In this article I wrote about a study that showed an anabolic rebound effect during feeding after fasted weight training. The title of that article ended with a question mark, since it was a (very) short-term study with results that were far from conclusive in regards to fasted training being superior to fed state training.

However, when it comes to endurance training, there’s compelling evidence showing that fasted state training is superior, or complementary, to fed state training.

Now, for those of you that have no interest in endurance training, bear with me. At the end of this article I will tell you how fasted state weight training may provide additional benefits, not discussed before, based on some new findings.

Fasted endurance training

One idea that has been tossed around in the endurance training community is that training sessions should be performed in the least performance-enhancing situation to ensure the most potent training stimulus. Training fasted, or under conditions with low muscle glycogen, could be superior to fed state training when it comes to inducing the fastest adaptations to training.

Two fairly recent studies has lent credence to this notion (De Bock and Nybo). While the researchers didn’t find any significant differences in some of the measured variables, it’s interesting to note that the fasted-trained groups in both studies showed higher levels of resting muscle glycogen concentrations after training. Similar to the anabolic rebound for fasted weight training, there seems to be an anabolic rebound during feeding after fasted endurance training through more efficient glycogen storage.

Looking at real world examples, the Kenyans, hailed for their superior endurance in running events, are known for doing a brunt of their training in the fasted state. They also follow a high carb diet to maximize muscle glycogen storage. According to experts, this pattern of “training low” and “competing high” might provide a distinct advantage. Muscles that are well stocked with glycogen can simply outwork the competition.

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The new study

Results of a new study on fasted endurance training was released just a few weeks ago. The primary aim here was to test the hypothesis that fasted state endurance training would yield greater improvements in fuel utilization and boost muscle glycogen storage efficiency. A hypothesis that was based on results seen in prior studies on this topic. The secondary aim was to see if the effects differed between genders, since men and women favor slightly different fuels during exercise. Men tend to utilize more glucose, while women tend to burn more fat.

This study lasted four weeks and had all subjects cycling 25 minutes at 65% VO2Max five mornings the first week. The duration was then increased by 25 minutes per week, so that subjects were cycling 100 minutes in the final week.

The cycling was either done in the fasted state or one hour after a cereal-based breakfast (1.5 g carbs/kg). In weeks three and four the fed group also received 30 g of maltodextrin during training. The fasted group received the breakfast, and the maltodextrin, after training.

  • Week 1, fasted: 25 min cycling followed by breakfast.
  • Week 2, fed: breakfast followed by 25 min cycling.
  • Week 4, fasted: 100 min cycling followed by breakfast and maltodextrin.
  • Week 4, fed: breakfast followed by 100 min cycling and maltodextrin.

With regards to the diet maintained outside the laboratory, weighed food records was collected to ensure that potential differences could not be explained by differences in diet. This was done pre-training and in the final week. The findings showed that calorie intake increased in both groups, with an increase coming mainly from carbs and protein. But no difference in total calorie intake or macronutrient intake existed between groups.

The results

After the study, the researchers summed up the improvements in a few relevant variables related to performance, muscle glycogen and fuel utilization. I’ll give you a brief rundown of what each of these variables means before showing the changes in the fasted and fed groups.

“The highest rate of oxygen consumption attainable during maximal or exhaustive exercise” (Wilmore & Costill, 2007). This is a rough measure of fitness.

Both groups started out with levels around 3.5 liters per minute (l/min), which is close to standards for untrained individuals. To put this into perspective, elite endurance athletes have about twice that capacity. One Norwegian skier topped this chart at 7.3 l/min. A more accurate measure of VO2Max is ml/min/kg, but in this study l/min was noted.

  • Fasted: +9.7% increase
  • Fed: +2.5% increase

The fasted group increased their VO2Max significantly more than the fed group. Interesting.

It’s also noted that “Whilst peak power increased in both groups, there was a strong tendency for FAST to improve their peak power more than FED”.

Muscle glycogen content: This is measured in millimoles per kilo dry muscle and shows how much glucose is stored in the muscle. The sample was taken from vastus lateralis, a portion of the quads, since this was the main muscle exercised during the cycling sessions.

  • Fasted: +54.7% increase
  • Fed: +2.9% increase

As you can see, the fasted group showed a dramatic increase in muscle glycogen content compared to the fed group. It’s almost too good to be true.

Citrate synthase (CS): This enzyme is critical for the initiation of the citric acid cycle, which regulates the mobilization of fat and converts glycogen into glucose for use during exercise. Think of it as a marker for fuel utilization efficiency.

  • Fasted: +17.9% increase
  • Fed: +19.1% increase

While the differences between groups, on average, did not show any significant differences, these appeared when comparing the results obtained from the women with those of the men. When this comparison was made, fasted training was found to stimulate significantly greater increases in CS in men (+35%) than in women (+10%).

On the other hand, fed training stimulated significantly greater increases in women (+25%) than men (+10%).

Men attained a more much better response from fasted training, while women received a more favorable response from fed training.

3-hydroxy-CoA dehydrogenase (HAD): Also a marker for fuel utilization efficiency, but this one is specifically involved in fatty acid metabolism. Think of it as a fat burning enzyme.

  • Fasted: +3.5% increase
  • Fed: +9.1% increase

As was the case with CS, the mean increase above is a bit misleading, since there were big differences in between fasted and fed groups depending on gender.

When looking at gender differences, females showed a stronger response than males (+5% fasted and +25% fed). This goes in line with prior studies which show that the HAD activity of female muscle is more responsive to the same training stimulus. Males in both groups showed only subtle change that was deemed non-significant (+3% fasted and -10% fed). However, fasted training seems to provide a slight edge once again.


