind, even if fasted cardio was as good as your favorite influencer says it is, it doesn’t mean a thing if you can’t adhere to it. The same goes for dieting with regular calorie control or any other method you see.
As of today, we aren’t aware of any dieting method that is superior to another, assuming the calories are controlled for. Therefore, if you find one method that works better with your lifestyle, go for it. In fact, we believe this is one of the reasons there are so many diets out there that are the best. People just assume what works for them will work for everyone else. It might, and it might not. Fasted workouts might work for your buddy but not for you. Or it might just be the best thing ever.
Related:
References:
- Horowitz JF, Mora-Rodriguez R, Byerley LO, Coyle EF. Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise. American Journal of Physiology-Endocrinology and Metabolism. 1997;273(4):E768-E775. doi:10.1152/ajpendo.1997.273.4.e768
- Paoli A, Marcolin G, Zonin F, Neri M, Sivieri A, Pacelli QF. 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. 2011;21(1):48-54. doi:10.1123/ijsnem.21.1.48
- Schoenfeld BJ, Aragon AA, Wilborn CD, Krieger JW, Sonmez GT. Body composition changes associated with fasted versus non-fasted aerobic exercise. Journal of the International Society of Sports Nutrition. 2014;11(1). doi:10.1186/s12970-014-0054-7
- Hackett D, Hagstrom A. Effect of Overnight Fasted Exercise on Weight Loss and Body Composition: A Systematic Review and Meta-Analysis. Journal of Functional Morphology and Kinesiology. 2017;2(4):43. doi:10.3390/jfmk2040043
- Edinburgh RM, Hengist A, Smith HA, 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;149(8):1326-1334. doi:10.1093/jn/nxz018
- Collado-Mateo D, Lavín-Pérez AM, Merellano-Navarro E, Coso JD. Effect of Acute Caffeine Intake on the Fat Oxidation Rate during Exercise: A Systematic Review and Meta-Analysis. Nutrients. 2020;12(12):3603. doi:10.3390/nu12123603
Garett Reid
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