Most of the time, runners and other endurance athletes hear about the importance of eating carbohydrates before a workout to ensure that glycogen stores are topped off to have the energy needed to work out and afterward help replenish depleted glycogen stores.
However, some experts say that deliberately running in the glycogen-depleted state occasionally can induce beneficial physiological adaptations.
Glycogen depletion workouts aren’t fun, but depending on your goals as a runner, shocking your body by forcing yourself to run in the fasted state may be something you want to consider once in a while.
In this article, we will discuss glycogen depletion workouts and running, including how glycogen depletion running workouts work, the pros and cons of glycogen depletion workouts, and how to do glycogen depletion runs.
More specifically, we’re going to look at:
- What Are Glycogen Depletion Workouts?
- How Does Glycogen Depletion Training Work?
- What Is the Purpose of Glycogen Depletion Runs?
- How to Do Glycogen Depletion Workouts
Let’s get started!
What Are Glycogen Depletion Workouts?
Glycogen depletion workouts involve running or exercising when your glycogen stores are essentially empty because you’ve not consumed any carbohydrates before or during your run.
How Does Glycogen Depletion Training Work?
Glycogen is the storage form of carbohydrates in the body.
When you consume any source of carbohydrates, the starch and sugar molecules are broken down into simple sugars, such as glucose.
These small glucose molecules are circulated throughout your bloodstream, fueling different tissues such as your brain, liver, heart, and muscles.
Excess carbohydrates that you eat during a meal or snack are converted into glycogen, a much larger molecule that is formed by synthesizing many simple sugar molecules together.
These glycogen molecules are primarily stored in the skeletal muscles and liver.
When your body has used the available blood sugar or needs additional energy—such as when you exercise—the stored glycogen can be broken down into simple glucose molecules, which can then be used by your muscle cells (or other tissue) to generate ATP (cellular energy).
The body has limited glycogen stores in the liver and skeletal muscles, and these levels deplete overnight during your fast.
According to the Academy of Nutrition and Dietetics, an endurance-trained athlete can store up to 1,800 to 2,000 calories of fuel as glycogen in the muscles and liver, though smaller runners might store closer to 1,500 calories or so.
This is said to be adequate to support about 90-120 minutes of running at your marathon race pace effort.
Therefore, when you run without eating beforehand, your body tries to conserve the limited glycogen remaining by shifting to oxidize more fat for energy instead.
For example, some studies suggest that you may burn up to 20% more fat when exercising on an empty stomach.
However, it’s important to make the distinction that you are not burning more calories overall when running in a glycogen-depleted state; rather, the source of those calories shifts to include a greater reliance on fat rather than carbohydrates.
What Is the Purpose of Glycogen Depletion Runs?
The theory behind glycogen depletion workouts is that running or performing some sort of endurance exercise when your glycogen stores are low improves your body’s ability to burn fat for energy at higher workloads.
Glycogen depletion training is also said to condition your muscles to conserve glycogen—termed glycogen sparing—earlier on in workouts.
Essentially, your muscles and heart require energy to continuously contract while you exercise.
That energy has to come from somewhere, whether it be from burning carbohydrates (circulating blood glucose from the food you’ve just eaten or stored glycogen from previous meals), fat (triglycerides in adipose tissue), or protein (in muscles).
According to research, during exercise at intensities greater than approximately 60% of your VO2 max (maximum oxygen consumption), blood glucose and muscle glycogen are the primary fuels being oxidized to produce the necessary ATP for your muscles.
However, when you’re in a glycogen depletion state, your body has to conserve any remaining glycogen by shifting to burning primarily fat for fuel.
Normally, if you haven’t been doing glycogen depletion workouts to train your body to be more efficient and effective at oxidizing fat for fuel at higher intensities of exercise, you may find yourself “bonking” or “hitting the wall” when your glycogen stores are depleted.