Quoting straight from the discussion in the full text paper:

The main findings of the present study were that: training in an overnight-fasted state enhances storage of muscle glycogen compared to training in the fed state; skeletal muscle of men and women respond differently in terms of oxidative activity to training in the fed and overnight-fasted state; and peak VO2 and peak power improved more when training in the fasted state compared to the fed state.

Questioning the dramatic increase (+54.7%) in muscle glycogen in the fasted group, the researchers were not able to find an answer based on unexpected confounders or behaviors between groups. Muscle biopsies were taken at the same time and there were no difference in diet in between groups.

…it is highly likely that the differences in glycogen stores between groups reflect the training intervention and not exercise timing or pre-biopsy diet.

Moreover, these results are in line with a prior study that found similar results for fasted training.

Importantly, our findings correspond to that of De Bock et al. confirming that training whilst circulating CHO levels are low increases the capacity to accrue glycogen in the trained muscles.

What might be the reason for the different effects between genders on oxidative enzymes? As mentioned previously, differences in fuel utilization. Males rely less on intramuscular triglycerides and fatty acids and more on glucose, while females burn a higher percentage of fat at any given exercise intensity. But why fed state training would then be more beneficial for females when it comes to “oxidative adaptation” requires further investigation.

My thoughts

This study is great news for anyone doing fasted training, particularly the kind that involves elements of conditioning and glycogen depletion such as CrossFit, kettlebell training, PX90. Or just about any kind of endurance training. Based on feedback from readers and clients, it reflects my personal experiences in this area as well (performance enhancing effects of fasted state training with little or no dietary intervention).

I would expect the effects seen here to be similar for weight training, just of a much lesser magnitude. Traditional weight training doesn’t improve VO2Max nearly as much as the aforementioned activities, nor is it as glycogen intensive* – but still, boosting your ability to soak up carbs as glycogen should have benefits for nutrient partitioning and performance. Not to mention the ability to eat more carbs without triggering de novo lipogenesis (the conversion of glucose into fat).

* Some numbers: For an average weight male, 25 mins cycling at 65% VO2Max expends roughly 250 kcal. At 65% VO2Max, fuel utilization is half glucose and half fatty acids, so each session depleted about 30 g glycogen in week 1 and 120 g glycogen in week 4. Rough numbers for weight training is 2.5 g glycogen per set of 10 reps at 70-75% 1RM. 25 mins of cycling is approximately the equivalent to a weight training session of 10-12 sets in terms of glycogen depletion (not counting excess post-exercise oxygen consumption which is small, but a tad higher for weight training).

It would be interesting to see whether competing bodybuilders, for whom size matters more than strength, could benefit from some higher volume weight training in the fasted state. On a cyclic diet, where higher carbs are consumed on training days, the improved ability to store carbs as glycogen will give the appearance of fuller and larger muscles – a clear advantage on the stage.

It would also be interesting to see whether such a protocol, fasted state high volume weight training, would provide benefits in regards to hypertrophy*. One theory that has been floating around is that of the anabolic effects of glycogen supercompensation. The Ultimate Diet 2.0, based around low carbs and concomittant glycogen depletion with a supercompensation phase, is designed partly around this concept.

* However, while this is an interesting thought, I still don’t think glycogen depletion should play an important part in the natural weight trainer’s lifting regimen. I consider it arguably more important to focus on progressive overload in the 4-8 rep range first and foremost. That being said, I have experimented with a few higher rep back-off sets (to induce modest glycogen depletion) following the heavier sets with good results. But I digress. Let’s get back on topic.


By what mechanism does fasted training lead to fuller muscle glycogen stores? I think the effects seen here might be explained by an increase in glycogen synthase, which is an enzyme involved in converting glucose to glycogen. Endurance training increase glycogen synthase, as would I expect any other form of activity that draws upon glycogen stores. On top of that, studies on intermittent fasting show a similar effect via phosphorylation of glycogen synthase kinase. Training in the fasted state might provide a synergistic effect, or at least be a double whammy, since both short term fasting and training independently induces adaptations that favors glycogen replenishment.

And what might explain the greater improvements seen in VO2Max? On that topic, I have no clear cut explanation, nor do the researchers in this study. If anyone is well versed in the scientific literature on endurance training, feel free to chime in and speculate.

Interrupting the homoestatic machinery

To explain these results in a broader framework, it might be fruitful to think of the fasted state as an additional stressor, on top of the training itself, that interrupts the homeostatic machinery of the body to a greater extent than that of fed state training. Greater interruption means greater adaptation in the recovery phase.

A similar pattern can be seen in some other phenomena. In my article on fasted training and muscle growth I mentioned that “studies show that ingesting antioxidants from supplements weakens the body’s own response to deal with free radicals created by training. ”

My point here being that if you make it too easy for the body to adapt, it won’t see a need to adapt, or the adaptation might not be as powerful. Force it to adaptation while training under more strenuous conditions and you will reap the benefits. This is what this study shows and what the study on fasted training and muscle growth hints at.

Coming up

While there seems to be some clear benefits of fasted cardio in terms of improving endurance and muscle glycogen storage, this form of training may hamper muscle growth by a few different mechanisms. Besides being potentially catabolic to muscle growth by accelerating de novo gluconeogenesis (the conversion of amino acids to glucose), it may also interfere with cellular adaptations to weight training.

Someone interested in preserving or gaining muscle while using cardio for improving conditioning, or as means to speed up fat loss, need to be cautious and implement strategies to sidestep the negative effects. This will be the topic of the next article on fasted state cardio (ETA: Sept).