Understanding Glycogen

Change The Way You Think About Carbs & Fitness

Carbohydrates tend to get a bad rap when it comes to building a lean and healthy body. Indeed, too many carbs (or the wrong kind) will contribute to weight gain and obesity.

But carbs aren’t inherently bad. In fact, carbs are an awesome energy source that fuels feats of strength and endurance.

What’s more, carbs can be converted into glycogen – a powerful substance that boosts energy and pumps up your muscles.

In this article, I’m going to teach you all about glycogen. Because when you understand glycogen, you’ll never demonize carbs again!

Glycogen

What Is Glycogen?

Glycogen is a form of stored carbohydrates found in the human body. And it acts as an energy reserve to fuel your activities.

But the carbs you eat don’t turn directly into glycogen. There’s a middle man called glucose.

Glucose & Glycogen

When you eat carbohydrates, your stomach breaks them down into glucose, the smallest molecule of sugar. Once digested, glucose enters your bloodstream (aka blood sugar) and flows around the body.

From there, a hormone called insulin enables glucose to enter cells where it can be used for energy. If there is no immediate need for energy, glucose molecules can bond together into glycogen to be stored for later use.

Glycogen vs Glucose

Figure 1. Glucose & glycogen explained in terms of chemical composition.

Glycogen Storage

Glycogen is stored in the cells of the liver and skeletal muscles. The liver can hold about 100 grams of glycogen. Whereas each pound of skeletal muscle holds an average of 5 to 6 grams of glycogen according to a study of 228 subjects1.

Therefore, most people store 500 to 800 grams of total glycogen depending on body size. Since there are 4 calories per gram of carbohydrate, that means your body can store 2,000 to 3,200 calories worth of quick-burning energy.

How Your Body Uses Glycogen

Imagine you want to build a campfire. First, you need to split big logs into smaller pieces of firewood that will burn easily. Then you put all those smaller pieces in a stack until you’re ready to use them.

Similarly, your body breaks down carbs into smaller pieces of glucose. And that glucose either gets burned right away or added to the stack to burn later.

What Is Glycogen

Figure 2. Glycogen is like your body’s firewood bundle.

Whether glycogen gets burned or stored depends on several factors. Including, when you ate last, what kind of food you ate, and how much glycogen you have available.

However, the primary factor that determines how much glycogen you use is your activity level. The more intense the activity, the more glycogen you will use.

Normal Activity

When you’re resting, almost none of the calories you burn come from glycogen. Even normal activities like walking only burn about 30-40% carbs.

That means you don’t burn much glycogen without deliberate exercise.

Aerobic Activity (Cardio)

As the intensity of your activity increases, you start tapping into more carbs for energy. For example, aerobic activities like jogging burn 50-60% carbs.

Cardio exercise is a true mixture of fat burning and carb burning. And you definitely tap into some of your glycogen reserves.

Glycogen Percent Carbs Burned By Exercise Intensity

Figure 3. Carb (glycogen) usage increases exponentially with exercise intensity.

Anaerobic Activity (Weight Lifting)

As illustrated in Figure 3, anaerobic activities like weight lifting burn 60-80% carbs. This means high-intensity workouts make a much bigger dent in your glycogen stores.

With this in mind, it’s important to understand how to adjust your diet in order to get results from your activity and workouts.

How Diet Affects Glycogen

As you’ve seen so far, your daily activity and exercise use glycogen. While the carbs you eat replenish it. But it’s also important to realize that your glycogen stores are limited.

Once your glycogen stores are full, excess carbs will be stored as body fat. Like chopping more firewood than you need, the pile overflows and you end up with wood scattered all around the yard.

Keeping your woodpile tidy is a matter of replenishing only what you’ve burned. And the same can be said for maintaining glycogen levels.

Maintaining Glycogen

Most people don’t pay attention to glycogen levels or grams of carbs. They just eat based on what their gut tells them.

