Some people swear by fasted cardio for fat loss. Others think it's a bunch of nonsense. If you want to know the truth (based on facts) read this article...
Ah, fasted cardio.
Hated by all, used by some in the hopes of losing fat faster, but often dismissed by bodybuilders as “too catabolic” to be worth it.
Yep, the fitness community is pretty divided on whether or not fasted cardio is really a practical fat-loss solution. It’s one of the few issues that even some of the most notable experts disagree on.
Some say it’s the secret to stubborn fat-loss. Some say don’t bother.
So who’s right?
Well, that’s what we’re going to discuss in this article. You can skip to the bottom line if you want, but if you really want to level up your fitness knowledge, you may just want to take a couple minutes to read the whole thing.
Once you know the facts (based on what the research says), you’ll be more equipped to decide whether fasted cardio is really for you.
The truth is that–like most things in life–there are pros and cons to consider.
Most articles you read on fasted cardio focus only on one or the other, but that’s no way to help you make an informed decision!
A fair amount of research has been conducted on fasted cardio–specifically, how it relates to fat loss–so the facts are right there. All we need to do is take a look…
In this article we will:
- Define Fasted Cardio
- Examine The Research Behind It
- Discuss The Pros and Cons
- Introduce Solutions To Mitigate The Downsides (While Preserving The Upside)
Once you’ve read this article, you’ll know everything there is to know about fasted cardio, including whether or not you should be working it into your routine.
What Is Fasted Cardio?
If you ask someone from your gym what fasted cardio is, the most likely response is going to be:
“Fasted cardio just means cardio on an empty stomach”
But that’s actually not true.
Your stomach is certainly empty when you do fasted cardio, but an empty stomach is not what defines it.
You’re in a fasted state when your Insulin levels are at baseline (low). This occurs when you haven’t eaten in several hours.
You see, when you eat a meal consisting of carbohydrates, protein, and fat, those nutrients get broken down.
- Carbs into glucose.
- Protein into amino acids.
- Fat into fatty acids.
These nutrients are then released into the blood stream where they’re met by Insulin.
Insulin’s job is simply to send those nutrients into muscle and fat cells where they’re either used for energy or stored for later.
When you eat a meal, your Insulin levels rise. As the nutrients you’ve just eaten are pushed into cells, Insulin levels begin to decline.
Insulin does more than just escort the nutrients you consume into cells though. It also signals the body to stop burning fat and start storing it.
When your Insulin levels are high, you’re not burning much fat.
When you’re Insulin levels are low (like while you’re sleeping), your body turns to fat for energy.
Here’s a visual representation:
As you can see, when you eat meals throughout the day, you’re Insulin levels rise and decline and then drop to fasting levels at night (while you’re asleep).
- When Insulin levels are high, you store more fat.
- When Insulin levels are low, you burn more fat.
The basic premise of fasted cardio is this:
Since your Insulin levels are low, you burn more fat with fasted cardio than you would if you did your cardio in a fed-state (having eaten recently).
Sounds good, but is it actually true?
Does Fasted Cardio really help you burn fat faster?
The idea that fasted cardio is ideal for fat-loss is definitely rooted in science. Our basic physiology suggests it.
Glucose is the preferred source of energy for muscles. It’s quick. It’s easy. That’s what your body likes to run on.
Fat can be burned for energy as well, but it’s not the preferred form of energy for muscle.
Your body only turns to fat as an energy source when there’s not enough Glucose (stored as Glycogen).
When you eat a meal, you have plenty of Glucose to be used as energy. When you haven’t eaten in a while though–and your Insulin levels are low–your body must turn towards fat for its energy needs.
Insulin levels low = more fat burned
But is it really that simple?
Even if fasted cardio increases the amount of fat you burn during exercise, that doesn’t necessarily make it a practical solution for long-term, sustainable fat-loss.
Let’s stop speculating about what fasted cardio might do, and take a look at the actual research.
The Research on Fasted Cardio and Fat Loss
If you Google “fasted cardio” and click on a few different results, you’ll quickly find out that the fitness community is split right down the middle when it comes to whether fasted cardio is beneficial for fat-loss or not.
Some say it helps you burn fat faster. Some say it’s not all it’s cracked up to be and you shouldn’t bother because the downsides outweigh any potential benefits.
