Glycogen Depletion Runs: The Purpose, the Benefits, and the Risks

Last week we talked about fueling during runs and you can train your stomach to handle carbohydrates on a run. This week, I want to discuss a related topic: glycogen depletion runs.

Glycogen is how your body stores carbohydrates in your muscles for energy. When you run, your body relies on a combination of glycogen, stored fat, and immediate available carbs (what you eat before and during your run) to provide energy for your muscles.

Glycogen depletion runs came into practice based upon scientific findings (see one of the studies here) that endurance training with low muscle glycogen increases fat metabolism and thus makes you a more fuel-efficient runner.

Do glycogen depletion runs offer value for some runners by helping them become more efficient in burning fat for long distance races? Or are the risks greater than any of the possible rewards? Let’s take a look. 

How do you perform a glycogen depletion run?

A glycogen depletion run is easier said than done. You avoid eating or eat very little beforehand and then complete a long run (90 minutes or longer) with no fuel. In other words, glycogen depletion runs are fasted long runs.

Glycogen depletion runs, if you chose to do them, should be done early in your marathon or half marathon training segment and only done for 50% or less of your runs over an hour. Runner’s Connect and The New Rules of Marathon and Half Marathon Nutrition both offer excellent guidelines for balancing fasted or low-carb runs in your training with carbohydrate-loaded runs.

It’s important to note here that Fitzgerald recommends not taking mid-run fuel in only on runs 1-2 hours in length and does not discourage a pre-run snack. On all long runs over two hours, you should definitely eat something mid-run.

Other proponents of glycogen depletion runs (such as the author of this Running Times article) claim you must run for 2-3 hours without fuel to reap the benefits. 

To properly do a glycogen depletion run, you need to refuel with plenty of carbohydrates afterwards (you should be refueling with carbohydrates after all of your runs, actually). Without glycogen replenishment, you lose any benefits from the glycogen depletion run and set yourself up for exhaustion, stomach issues, and a bad training runs. You wouldn’t try to keep driving your car for long after your empty light turns on; likewise, you need to fill your body’s gas tank after depleting your carbohydrate stores.

Do NOT fast during a hard long run. All glycogen depleted runs should be done at a conversation pace (see again this article from Running Times for more information). Your body needs carbohydrates, whether in the form of stored glycogen or quick-releasing sugars), to run far; it needs even more carbohydrates to run the same distance faster.

What’s the purpose of a glycogen depletion run?

Consuming carbohydrates during your long runs, whether in the form of a sports nutrition product or whole foods, improves your athletic performance, especially in hard workouts and races. However, some runners overuse sports nutrition products so much that they decrease their fat-burning capabilities and hinder their ability to draw from stored glycogen. Thus, they become reliant on mid-run carbs even during short runs.  

Matt Fitzgerald explains in his book The New Rules of Marathon and Half Marathon Nutrition that “heavy reliance on supplemental carbohydrate acts as a metabolic crutch that limits fat-burning practice and thus limits improvements in fat-burning capacity.” (Page 81)

Your fat-burning capacity directly impacts when you hit the wall in an endurance event. If your body is able to convert both fat and glycogen into energy, then you’re less likely to hit the wall in a marathon.

What are some of the drawbacks of glycogen depletion runs?

If you do too many glycogen depleted runs in training and are not careful to replenish your carbohydrates after your runs, you risk depleting your glycogen stores too much. Simply put, doing too many glycogen depletion runs will just keep emptying your tank until it has nothing left to give.

How do you know if you’ve taken glycogen depletion training too far? You’ll experience fatigue during the day and struggle during even shorter runs. Your training will suffer as your body attempts to replenish and conserve your glycogen stores.

Michele at NYC Running Mama wrote a great post on problems she encountered from doing too many glycogen depletion runs. I highly recommend your read it for her valuable insight from her experience.

Another drawback of glycogen depleted long runs is the fact that you risk not completing vital long runs. If, for example, you decide to try 18-20 miles without any fuel during, there’s a chance you’ll bonk and be unable to finish that run.

In which case you have to ask yourself the question of which is more detrimental to your race goals: not training your body to rely possibly more on fat as fuel, or not completing your most important long training runs?

Should you do glycogen depletion runs?

If you do add glycogen depletion runs to your training, I highly recommend working with a running coach so that these runs are done during the appropriate phase of training and so you have someone to talk to if anything goes awry. 

As I stated last week with fueling on your runs, every runner is different. Some runners see improvements in their running from glycogen depletion runs, while others may struggle through them and see little difference in your training and racing.

Do what works best for your body, and be honest with yourself with what actually does work and what doesn’t. Don’t let the comparison trap snare you here; glycogen depletion runs do not make you more of “real runner.” If you run, you are a real runner!

Of course, there’s so much more to be said about glycogen depletion runs, including their impact on your weight, how they alter your recovery period, whether they train you to run faster, and how to push back the wall in marathon training. Want to learn more? Subscribe to my email for exclusive content and updates on my upcoming Master Your Fueling and Hydration e-course! 

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Disclaimer: I am not a doctor or a sports nutritionist, but I am a certified running coach. You should always consult your coach and medical professionals before adding glycogen depletion runs to your training. If you have low blood pressure, diabetes, or other health conditions, or are pregnant, please do not do glycogen depletion runs for the sake of your overall health.

Linking up with Jill for Fitness, Health, and Happiness! 

Have you done glycogen depletions runs in your training for endurance events? What were the results?
Do you ever write a word so many times that you question its existence? That totally happened to me after typing “depletion” so many times.