And this intuitive eating works for some people. But when your body weight fluctuates or you don’t like the way you look, it’s time to pay attention to carb intake.

When you eat about the same number of carbs as you burn, you will maintain your body’s glycogen levels.

Carb Intake ≈ Glycogen Usage = Glycogen Maintenance

To figure out how many grams of carbs you should eat, you first need to estimate how many grams of carbs you burn. Table 1 below helps you estimate how many grams of carbs you need to eat to maintain your glycogen levels.

Activity Intensity (Max HR) Carbs Burned
Sleeping 25-35% 0 g/hr
Sitting 35-45% 0-5 g/hr
Walking 45-55% 20-40 g/hr
Jogging 55-65% 60-90 g/hr
Weight Lifting 65-75% 90-130 g/hr
Sprinting 75-85% 140-180 g/hr

Table 1. Approximate grams of carbs burned per hour of exercise at various intensities.

After looking at these numbers, you may be surprised by how hard it is to burn carbs. Or how easy it is to eat carbs in excess.

For example, to burn the number of carbs you get in McDonald’s Big Mac Combo Meal (which has 144 grams), you would have to jog at a decent pace for 1.5 to 2 hours!

For the average Joe, a target carb intake (and calorie intake) may be all you need to manage your weight. But if you’re on my site, I assume you’re looking to transform your body or increase performance.

That’s when a more advanced understanding of glycogen is necessary.

Carb Cycling

If you’re like most people, your activity level changes from day to day. Some days you work out and other days you rest.

So, if you want to optimize your performance and results, it’s best to adjust your daily carb intake up and down with your activity level. This is called carb cycling.

zig zag diet carb cycling

Figure 6. Illustration of the concept of cycling carbs based on activity level.

When it comes to carb cycling, there’s no single right way to do it. The best daily carb intake for you depends on your fitness goals.

Generally, you want to eat more carbs on days you exercise in order to fuel workouts and replenish the glycogen you burn. While your carb intake should be much lower on rest days.

In addition, it’s advisable to include at least one high carb day per week if you’re restricting calories. This ensures your glycogen levels stay topped off while also giving your metabolism a boost – also known as a refeed day.

During a muscle growth or strength-building program, your carb intake will be relatively high every day that you workout. But somewhat lower on rest and recovery days.

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Carb cycling is easy and effective once you get the hang of it. And with this technique, you can get into amazing shape while fueling performance.

But there may be times when you want to take things up a notch. That’s when knowledge of glycogen depletion and loading comes in handy.

Glycogen Depletion

Glycogen depletion is the process of using up your glycogen stores. One reason to deplete glycogen is to increase fat burning. Because when less glycogen is available, your body has to tap into more body fat for energy.

To deplete glycogen, your carb intake must be less than the amount of glycogen you burn each day.

Carb Intake < Glycogen Usage = Glycogen Depletion

In other words, you have to eat very few carbs, exercise a lot, or both. As an example, let’s say you do 1 hour of cardio plus 1 hour of weight lifting per day.

From Table 1, we know cardio burns an average of 75 grams of carbs per hour. And weight lifting burns about 110 grams of carbs per hour. So that’s 185 grams of carbs per day through exercise.

For the rest of the day, we’ll say you have a desk job and only burn an additional 25 grams of carbs. That brings the total up to 200 grams of carbs per day.

Now, if you eat a relatively low carb diet of 150 grams per day, your glycogen will deplete at a rate of 50 grams per day. That means it would take 10 days to deplete 500 grams of glycogen.

Glycogen Depletion

Figure 4. Estimated rates of glycogen depletion based on 50 and 150 g/day carb deficit.

Alternatively, you could deplete your glycogen in as little as 3 days if you lower your carb intake to 50 grams per day along with the same exercise protocol.

This example is an oversimplification, but it shows that glycogen depletion takes a combination of low carb intake and high activity output. And even then, it doesn’t happen overnight.

