Should you perform fasted cardio?

Should you perform fasted cardio?

Something that comes close to being universally accepted within the fitness community is the benefits of performing cardio in a fasted condition when getting ripped is the goal. According to proponents of fasted cardio nothing else comes close to this approach for burning fat, specially stubborn fat.

Fasted Cardio Theory

When we look back at the origins of the fasted cardio theory we can trace it to the writings of former Musclemedia and EAS owner, Bill Phillips who postulated that 20 minutes of fasted cardio was more effective than 60 minutes of cardio in the fed state. The reason Phillips gave which is repeated by those arguing in favour of fasted cardio is that when glycogen levels are low the body will be forced to rely upon its fat reserves to provide the energy to power cardiovascular activity. The theory states that as soon as you have a meal containing carbohydrates in particular you are providing the body with an exogenous source of fuel to power exercise activity meaning that there is less reliance on stored energy reserves such as body fat.

This certainly appears to make sense and the science behind the statements above is sound. By training in a fasted state your body will increase its utilisation of stored body fat when compared to if you had eaten breakfast. In other words, during your workout you will burn more body fat when you train fasted than when you first have a meal before your workout.

However, should we be focusing so much on the fat burned during training?

Another way to look at this is to consider that by providing some energy before your session you will improve performance during the workout and, at the same time you are providing carbohydrates at a time when your body is most efficient at using them, where insulin sensitivity and uptake into muscle cells is highest. What if training in a fed state led to higher performance over the workout which, in turn, led to greater burning of body fat during the rest of the day?

As you can see, far from being an open and shut case in favour of fasted cardio as so many bodybuilders would have you believe the scenario is more complex than commonly understood.

What does the research say?

A number of studies have been performed (1,2) which showed that carbohydrate ingestion prior to exercise reduced fat oxidation at a range of 25-50% of VO2 max. This is what we would expect although we must note these subjects were untrained individuals. However, research where carbs provided before training at higher intensities showed mixed effects. Coyle (3,4) demonstrated that in trained men who performed cardio at 65-75% of VO2 max the ingestion of carbohydrates did not reduce fat oxidation during the first 120 minutes of exercise. Horowitz (5) compared the effects of high GI carbs on moderately trained men who underwent either low intensity (25% VO2 max) or moderate intensity (68% VO2 max) exercise. The work produced similar results to Coyle’s. In the low intensity group fat oxidation did not drop below the fasted control group’s level of fat oxidation until the subjects had been training for over 80 minutes. In the 68% group there was no difference at all in fat oxidation between the fed and fasted groups.

So far it appears to be the case that so long as you take untrained individuals, performing cardio on an empty stomach leads to greater fat oxidation while there is no difference in fat oxidation seen in trained subjects. However, nothing is ever as simple as we would wish. A study conducted by Coyle (6) in 1997 showed that at 50% of VO2 max carbohydrate availability did, in fact, lead to less fat oxidation in trained subjects. The work by the same researcher which showed no effect on fat oxidation when comparing fasted and fed subjects may then be related to the higher intensity used (65-75% versus the 50% here)?

If that was so, a more recent study conducted by Wallis (7) sheds further light, or rather obscures the facts yet further. In their analysis they compared water versus glucose ingestion at a dose of 1.5g per minute in moderately trained subjects exercising at 67% of their VO2 max. Confounding the results that Coyle produced with trained men at this intensity zone, Wallis found that for both men and women carbohydrate ingestion during exercise inhibited fat burning.

So, the research on the subject of fasted cardio is open to interpretation with different studies producing different results when providing carbohydrates versus making subjects train in a fasted state.

All of these studies though consider only fat loss during exercise. This is where we run into a limitation. What happens after the training bout?

When we are looking at burning body fat what is the use of burning more body fat during training, if, on a 24 hr basis we end up in the same place? Put another way, fat stored versus fat oxidised over 24 hrs will determine whether you get leaner or not. Looking at fat loss from the point of view of any time period as short as a single workout is meaningless as so many variables (enzymatic, transcriptional and hormonal influences have a potent effect) determine the rate at which your body burns fat. Merely looking at fuel source is far too simplistic.

A recent review by Schoenfeld in the Strength and Conditioning Journal explored the topic of fasted state cardio. In addition to highlighting the importance of looking at fat balance (fat stored vs fat oxidised) Schoenfeld made the following points:

