Fasted Cardio Or The Thermic Effect Of Food?


Loosing Excess Body Fat

Frequently, I come across people telling me that fasted cardio is absolutely awesome if you are trying to lose body fat. In fact, some people rave about it so much, it’s become like a religion for these people in how they manage their cardio (and for some coaches who insist that your cardio should be done in a fasted state).

It also makes for some great fitness marketing as well, especially if you’re a fat loss guru. For some people who are on the path to lose weight, they are convinced that it’s the holy grail for their weight loss efforts. It’s a must in their cardio program. People have the idea that by exercising on an empty stomach (fasted), the body will immediately tap into it’s fat stores and you will begin to strip away those extra pounds. Behold, increased fat loss! I wonder though, is fasted cardio really that great and the answer to accelerating fat loss? Is it really just a case of not eating before you do some cardio?

What The Science Has Said

Ok, so by now you guys are probably aware that in my articles, I don’t like to bombard you with too much scientific literature, I don’t want you guys feeling like it’s homework or make simple concepts sound harder than they actually are. But, since hearing so much recently about the apparent benefits of fasted cardio, I decided to do some digging into the subject. Simply to see if I was missing something. 

So alot of people who follow fasted cardio like a religion go by this idea (this is why it’s so appealing! But really only on paper): not eating before your cardio session will cause your body to shift away from carbohydrate utilisation to fat stores for energy production. It has to do with the fact that by not eating, your glycogen stores are depleted, insulin levels are low, genes involved in fat oxidation and transport are up-regulated and lipolysis (breakdown of fats into their basic building blocks) is stimulated.

The thing is, most of this is actually just theoretical and very few studies have been able to back any of this theory up. We really don’t know how fat loss is impacted over a long period of time when non-fasted cardio is switched out for fasted.

Brad Schoenfeld (you might know him online as the hypertrophy specialist) made a very good point in saying that you can’t look at fat burning on an hour-to-hour basis but more a day-to-day, week-to-week, month-to-month basis. Fat burning is too complex a process and highly dynamic, that is regulated constantly through many different physiological processes. Not to mention, your bodies utilisation of fat at one given time period, will be compensated for through increased carbohydrate utilisation later in the day.

The bottom line is that your body is constantly adjusting it’s use of fat for fuel. Therefore, in the context of fat loss and body composition, one exercise session will tell you very little, if anything meaningful about how fasted cardio might impact fat utilisation over time.

If you look solely at the science that has been conducted over the last few years, then it seems that there is no statistically significant differences in losses in bodyweight or fat mass over a short period of time between those that underwent fasted and non-fasted cardio. Yes, both groups were found to lose bodyweight and fat mass over the course of observation, but then they were both kept in a calorie deficit throughout the entire duration of observation.

Given the fact that there were no major differences in weight loss between those that underwent fasted cardio and those that underwent non-fasted cardio, it seems that the changes occurred simply through the good old calories in/out principle (shifting energy balance in favour of weight loss).

Check out this study which gives a nice collection of experiments and explanations over fasted cardio. The Schoenfeld et al, 2014 paper does well to explain this topic in further detail. 

The bottom line:

Whether you perform fasted cardio or non-fasted cardio, you will loose weight, as long as you are in a calorie deficit. The extent of fat loss between those that performed fasted and non-fasted cardio was not significant enough to support the potential ‘increased fat burning effects’ of fasted over non-fasted cardio. At this point in time, shifting energy balance through calories in/out manipulation is your strongest tool in loosing body fat. 

Intramuscular Triglycerides (IMTG)

There are a couple of other things mentioning regarding fasted cardio and that is something known as intramuscular triglycerides. Sounds complex but essentially they are just fat stored within the muscle fibres themselves. They can provide approx 2/3 of the energy  of muscle glycogen. This is completely different to subcutaneous fat (found just underneath the skin – that which influences body composition), which is why I find it interesting.