Another reason to deplete glycogen is so that you can reload it. This has benefits for both physical performance and appearance.

Glycogen Loading

Glycogen loading is the process of refilling your glycogen stores after they’ve been depleted. Some endurance athletes use glycogen loading to build up energy reserves before a big race. And fitness athletes use it to get in peak condition for a contest or photoshoot.

To load glycogen, your carb intake must be greater than your glycogen usage. And remember, this is only recommended following extensive depletion.

Carb Intake > Glycogen Usage = Glycogen Loading

During the loading phase, it’s common to consume 500 to 700 grams of carbs per day. But the optimal amount depends on your body size and depletion level.

Glycogen Supercompensation

Glycogen supercompensation is when muscles are able to hold more glycogen than they normally would. Studies show the effect is most pronounced with a loading phase after glycogen depletion to 25% or less of baseline2.

It’s sort of like being dehydrated and then drinking a bunch of water. Your body will hold on to the excess in case you don’t find water (or carbs) again for a while.

Through the process of depletion and loading, muscles actually overcompensate and store 50-100% more glycogen3.

Glycogen Supercompensation

Figure 5. Illustration of glycogen supercompensation following depletion and loading phases.

Figure 5 shows a typical depletion and loading protocol used by bodybuilders and athletes. To explain, let’s walk through each phase.

The depletion phase typically lasts between 4 and 7 days depending on how much glycogen you’re starting with. As well as your diet and exercise routine.

During depletion, you should target 50 grams of carbs per day. And perform 1-2 hours per day of exercise at 65% or more of your maximum heart rate. Make sure you hit every muscle group to deplete local glycogen stores.

The loading phase begins immediately after your last depletion workout. This is when glycogen synthesis is at its greatest. While loading, rest or perform no more than 20 minutes of low-intensity cardio.

The amount of carbs you eat each day of the loading phase depends on your muscle mass (aka lean mass). Generally, 3.5 to 5 grams of carbs per pound of lean mass per day results in maximum glycogen uptake.

For example, a 180 lb person with 20% body fat has 144 lbs of lean mass (180 – (180 x 0.2)). That means they should eat 504 to 720 grams of carbs per day while loading (144 x 3.5 to 144 x 5).

Depletion & Loading Expectations

Keep in mind, depletion and loading are for reaching peak performance or dialing in your conditioning. And you should only go to this extent once every few months at most to avoid diminishing returns.

In addition, you can expect large fluctuations in body weight. A larger person will lose 800 grams or nearly 2 lbs of glycogen. And each gram of glycogen takes about 3 grams of water with it. So you could lose 5 to 6 lbs during depletion.

Actually, this is the reason the keto diet seems so effective at first! But don’t confuse this drop in weight with fat loss or muscle loss, and realize that it’s only temporary. 

You will gain that weight back during loading. And the supercompensation effect will make your weight increase above baseline for up to 7 days.

All that weight goes into your muscles, making them look big and full. Plus the extra glycogen sucks up excess water and makes you look leaner. This is why bodybuilders use depletion and loading leading into a contest!

In terms of performance, the 50-100% increase in glycogen gives endurance athletes 1,000’s of additional calories to burn during an event.

Key Takeaways

This article was meant to be a deep dive into the concepts surrounding glycogen. And I’m aware it’s a lot of information to process all at once. So here are some of the key points to remember.

  • Glycogen is carbohydrate stored in your muscles and liver
  • Your body uses glycogen for energy during exercise
  • The higher your heart rate, the more glycogen is used
  • Carb cycling is matching your carb intake to your activity level
  • Eating fewer carbs than your burn depletes glycogen
  • Refilling glycogen after depletion is called loading
  • Supercompensation is when your muscles store more glycogen than before

Now you know as much about glycogen as the most seasoned athletes and bodybuilders. And hopefully, this information will help you make adjustments to your diet while reaching your fitness goals.

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