Runners experience this feeling as sudden fatigue, a substantial decrease in your speed, and uncomfortably heavy legs.
Essentially, your muscles can generate the ATP (cellular energy) they need to allow you to keep running from carbohydrates and fats (and, to a lesser degree, proteins).
Glycogen (carbohydrates) can be oxidized for fuel more quickly, which means that your muscles can create energy quickly enough to keep up with the demand for ATP.
This means you can keep running at a faster pace.
In contrast, fat is oxidized more slowly, so your muscles can only keep up with generating energy at the rate they need it when you are running at an easy pace or exercising at a low intensity that you’re able to keep.
This is why your pace can drastically slow down once your glycogen stores are empty.
The entire premise behind glycogen depletion workouts is that you are training your body to conserve your precious resource of carbohydrates during high-intensity exercise and become better at burning fat quickly and efficiently enough for your muscles to be able to meet their energy needs through fat oxidation at faster running paces.
How to Do Glycogen Depletion Workouts
Marathoners, ultramarathoners, and other endurance athletes whose event lasts significantly longer than 90 to 120 minutes have the most to gain by incorporating glycogen depletion workouts into their training programs.
There’s very little to gain from this training strategy if your target race is shorter than a full marathon because your body should not run out of glycogen during a shorter event as long as you have fueled properly beforehand.
If you are going to try glycogen depletion workouts, the best time to do them is during long runs because your body will naturally run out of glycogen after 90 minutes or so.
When considering where in the training block to try a glycogen depletion long run, it generally makes sense to incorporate these workouts earlier on for numerous reasons.
For one, glycogen depletion workouts are very taxing for the body, and you don’t want to combine this type of stress with heavier training loads that occur closer to your race.
Doing so can increase the risk of injury and suppress your immune system, leaving you prone to illness.
Additionally, the adaptations from glycogen depletion training aren’t immediate, so doing glycogen depletion workouts too close to your race is unlikely to induce appreciable benefits for an upcoming marathon.
Moreover, doing glycogen depletion workouts earlier on in your training block will potentially improve your fat-burning efficiency and glycogen-sparing capacity on future long runs in your training block leading up to your race.
In other words, you can capitalize on the beneficial adaptations of glycogen depletion runs more fully if you do them early enough since the adaptations will affect your race as well as upcoming long training runs.
Finally, it’s really important to practice your fueling strategy on long runs.
It can take several runs of at least 18 to 20 miles to really get a sense of how your digestive system responds to what you are eating and drinking and how your body performs with different types and timing of fuel in order for you to dial in the best nutritional strategy for your race.
Although it’s important to practice your fueling strategy during pretty much any long run, it becomes particularly important for the longer long runs on your training plan, as these best simulate the actual needs your body will face during the race.
The 18-20+ mile long runs usually fall later in the training block after you’ve built up sufficient endurance.
For most runners, long runs that fall somewhere within the 14 to 18-mile distance work pretty well for glycogen depletion workouts.
This distance is long enough that you will become glycogen depleted if you do not take in any sort of carbohydrates before or during your run, yet they aren’t so long that it’s one of the crucial 20-mile long runs you need to use as a race rehearsal.
Avoid doing glycogen depletion runs when you have speed workouts, tempo runs, or other key workouts on your training plan.
You need to be sharp for these workouts, but running them without fueling properly can be deleterious to your athletic performance.
To do your glycogen depletion run the day before your targeted workout, perform a moderately-difficult workout.
Afterward, refuel only with protein and fat. Eat a low-carb dinner the night before your glycogen depletion workout.
Do not eat carbohydrates before or during your long run, though you should fuel with fats such as nut butter and/or protein.
Hydrate with plenty of water and electrolytes, but keep the glucose levels low.
It’s important to refuel afterward as soon as possible with plenty of carbohydrates, protein, and fluids.
Looking to try glycogen depletion long runs? Here are some high-protein foods to kick the carbs the day before!