Intermittent Fasting and Glycogen Depletion

In this article we will discuss two topics that are circling the minds of strength, power, and fitness athletes alike these days…

Intermittent fasting and glycogen depletion.

Below, we will briefly describe each, discuss some potential benefits (and limitations), and go deeper into why some athletes may be for (and against) my conclusions…

What Is Intermittent Fasting?

Intermittent fasting is a type of eating practice that has you go prolonged periods of time without eating (which some studies have shown benefits of) followed by re-feeding times. There are a wide array of eating strategies and various intermittent fasting formats, and some of the more popular ones are discussed in this intermittent fasting article.

*I want to say that I have tried to stay as unbiased as I could throughout this piece, presenting research and conclusions both for and against, and offer you some compelling food for thought before you dive into (or if you disagree) with intermittent fasting and strength training.

Potential Benefits of Intermittent Fasting

Some research suggests that intermittent fasting can offer benefits to athletes (described in more detail below). Please note that the below benefits are discussed deeper in the above article, and have been linked to clinical research findings. In no way is this article making a case for the effectiveness (or lack of) for intermittent fasting, but rather simply reiterating what some research has found and how it can be used for strength, power, and fitness athletes if they are curious/interested in learning more about intermittent fasting.

  • Increased Human Growth Hormone (HGH): HGH has been shown to increase as much as five times during fasting periods. *To play devil’s advocate here, HGH has also been shown to increase via hard, strenuous training. This research only compared HGH secretions in non-training states.[ * ]
  • Improve Cellular Repair: One study found that prolonged periods of fasting shifted stem cells from a dormant state to a state of self-regeneration and repair. *Note that the research article used here tested this in mice, and therefore further research may be needed in humans and the cellular recovery from training) [ * ]
  • Reduces Inflammation: Some research suggest that fasting can decrease oxidative stress and prevent/fight inflammation throughout the body.
  • Improved Cognitive Health: In addition to potentially fighting inflammation, fasting has been shown to increase levels of brain-derived neurotrophic factor. Be sure to read more about the above intermittent fasting benefits and research here.

What Is Glycogen Depletion?

In an earlier article we discussed the ins and outs of glycogen, how it is beneficial for muscle growth and performance, and signs and symptoms of glycogen depletion. In short, glycogen depletion is a state in which the skeletal muscles are unable to perform higher intensity movements due to decreased availability of stored glycogen. This is often this case during prolonged periods of higher intensity training sessions and/or chronic malnutrition (lack of adequate carbohydrate and/or caloric consumption).

How Does Intermittent Fasting Affect Glycogen Depletion?

When we take a look at intermittent fasting and glycogen depletion, the most important aspect we need to consider is someone’s overall ability to provide enough calories and carbohydrates throughout a 24-hour period to allow for muscle growth and training responses to occur. If someone is under eating (not enough calories) over the course of the day, he/she will be unable to train at increasing intensities and/or build muscle mass.

That said, most athletes who intermittent fast will have a shortened refeeding window, which will have them fit their entire daily caloric needs (and macros) into a smaller window (typically most of us will fast 6-8 hours when we sleep, leaving us with a 16-18 hour eating window). Those who fast, however, may have a shortened eating window (for example, The 16/8 Method has only an 8-hour fasting window). The need to meet adequate caloric intake is paramount for athletes who are doing intermittent fasting, as they must fit a day’s worth of eating within a smaller time period. If they do not consume enough calories that their body needs (this goes for whether you are fasting or simply not fasting), muscle mass, strength, and performance could be hindered (and glycogen stores not repleted).

The Effect of Insulin Sensitivity on Glycogen Repletion

Intermittent fasting has the ability to increase one’s insulin sensitivity, which can be heightened when coupled with the natural insulin sensitivity benefits of resistance training. When training in a fasted state (which can have some potential side effects as well), glycogen levels are often depleted by the end of a training session, with a high spike in insulin sensitivity. At this time, a lifter can increase caloric intake (preferable carbohydrates and proteins) which will increase glycogen stores and enhance protein synthesis (however this is also the case for non-fasted lifters who train hard, presumably increasing glycogen stores and replenish them in a heightened state of insulting sensitivity).

Playing the Devil’s Advocate…

To play Devil’s Advocate here, training hard using moderate to heavy intensity, higher volumes, and shorter rest periods have also been shown to have profound effects on increases in HGH, insulin sensitivity, and decreased glycogen depletion. When an athlete simply re-feeds post-workout and replenish glycogen stores, similar effect in terms of protein synthesis and insulin sensitivity/IGF factor have been seen. 

Training in a fasted state may not have its limitations in less intense sessions (such as ones that do not rely on power output, maximal strength, and/or anaerobic processes). The risks of training in a fasted state (especially lack of glycogen/blood sugars) are that a lifter may not be able to train with as much intensity and/or perform as much training volume per session (which both are huge keys for strength, power, and even fitness athletes) than if they were well nourished throughout the day and before training sessions. 

I would love to hear what you all think about this topic in the comments below, as there’s so much research circulating about intermittent fasting, muscle growth/loss, and how it can be used by strength/power/fitness athletes (seeing it is more used for aesthetic purposes).

*This was noted above, but I’ll say it again: I want to say that I have tried to stay as unbiased as I could throughout this piece, presenting research and conclusions both for and against, and offer you some compelling food for thought before you dive into (or if you disagree) with intermittent fasting and strength training.

Featured Image: @lisahaefnerphoto on Instagram