Understanding Glycogen, Your Body’s High-Performance Fuel

Published November 22, 2022 01:35AM

Put 100 runners around a pre-race dinner table, and they won’t agree on much. Training talk may cause shouting matches; shoe talk could cause friends to come to blows over terms like “drop” and “stack height.” But one thing most would agree on is what to eat—a big pasta buffet.

Pasta has earned its vaunted place at pre-race meals everywhere because of its impact on glycogen. You have probably heard the term “glycogen” bandied about before, and you may have even used it yourself when ordering your pre-race meal.

I know that when I order at restaurants, I am implicitly saying, “I’ll have the large glycogen pizza, please, with a side of glycogen breadsticks.” But what is glycogen, and how can you use it to avoid the dreaded bonk?

The Basics

Glycogen is a branched polymer of glucose stored in the liver and muscles that acts as a fuel source for exercise. Glycogen metabolism is the process by which these stored carbohydrates are used as fuel, involving many enzymes with chemical compositions that fill a quarter of a page. While the glycogen-to-fuel process is complex enough to fuel many PhD dissertations, the basic takeaway is that excess carbohydrates are stored as glycogen in the liver and muscles, turning pasta into performance.

Among other things, the breakdown of glycogen is used in ATP synthesis, which is essentially how energy transfer happens in cells. Even though glycogen only accounts for a minimal amount of total stored energy in the body, lower-burning fat takes too long to go through the same process to fuel sustained moderate exercise, so at around 55 to 65 percent of VO₂ max for most athletes, glycogen stores become essential to maintain performance levels (though exact intensity depends on many athlete-specific factors).

Think of fat and carbohydrate burning on a spectrum, with high-intensity efforts involving mostly carbohydrates and low-intensity efforts involving mostly fat. Both energy sources are important for running.

According to cycling expert Dr. Iñigo San Millán, at race pace, most athletes will burn two to three grams of carbohydrate per minute. Even at lower intensities, most athletes will burn one to two grams of carbohydrate a minute (though this rate can be adjusted with training). Most athletes store 300 to 500 grams of glycogen when fully fueled, equating to about 90 to 120 minutes of intense exercise.

Glycogen burns rapidly but is refilled at a drip, usually replenishing at a rate of two to five percent per hour after exercise. Empty glycogen stores can take a full day or more to restore.

Three ways glycogen is important for endurance athletes.

If your training and racing goes beyond low-level aerobic exercise, you will need to use glycogen to perform at your peak potential. According to a paper in Medicine and Science in Sports and Exercise, chronically low glycogen stores in athletes can cause fatigue and even induce a “catabolic” state involving muscle breakdown by requiring the body to rely on proteins and amino acids for fuel. That is one reason why low energy availability over time may contribute to a reduction in performance, and even overtraining syndrome.

As outlined by an article in the Journal of Sports Science and Medicine, because glycogen helps muscles recover and avoid cannibalizing themselves for fuel after high-intensity exercise, replenishing glycogen can preserve muscles and accelerate recovery. Topping off glycogen stores will help you get ready for another run sooner.

3. Training in a glycogen-depleted state can enhance some training adaptations and improve aerobic efficiency.

While the body generally needs glycogen to perform at a high level, it can be trained to use its glycogen stores more strategically. An article in the journal Sports Nutrition outlines how running in a glycogen-depleted state can enhance markers for adaptation to training and make the body better at burning fat. Some top ultrarunners like Zach Bitter and Jeff Browning take it one step further, using a low carbohydrate high fat (LCHF) diet to train their body to burn mostly fat at relatively fast paces. However, LCHF diets are complex and controversial, and should be undertaken solely for training purposes when preparing for low-intensity events, at the advice of an expert.

How should you maximize glycogen fueling in your own training?

1. Train with adequate glycogen stores by eating carbohydrates in your daily diet.

Don’t overthink things. Just prioritize a balanced diet rich in healthy carbohydrates like whole grains, along with plenty of good fat and rich protein. Since glycogen levels take many hours to fill up, what you ate yesterday is often more important than what you eat the morning of. Avoid food restriction and eat guided by hunger.

2. After runs, prioritize replenishing glycogen through carbohydrate intake.

After exercise, the body is aching to top off glycogen stores. Chocolate milk is often cited as a good post-run drink due to its mix of carbohydrates, protein and fat. While it’s hard to think of a more delightful nutrition suggestion, any similarly carbohydrate-rich food will work.

As glycogen levels drop, so, too, does performance for most athletes. For runs long enough to begin burning stored glycogen (usually 60 to 90 minutes or longer), practice refueling as you go. For most athletes, 200 to 300 calories per hour of mostly carbohydrates—like gels or sports drink—is a safe bet, adjusting for body type and background.

4. You can’t replenish glycogen as fast as you can burn it, so temper your effort level.

In events 90 to 120 minutes and below, if you start with full glycogen stores, you can pretty much go as hard as you’d like and avoid low-glycogen bonking. Over that, you need to pace yourself to avoid running on empty.

As a thought experiment, imagine that a typical athlete has about 360 grams of carbohydrates stored as glycogen, and can replenish 60 grams per hour while burning 180 grams per hour. With no carbs, the athlete goes two hours until bonking. Even with adequate carbs, the athlete will bonk in less than three hours. So the key is to reduce the amount of carbohydrates burned per hour by reducing intensity so the body can burn more fat. In other words, pace yourself to achieve the optimal fat-to-carbohydrate burn rate to avoid bonking.

My general glycogen-related pacing guideline is: you can go hard in events less than two hours, moderately hard in events two to three-and-a-half hours (glycogen can mostly be replenished at strong efforts), moderate for events three and a half to five hours (when fat burning becomes more important), easy/moderate up to eight hours and easy and conversational in anything over that.

You can use glycogen depletion as a tool to enhance training adaptations. An approach I use with some of the ultra athletes I coach is to do every third long run at very low intensity in a glycogen depleted state (no carbohydrates since the evening before), doing back-to-back long runs every month (even with normal fueling, this type of schedule causes glycogen depletion naturally) and doing short doubles on one or two workout days most weeks. However, to keep it simple, you can just do your daily morning run without breakfast occasionally. (Note: Dr. Stacy Sims, author of the book “Roar,” indicates that glycogen depletion may be less useful—and possibly detrimental—for some female athletes due to hormonal shifts. Very few of the professional female athletes I coach aim for glycogen depletion in a structured way, though will sometimes do back-to-back long runs and doubles for other reasons.)

David Roche runs for HOKA One One and NATHAN, and works with runners of all abilities through his coaching service, Some Work, All Play.