It may seem strange that people who normally agree on matters of physical fitness are so divided on an issue where there’s plenty of data from which to draw conclusions, but it’s actually the data itself that poses the issue.
So that’s it, right?
Fasted cardio = fat loss. Case closed.
Not so fast!
It doesn’t matter if fasted cardio increases fat-oxidation during exercise unless that increase leads to a sustained, measurable decrease in body fat over time.
Does that mean they’re effective fat-burners? No!
Rather than looking at studies which measure fat oxidation during single bouts of exercise, we need to look at longer-term studies where fat loss from fasted cardio is measured over time.
The good news is that there have actually been a few studies published on this very topic.
The bad news is that some studies have reached different conclusions than others.
Pundits of fasted cardio as a fat-loss strategy tend to point to a 2012 study published in The Journal Of The International Society Of Sports Nutrition which found that subjects who performed fasted cardio lost more fat than those who did their cardio in a fed state.
What gets left out, unfortunately, are the multiple failed studies that showed no such benefit.
In a 2013 study published in the Journal Of The International Society Of Sports Nutrition, researchers tracked the body composition changes in a group of 18 bodybuilders during Ramadan (when they fast throughout the day).
Some of the participants in the study worked out during the day in a fasted state. Some of the men worked out at night after they ate.
Body composition measurements were evaluated throughout the study as well as at the end.
No meaningful difference whatsoever in terms of fat-loss or body composition between those who trained fasted and those who trained fed.
A year later, another group of researchers conducted a similar experiment…
Rather than measure fat oxidation rates in the short-term, the researchers in this study measured body composition over a period of 4 weeks.
The subjects were divided into 2 groups who followed the same hypo-caloric (negative energy balance) diet and exercise program. Some of them exercised fasted and some exercised after eating.
Total calorie intake was the same though.
As you would expect, after 4 weeks on a reduced calorie diet, all the subjects lost some fat. But the subjects who did their cardio fasted hadn’t lost anymore fat than the subjects who trained in a fed state.
What we have here are a bunch of studies which show fasted cardio can increase fat oxidation during exercise (in the short-term) but it just doesn’t add up to meaningful fat-loss in the long-term.
The Truth About Fasted Cardio And Fat Loss
Fasted cardio increases fat oxidation more than cardio in a fed state, but that doesn’t mean it’s a highly effective fat-loss strategy in the long-term.
While the notion that fasted cardio accelerates fat-loss seems logical, studies have failed to find any significant benefits with regards to body composition or fat-mass when measured after several weeks.
Fasted Cardio may tip the fat-loss needle in your favor, but it’s just not the miracle fat-loss solution so many people are a making it out to be.
When it comes to burning fat and losing weight, it’s not just a matter of how many calories you burn during your workouts. That’s where people get confused.
It’s possible to increase the amount of calories you burn all day long, even way after you’re done working out, depending on how you train.
For example, eating a meal prior to working out has actually been shown to increase 24 hour Resting Energy Expenditure (an indication of fat utilization).
This could very well explain why, despite fasted cardio increasing fat oxidation during exercise more than cardio in a fed-state, it doesn’t lead to additional fat-loss over time.
If fat loss is your primary objective, the timing of your meals doesn’t really matter. What does matter is how may calories you consume in a day and how many you burn.
The law of energy balance still applies.
In other words, if you want to lose fat fast, you need to place yourself in a caloric deficit.
Whether you’re training fasted or not, you’ll lose fat if you’re taking in less calories than your burning day after day, week after week.
Look at it like this:
If you and your friend both cut your daily calorie intake by 10%, but you do your cardio fasted and he does his cardio non-fasted, it’s unlikely that you’ll be noticeably leaner than him.
In fact, losing fat relatively quickly isn’t even that hard as long as you:
- Place Yourself In A Caloric Deficit (10-20% is fine)
- Train Hard (Almost Everyday)
- Take The Right Supplements
Don’t get me wrong…
If you just want to achieve a lean physique (say 7-8% body fat) though, it can be done with or without fasted cardio.
Of course, there are some other reasons why people might choose to do fasted cardio, even when fat-loss isn’t a primary concern.
other benefits of fasted cardio
We’ve established that fasted cardio may be slightly capable of increasing the amount of fat you burn during exercise, but probably won’t lead to meaningful fat-loss in the long-term, all else constant.