Brad Schoenfeld in his review paper over fasted cardio highlighted a few previous studies and one that found that in trained individuals, 120 minutes of moderate intensity training would draw most of it’s energy from fat (approx. 80%) from these IMTGs.

The point here is that yes, you might be increasing fat utilisation during your cardio, but is it the right kind of fat? Probably not by the looks of it. If you are wanting to look better, then it’s subcutaneous fat you need to reduce. Simply utilising these IMTGs during cardio will have no bearing on subcutaneous fat reduction and thus an improvement in body composition. From the looks of it, subcutaneous fat reduction will come through a maintained calorie deficit, while cardio will simply target mostly these IMTGs (and little subcutaneous fat).

Even if science does finally catch up and firmly prove all this heightened fat oxidation, gene expression, lipolysis stuff etc. does happen during fasted cardio, are we talking here about oxidation of subcutaneous fat or merely the IMTGs in muscle tissue? Who knows, it’s interesting stuff! (Can read more in Brad Schoenfeld’s 2011 review article).

Muscle Tissue

Let’s not forget about muscle tissue. No one likes to lose muscle tissue. The only way to improve your body composition is through losing fat and building/maintaining muscle tissue. So, losing muscle tissue pretty much goes against that goal!

Interestingly, in Brad Schoenfeld’s review on fasted cardio, studies have found that fasted cardio might not be very muscle sparing. Not great if your goal is to preserve or even build muscle mass (which you also need to keep the calorie burning furness going!).

Well, these studies have shown that training in a glycogen depleted state (in comparison to a glycogen loaded state), there is a significant increase in NITROGEN LOSSES. Well, that’s really not good news!

Simply put, nitrogen is an essential component in amino acid production and if amino acids can no longer be made, then it becomes to difficult to preserve and build new muscle tissue. So fasted cardio might be something to look out for if you are trying to hold onto that hard earned muscle! (Again, you can read more in Brad Schoenfeld’s 2011 review article). But again, any studies that show this tend to be short-duration in nature. We simply don’t know what would happen over the longer-term. Even if you do partake in fasted cardio, any losses in nitrogen through it, should be offset if your nutrition is consistent, on point, you supplement with plenty of protein, your daily calorie intake (thus energy balance) is in line with your goals and, you have a well planned training program.  

Exercise Quality

Finally and well, maybe pretty obvious is that people generally have the ability to train more intensely if they eat something before they exercise. I have observed this myself and with many other people that if you have something before you train (whether cardio or weights), your ability to push yourself harder and longer is just heightened.

What follows? Well, if you can do more work during your training sessions, then you will expend more energy (in terms of burning more calories, great!) and in the grand scheme of things, that will make it easier to maintain your calorie deficit. No, eating before a workout won’t blunt your fat loss efforts, but it will give you the extra energy to use that training session to burn some extra calories, which in turn will only serve to aid your overall fat loss efforts (Again, for a little more insight on the studies, please check out Brad Schoenfeld’s 2011 review article).  

Fasted Cardio Has A Few Potential Problems:

Firstly, if it does increase the oxidation of fat, what fat is being predominantly oxidised? If it’s IMTGs in muscle tissue, then it will do nothing for improving body composition. If it’s subcutaneous fat, then great! But at this time, we don’t have a clear cut answer.

Furthermore, fasted cardio is likely to make it more difficult to maintain or build muscle tissue due to potential nitrogen losses. However, the magnitude of this effect will depend on how much fasted cardio you do, your diet, protein intake and your training program.

Lastly, fasted cardio might make it more difficult to have an intense training session, reducing the number of calories you potentially burn overall. This can make it difficult to maintain an overall calorie deficit depending on other factors (such as nutrition, the magnitude of your calorie deficit and how often you train). This might make your fat loss efforts harder. 

The Thermic Effect Of Food, Maybe Fasted Cardio Is Not The Answer!

