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The Important Role of Glycogen in Running

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Runners have always looked for the best way to supply their bodies with the energy needed to go for a 10-mile run without completely depleting their stored energy. Our bodies have actually come up with a genius way of having an abundant supply of energy ready on demand in our muscle cells. It is called glycogen and its role in running is one of the centerpieces of modern-day endurance training and high-performance workouts.

In this article, we will talk about glycogen, how it is supplied to our muscle cells, how they use it, and most importantly – how can we maximize our glycogen supplies both before and after a long workout. Before we dive deeper into the topic, we must first answer the most important question here…

What exactly is Glycogen?

Think about it like this – take 55,000 glucose molecules and put them together in a single larger molecule. In its essence, that is what glycogen really is – a ton of glucose clumped together to serve as a compact fuel source in our bodies ready to be used at a moment’s notice. Glycogen is mostly stored in the liver and muscle cells and that strategic placement is carefully picked by our bodies for a few main reasons. First, the liver needs energy and glucose most of the time for various other enzyme reactions. Secondly, and probably more importantly, our muscles cannot exactly run on energy derived from our body’s fats since that takes too long to burn. Instead, these smart muscle cells dissolve the glycogen into thousands of glucose molecules and use them all as instant fuel. While I won’t get too in-depth into the whole glycogen metabolism, all you should remember is that it is our body’s main fuel source thanks to its compact and energy-dense nature.

The main reason between liver glycogen and muscle one is that the liver cells can release glycogen into the blood flow effectively supplying the whole body with glucose (energy). Muscle cells, on the other hand, have a more egoistical approach since they cannot release glycogen into the bloodstream. For that reason, their glycogen usage is strictly internal to the cells, hence the important effect glycogen has during a workout.

How Our Body Supply Themselves With Glycogen

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As with most other molecules in our bodies, glycogen is synthesized by our bodies to fit the body’s needs. The enormous amount of glucose needed for that process is supplied by us by eating foods rich in carbs. The two biggest moments in terms of glycogen production occur just after eating (postprandial production) and after we’re done training. This is the main reason many athletes munch of carbs before their workout. Whether eating carbs after workout contributes directly to that post-workout glycogen production process is still debated but some people still use it as an excuse to eat something sweet.

After glucose is absorbed in our intestines, it travels to the liver in our bloodstream. Inside the liver cells, it becomes a building block in the construction process of glycogen (glycogenesis). After glycogen is synthesized there, it will either remain stored in the liver or travel to the muscle cells to act as close-by energy storage in all muscles. Glycogen is also used by other organs and tissues such as our brain but in much smaller quantities.

Why Is It Important For Runners?

This is the most important part of the life-cycle of glycogen and arguably the thing that it was meant for. When you start running, your body will immediately start redistributing its energy consumption and creation process chains. At the beginning of your workout, the muscles will work primarily in aerobic conditions, meaning they will use glucose (mostly from the blood and fat cells) and, with the help of oxygen from our lungs, will burn that glucose to produce the energy needed for the muscle contraction. As you increase your pace, the muscle cells will begin to prioritize the glycogen inside of them compared to the free glucose in your bloodstream and energy derived from fat tissues. This is why people that want to prioritize fat burning workout mostly in the lower heart rate zones with a heart rate of around 50-60% of your HR Max.

The beauty of glycogen is that it will supply your muscle cells with glucose even if your lungs start to lag behind with the oxygen supply. Then, your body starts working out in anaerobic conditions (without oxygen). Those conditions will require different metabolic pathways for energy production but will still use the same old glucose that comes from the glycogen inside the muscle cells.

Those anaerobic conditions begin occurring once you enter the third and fourth heart rate zones, meaning it won’t happen straight away. In these conditions, your muscle cells will also begin using Creatine-phosphate to keep creating ATP (energy), which is the reason most athletes take a supplement called creatine.

How To Increase Our Glycogen Levels

The best way to supply your muscle cells with glycogen is to supply your body with glucose. And the best and fastest way to supply your body with glucose is through what are called high glycemic index foods. The carbs in these foods have the highest conversion rate to blood glucose and are, therefore, perfect for an almost immediate supply of glucose to our liver, which then starts synthesizing glycogen for our muscles. Example of high glycemic index foods are:

  • Baked and fried potatoes
  • Honey
  • White rice
  • Cereals
  • Bread (white)
  • All sorts of pasta
  • Watermelons
  • Ice cream

Eating those before and after your workout will supply your body with the much-needed glucose to synthesize glycogen. Still, if it is too close to the workout itself, the body will probably skip the glycogen-creation process and will use the glucose straight away as it is. Remember, glycogen is a molecule used for storage, so you have to plan in advance about having as much glycogen as possible in your muscle cells. More on that later.

Carbohydrate loading

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This is a very important process that is used by a lot of athletes. In short, carbohydrate loading happens when our bodies are supplied with large amounts of glucose after a period of glycogen depletion (either a long workout or a diet). Then, the body increases the muscle cells’ glycogen storage capacity allowing them to take more carbs in and store large amounts of glycogen afterward. The main mechanism through which that works is by increasing the sensitivity of the muscular insulin on the walls of these cells.

Click here to learn more about some post-workout foods that promote fat burning!

What Happens When We Run Out Of It?

Long-distance runners, cyclists, and other athletes often deplete their glycogen levels. This is referred to as “hitting the wall” and is quite dangerous since it leads to extreme fatigue and failure of the body to work properly. To counter that, athletes use one of the following three strategies (or all three at once):

  • Continuous eating of foods with a high glycemic index – Eating small but frequent portions of high-glycemic-index foods will help your body postpone “hitting the wall” a bit (estimated around 35% at training at the higher heart rate zones).
  • Through fasting combined with low-intensity training
  • Through carbohydrate loading

Cyclists are well-known for using all of these methods combined, with the carbohydrate loading happening after their training sessions combined with fasting and low-intensity endurance workouts embedded into their schedule. During their famous 5-hour races, they bring along snacks that have a lot of carbs with high glycemic indexes.

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How Many Carbs Do You Need?

How much daily carbs you eat both during your workout and before/after it mainly depends on your physical activity. Low-intensity sports and exercises require low carb intakes. High-intensity workouts, especially ones that are stretched over longer periods, will require a ton more carbs throughout your day. Remember, the reason for eating as many carbs as your body needs is to keep your glycogen supplies high. In general, try supplying your body with carbs that have calorie worth of around 40-50% of your daily calorie intake. If you aren’t training, then there is no need to prioritize carbs with a high glycemic index. During a race, however, one might need to take as much as 60-100g/hour in order to sustain their glycogen levels. This is especially valid with races longer than 3-4 hours be it in running, cycling, or some other sport.

Frequently Asked Questions

Why do our bodies choose glycogen over glucose for their energy needs?

Glucose, as a whole, is much more reactive than glycogen. It is also far less stable. In other words, glycogen is the more cell-friendly way of storing high-energy glucose molecules in bulk. Glycogen can also be only broken down by a few tissues and organs, making it highly specific, which makes things easy for our bodies organization-wise.

Do our bodies burn fat or glycogen when exercising?

This is a great question! During a workout, the lower your heart rate is, the more energy your body will be able to get from fats. In other words, energy from fats is harder to convert and takes longer, meaning the body won’t be able to keep up if you exercise in the higher heart rate zones. In these cases, the body will logically prioritize glycogen over energy from the fatty tissue.


The role of glycogen in running is equally important and complicated. The main takeaway from this article is that you should always keep carbs in mind, especially if you’re into longer workouts. During those, your body will use mostly glycogen to supply its muscle cells with glucose, even if you enter anaerobic training zones.