But that doesn’t necessarily mean it’s not worth doing (for some people).
Fasted Cardio Improves Insulin Sensitivity
In a 2010 study from the Journal of Physiology, researchers sought to determine if fasted cardio would influence Insulin sensitivity (Glucose tolerance) in subjects fed a high fat diet, compared to subjects on the same diet but training in a fed state.
Not only did they note an increase in fat-oxidation (as expected), but subjects also experienced significant improvements in Insulin Sensitivity.
The term Insulin Sensitivity refers to the degree of Insulin secretion needed to shuttle the nutrients you consume into cells.
- If you’re Insulin Sensitivity is high, you’re body doesn’t need to produce as much Insulin to get the job done.
- If you’re Insulin Sensitivity is low, more Insulin must be secreted.
So, those at risk of developing Insulin Resistance (which is an alarmingly high percentage of the population of the US) could stand to benefit immensely from doing some fasted cardio.
Fasted Cardio Enhances The “Anabolic Response”
In another study, subjects who trained fasted experienced a heightened anabolic response to post-workout nutrients (carbs/protein/leucine), compared to subjects who trained in a fed state (carbs prior to exercise).
These results suggest there may be favorably training adaptations associated with training fasted. Whether those adaptations make much of an impact on body composition in the long-term remains unclear.
There is certainly a positive correlation between Insulin Sensitivity and being lean though, and enhancing the anabolic response may promote muscle-building.
So it make sense that fasted cardio could contribute to a favorable body composition over time due to improved Insulin sensitivity, but we’re talking over many years.
In the meantime, you have to consider the downsides to fasted cardio…
The Downside to Fasted cardio
Wow, so fasted cardio/training sounds great! Slightly more fat-loss, better Insulin sensitivity, and a more pronounced anabolic response to food?
Why would anyone NOT train fasted?!
Because fasted cardio–or any kind of training in a fasted state–poses 2 issues.
If you’re body runs out of glycogen for energy, it doesn’t just turn to fat right away. It starts to harvest amino acids from muscle tissue as well (i.e. muscle breakdown).
Fasted cardio accelerates muscle loss.
If your goal is to gain muscle, that’s not good!
Another negative aspect of fasted cardio is that you’re most likely going to perform worse than you usually would.
That means less endurance whether you’re running or lifting.
Taking in carbs before or during you exercise improves performance, so if you’re fasted, you just won’t perform as well.
These major drawbacks are enough to make the average person say “forget about it!” and go load up on carbs before they workout.
If you do:
- you’ll be stronger
- have a bigger pumps
- get more sets
And ultimately, get bigger and stronger faster than you would be able to with fasted cardio.
If you want to get bigger, you have no business doing fasted cardio.
Possibly burning a little extra fat may sound great, but not at the expense of losing muscle.
Fortunately, there are some things you can do to reduce muscle loss and perform better, without spiking your insulin levels and breaking the fast.
How To Limit Muscle Loss From Fasted cardio
There’s no denying the drawbacks to fasted cardio.
Compared to eating a nice solid meal 1-2 hours before training, you’ll lose more muscle and you won’t hit the numbers you normally would in a fed state.
Does that means it’s not worth doing though?
Now that we’ve identified the issues that fasted cardio poses, let’s discuss ways to minimize them…
Eat a high protein diet
Research has consistently demonstrated that the absolute best way to maintain lean muscle mass is to eat a high protein diet.
Although a lot of bodybuilders will tell you that “you need to eat every 2 hours to maximize muscle growth”, keep in mind that these guys are all on steroids.
Steroids alter many of the physiological reactions in the body, including how much muscle protein synthesis can occur.
For normal people (who aren’t on steroids), the timing of your meals doesn’t really matter. What matters is how much protein you’re taking in on a daily basis.
High protein diets are ideal for maintaining muscle mass, especially during a caloric deficit.
How much protein you need depends on:
- your level of physical activity
- your personal muscle-building goals.
If you hit the gym several times a week, and you want to actually BUILD muscle, you need to take in anywhere from 0.9-1.1 grams of protein per pound of bodyweight.
During caloric restriction (as would be the case if you’re trying to lose fat), you should take in at least 1.1-1.3g of protein per pound of body weight to maintain muscle mass.