  • As a general rule if you burn more fat during training you will burn more carbohydrate afterwards. The reverse also applies so if you burn more carbohydrate during training you will burn more fat in the period after the workout.
  • Schoenfeld illustrates that high intensity exercise inhibits the ability of the body to burn fat during the workout due to diminished blood flow. This provides the basis for the fat burning zone advocates who maintain that cardio should be kept at a low intensity to allow for optimal fat loss.
  • In spite of the evidence above, numerous studies support the greater efficacy of high intensity cardio over the low intensity cardio popularly advocated by fitness professionals.
  • Schoenfeld reiterates the point already made that while low intensity cardio shows greater fat burning during the workout in untrained subjects, this difference evaporates once we examine those with considerable training experience under their belts.
  • Oxidation, NOT lipolysis is the rate limiting step when it comes to fat loss. That is, the body is able to liberate the fatty acids into circulation but its ability to oxidise, or burn, them off becomes the limiting factor. Schoenfeld shows how while lipolysis is sometimes higher in fasted cardio subjects, oxidation (the amount of fat burned) is the same. This implies that the fat not oxidised would be re-esterified and deposited back on to the body.
  • Interestingly enough, Schoenfeld points out that having a meal before training leads to a greater EPOC (post exercise oxygen consumption). In other words, more calories get burned after training when a meal is consume beforehand. While this sounds exciting the actual increase is relatively minor and amounts to around an extra 23 calories burned in the two hour period after training. Furthermore a review by Laforgia et.el (8) demonstrated that the impact of EPOC is relatively unimportant amounting to between 6-15% of the net oxygen cost of the exercise.
  • Schoenfeld highlights that during steady state cardio the percentage of calories which come from fat amounts to 40-60% of total energy expenditure. While that sounds promising, over half of this total comes from intramuscular triglycerides as opposed to free fatty acids. The latter is what we want to burn off to create a lean look but in untrained subjects at least, the majority of fat is burned is the former (9). . Intramuscular triglycerides are burned within muscle similarly to glycogen and in trained subjects an even greater proportion of fats burned during moderate cardio are from this fuel source. In endurance trained subjects the proportion of fats burned from this source can be as high as 80% (10).
  • Importantly, Schoenfeld highlights the fact that the burning off of IMTG’s has no bearing on your bodybuilding condition. Rather it is the fat stored in visceral fat and subcutaneous fat that has a bearing on how lean you are. While this is true, it must be pointed out that other research by Van Loon (11) showed that the contribution of intramuscular triglycerides was around 50% or fat reserves. Therefore while Schoenfeld’s assertion that IMTG’S being oxidised does not improve body appearance, it must be stressed that this would only apply if all the fat burned off during cardio was from these stores. This is not the case though.
  • A key point for bodybuilders to consider is the effect of fasted cardiovascular training on the body’s nitrogen stores. He quotes research from Lemon and Mullin (12) which showed that training while glycogen depleted (such as after a fast) led to more than double the rate of nitrogen losses compared to training after taking on carbs. After one hour of cycling at 61% of VO2 max it was estimated that the protein loss from the body was around 10.4% of the total caloric cost of the exercise. This is very worrying for trainees such as bodybuilders who are concerned with not losing any muscle mass if they can help it. While it does not follow that the loss in protein came from muscle protein exclusively it is nevertheless something which should be a concern. To offset this it would be prudent to supplement with BCAA’s or an essential amino acid product prior to training if performing fasted cardio. Although you would technically no longer be training fasted research (13) shows that it can actually lead to greater fat oxidation as well as improve performance with the former probably being related to the latter.
  • Schoenfeld rounds off his argument by stating that training while fasted will impair performance. While this is true for endurance training over a long period of time the evidence showing fasted training impairs performance does not hold up for shorter and higher intensity training such as weight training. The evidence from studies in this case is decidedly mixed. Furthermore, those who perform endurance training fasted consistently show positive adaptations to their training which leads to greater fatty acid oxidation (14).
  • Schoenfeld concludes by stating that training fasted will not only not produce better fat loss results, but possibly inferior results compared to training after a meal. In addition, he states training while fasted will lead to greater potential muscle loss.


So what can we take away from the evidence to date concerning fasted cardiovascular training? In one respect an analysis of the work to date demonstrates that all approaches can potential work when it comes to fat loss as the primary goal. If you currently employ fasted cardio to try to mobilise and then oxidise stubborn body fat there is nothing wrong with this approach. If you reject this and prefer to train cardio after a meal, there is evidence to support this also such as the slightly increased EPOC seen after a pre-workout meal.

For bodybuilders concerned primarily with appearance it would be wise to ingest some amino acids before fasted training to mitigate the potential for muscle loss which might otherwise occur. Ultimately though the single most important determinant of the success or otherwise of your plan will come down not to the 1-2 hours of cardio you perform but rather your ability to hit your daily macronutrient goals. In this respect we have to remember that studies performed over a short time period do not necessarily give the detail that those performed over a longer period of time where we consistently see issues of training and nutrient timing to be secondary to the overall workload used in training or the daily number of macronutrients ingested.

Of consequence to those performing fasted training to try to improve performance, the data shows that the body makes favourable metabolic adaptations in response to fasted cardio although performance may be hindered potentially in the short term. At this stage there is an absence of information on whether training fasted leads to greater improvements in subjects who then perform the same training in a fed state.

In our opinion, while the mythology behind fasted cardio training leading to greater fat loss can probably be rejected, those incurable fasted cardio fiends among you can safely carry on using this method if you wish. Ultimately, it comes down mainly to individual preference but at least you now know that the next time someone tells you that you need to perform fasted cardio to get lean you can see that they are wrong and that if you do not perform it you will not be missing out.


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2. De Glisezinski I, et al. Effect of carbohydrate ingestion on adipose tissue lipolysis during long-lasting exercise in trained men. J Appl Physiol. 1998 May;84(5):1627-32.

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4. Coyle, et al.. Carbohydrates during prolonged strenuous exercise can delay fatigue. J. Appl. Physiol. 59: 429-433, 1983.

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11. Van Loon LJ, et al. Intramyocellular lipids form an important substrate source during moderate intensity exercise in endurance-trained males in a fasted state. J Physiol. 2003 Dec 1;553(Pt 2):611-25.

12. Lemon PW, Mullin JP. Effect of initial muscle glycogen levels on protein catabolism during exercise. J Appl Physiol. 1980 Apr;48(4):624-9.

13. Gualano AB, et al. Branched-chain amino acids supplementation enhances exercise capacity and lipid oxidation during endurance exercise after muscle glycogen depletion. J Sports Med Phys Fitness. 2011 Mar;51(1):82-8 14. Van Proeyen K, et al. Beneficial metabolic adaptations due to endurance exercise training in the fasted state. J Appl Physiol. 2011 Jan;110(1):236-45

© 2012, Reggie Johal. All rights reserved.