The thermic effect of food (TEF) is pretty simple to understand, it essentially says that when you ingest food, your body must utilise energy to break that food down, transport it as well as absorb it. All these processes require energy. Since this requires energy, your body is going to be expending calories. Seems pretty appealing right, eating more to burn more calories? Sounds pretty crazy when you talk about it in the context of fat loss. Well, let’s not get ahead of ourselves to quick! So it’s pretty well understood that when you eat something, your body burns calories to process it. The more you eat (more calories you consume), the magnitude and duration of the TEF will also increase. But, how does this relate to exercise?


Interestingly, it has been demonstrated that the TEF is actually greater in magnitude (both following low- and high-intensity exercise) than the TEF measured after just a meal (and no exercise following). Furthermore, it was found that high-intensity exercise resulted in an greater and sustained elevation in TEF in comparison to the TEF after low-intensity exercise and resting conditions (for greater insights into the study please check Kent et al, 1992). This is great news as an increased TEF means more calories potentially burned post-prandial exercise (meal just before exercise). Now, If you are wanting to loose weight, and need to maintain a specific calorie deficit to make that goal happen, then an increase in calorie expenditure, through an increase in TEF following exercise, can certainly make that process much easier.

Moreover, in another study using a glucose/milk beverage, it was found that the post-exercise oxygen consumption (basically your body keeping energy consumption high after exercise to return your body to homeostasis) following low- and high-intensity, to be significantly higher in those participants that consumed a glucose/milk beverage before exercise over those that did not (fasted exercise) (For more insights please check out Schoenfeld et al, 2014).

The Bottom Line:

Having a meal before exercise might increase the TEF following exercise, allowing for potentially more calories to be burned (in comparison to those that exercise fasted). This will aid in increasing overall calories burned during the entire day to help those to maintain a calorie deficit needed for their weight loss goals.

Pre-Exercise Food And TEF To Aid In Fat Loss?

If you look at these TEF studies, it lends support to the idea that fasted cardio might not be your best bet if you are looking to loose weight. Not because eating food before exercise has any direct influence on fat regulation, oxidation, transport etc, but simply because it helps increase ‘calorie burn’ after exercise through raising your TEF (in comparison to a fasted state). The result being that an elevation in calorie expenditure by the body after exercise (but also during exercise due to having more energy overall), helps increase total calorie expenditure throughout the day.

From a fat loss perspective this is great because it will help you maintain the calorie deficit you need, to meet your weight loss goals. Fasted cardio is attractive because people believe that it has direct effects during exercise itself on fat breakdown. While attractive on paper, this is really still to be seen. The evidence is just not there at the moment.

When it comes to TEF, the increase in calorie burn from exercise after a meal, seems attractive if you are trying to maintain a calorie deficit. While it’s pretty obvious that eating something before you exercise makes for a better workout (and thus more calories burned during it!), the attraction of also potentially burning MORE calories after exercise (with a pre-exercise meal) makes that far more appealing, and beneficial than fasted cardio. Especially from a long-term perspective. 

It’s All About Calories In/Out

Even when you look at all this research, there is still one thing that matters for your weight loss goals: energy balance (calories in/out). If you want to loose weight, then you need to  maintain a calorie deficit through taking in less calories than your body consumes.

The attraction of fasted cardio is that the increased fat oxidation during exercise is still to be seen, and at this time, we know that there is no real difference in fat loss between individuals who perform fasted vs. non-fasted exercise. The only difference is that you are more likely to suffer from poor workouts without any energy in a fasted state (for no extra benefit!). This is likely to reduce output during your fasted exercise reducing potential calorie burn during. But, what we do know is that eating a meal before exercise can help increase energy expenditure (calorie burn) after exercise (to what extent and in what magnitude still needs to be further investigated), but, it’s clear that there is an effect. An effect that might help you maintain a calorie deficit and thus aid in your fat loss efforts. Maybe then, having one of your planned meals before exercise isn’t that bad after all. Maybe something for the followers of fasted cardio too try?

Any questions, ask away! 


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this:
search previous next tag category expand menu location phone mail time cart zoom edit close