If you’re trying to minimize the muscle loss that generally occurs with fasted cardio, upping your protein intake throughout the day is a must.
It’s also worth mentioning that higher protein intake (1.5g/lb/day) has been shown to improve cognitive function, so just about anyone can benefit from eating more protein.
It’s a good idea even if gaining muscle isn’t you’re primary goal!
High Intensity Interval Training (HIIT)
High Intensity Interval Training (HIIT) is a form of short-duration exercise where you alternate between periods of intense activity and low effort activity.
You sprint for 30 seconds, then walk for a minute or two. Sprint for 30 seconds, then walk. Sprint, then walk.
You get the idea…
You may have heard that steady-state cardio is better for fat-loss because your body tends to use carbs for high intensity activities and fat for low intensity activities.
That’s why many bodybuilders walk 5 miles an hour on the treadmill and call it “cardio”.
Perhaps the most clear-cut example of this was a 2011 study from the University Of Ontario in which subjects performing 4-6 30 second sprints on treadmill 3 times a week lost significantly more fat than subjects performing 60 minutes of steady-state running on a treadmill.
That’s right, 3 minutes of HIIT 3 days a week resulted in more fat loss than 60 minutes of stead-state cardio.
Scientists have yet to fully understand why HIIT is so much more effective for fat-loss, but the research is quite unanimous…
High Intensity Interval Training burns fat faster and more efficiently than any other form of cardio.
Research also indicates that it results in far less muscle breakdown than steady state cardio. So, if you’re looking to maximize fat loss while keeping muscle mass, HIIT is the way to go.
Of course, any kind of fasted cardio will always:
- increase the amount of muscle you lose
- cause you to perform at least slightly worse
But there are some others ways to minimize muscle loss and improve performance (without breaking the fast).
SupPlements you should take for fasted cardio (AND WHY)
The truth is, most supplements are pretty useless…
Supplements alone won’t give you the body you’ve always wanted, but there are some that have been scientifically proven to help.
When it comes to fasted cardio though, there’s a few supplements that you should definitely use.
Leucine/BCAAs to preserve muscle
The Branched Chain amino Acids consists of:
Of course, this is assuming the correct dose…
Since Leucine is the most potent of the three BCAAs when it comes to stimulating muscle protein synthesis, you want to ensure that your BCAA supplement provides an adequate dose of Leucine.
3-5g of Leucine prior to your fasted cardio sessions will help negate the muscle loss that typically occurs with any kind of fasted training.
In other words, Leucine won’t break your fast.
You’ll still burn more fat, while limiting muscle breakdown significantly.
Citrulline Malate to improve performance
Recall from earlier that training in a fasted state tends to negatively impact your performance in the gym.
This doesn’t mean you’re not gaining muscle. It’s just that when your body is low on glycogen (energy from carbs), you won’t be able to bang out quite as many reps as you can when your muscles have tons of glycogen to use as fuel.
Citrulline Malate has been shown to enhance muscular endurance, or more specifically, to increase the number of reps performed toward the end of the workout when muscular fatigue would normally be setting in.
It doesn’t make you superman, but the difference can be quite noticeable.
Of course, nothing is going to replicate the benefits of good ol’ carbs when it comes to fueling you’re workout, but if you’re trying to keep Insulin levels as low as possible (as is the case with fasted cardio), carbs are not an option.
Citrulline, on the other hand, won’t break your fasted state at all. In fact, there is sound research to suggest that Citrulline Malate may even help keep Insulin levels low during exercise.
Furthermore, Citrulline Malate has been shown to favorably influence branched chain amino acid utilization during exercise, offering yet another compelling reason to use both prior to fasted training.
Again, dosing is everything here. Most pre or intra-workout supplements that use Citrulline contain far less than what has been used clinically to enhance performance. Some only contain 1-2g!
You should ideally be shooting for no less than 6-8g of Citulline Malate prior to/during your workout.
It’s possible that lower doses of Citrulline are effective to some degree, and that’s what most companies that under-dose Citrulline in their products will tell you, but almost every study involving Citrulline Malate has been performed using 6g and up.
Why gamble on less when theres tons of evidence that 6-8 grams is all it takes.
Phosphatidylserine to keep cortisol in check
If you can’t pronounce that word, don’t worry about it…
Here’s what you need to know:
Although it is currently being studied for several potential applications, one of the most notable qualities of PS is its impact on Cortisol.
To be clear, we’re talking both physical and mental stress. Unlike most stress supplements which tend to interact with neurotransmitters like GABA to reduce brain activity in general, Phopshatidylserine reduces stress through an entirely different mechanism.
It blunts Cortisol-induced activation of the Hypothalamo-Pituitary-Adrenal (HPA) axis, a set of complex processes involving three glands–endrocrine, hypothalamus, pituitary, and adrenal–which work tegether to regulate the stress response.
The result? Less Cortisol is produced in response to stress.
Stress still occurs. It just takes less of a toll on the body.
Now, you’re probably wondering what this strange, hard-to-pronounce substance has to do with limiting the downsides of fasted cardio.
Well, it’s actually quite simple…
There are a lot of ways to lower Cortisol, including
With the exception of music, however, it would be difficult to work any of these things into your fasted cardio sessions.
Phosphatidylserine has been shown to reduce stress and Cortisol at doses of 200-800mg/day, with 600mg being the preferred standard.
Caffeine To Burn More Fat
Caffeine triggers the release of fat-burning hormones, called Catecholamines, including Dopamine and Noradrenaline.
When you exercise intensely, these chemicals are released endogenously (by the body), but Caffeine can increase them even more.
This is the same group of chemicals that is primarily responsible for the feelings of increased alertness and mental energy that Caffeine provides.
In addition to making you feel more energized, Catecholamines also initiate Lipolysis.
More Lipolysis = more fat-burning.
Consider taking 200-400mg of Caffeine prior to your fasted cardio sessions to accelerate fat-loss.
Rauwolscine To Burn More Fat
Rauwolscine is a close relative of the popular fat-burning supplement, Yohimbine. They share the exact same mechanism of action–alpha-receptor antagonism–except Rauwolscine is much more potent and less likely to cause side effects.
You see, your body has two types of receptors located in fat tissue which regulate Lipolysis (fat breakdown).
- Alpha receptors – block Lipolysis
- Beta-receptors – trigger Lipolysis
By antagonizing (blocking) alpha receptors, Rauwolscine allows for significantly more Lipolysis to occur than would otherwise normally be possible.
That means you’ll burn more fat during your fasted cardio sessions with Rauwolscine than you otherwise would without it.
The Best Time To Do Fasted Cardio
A common misconception about fasted cardio is that it must be performed first thing in the morning.
Most people find it easier to train fasted in the morning because they haven’t eaten for 7-8 hours at least and don’t necessarily feel hungry yet.
However, you can still eat breakfast and train fasted later in the day. It just might be a little unbearable for some who are used to eating shortly before lifting.
Personally, I get hungry after 2-3 hours , irritability and on-edge after 4-5 hours. So, if i do train fasted, it’s going to be first thing in the morning.
You need to allow 5-6 hours after eating for your Insulin levels to return roughly to baseline. So, if you eat lunch at noon, you can be back in a fasted state and ready to do your fasted cardio by 5-6pm.
You just might be really hungry.
The Bottom Line 0n fasted cardio
Doing fasted cardio can potentially tip the fat-loss needle in your favor, but by itself, it’s no miracle fat-loss strategy.
Don’t get me wrong…
I believe that fasted cardio is better than non-fasted cardio if we’re strictly talking fat-loss, but I’m not convinced that the pros outweigh the cons in the long-run.
The truth is, if you just:
- place yourself in a slight caloric deficit
- maintain a high protein diet
- lift heavy 4-5 days a week
You can cut a lot of fat and get that muscular/defined look without fasted cardio. It’s something to consider if nothing seems to be working, but don’t expect a miracle.
If you want to give fasted cardio a shot, go for it. You have nothing to lose…except muscle. But again, that can limited.
Just don’t suffer through fasted session after fasted session under the delusion that it’s the only way to achieve a lean, shredded physique. It’s not…
ANy experience with fasted cardio? Anything you disagree with on? FEel free to comment below…
I’m Matt, Founder of Momentum Nutrition and SuppWithThat.com. I’ve spent the better part of the last decade researching and experimenting in the areas of fitness, nutrition, and supplementation, and this is where I write it all down.