The Importance of Glycogen for Athletes: A Complete Guide

Have you ever wondered what gives you energy during a workout or sporting competition? The secret lies not only in your heartbeat or muscle contraction, but also in a lesser-known element: glycogen. This molecule is the primary form of glucose storage in our body, acting as an essential energy reserve during physical activity. For athletes, managing glycogen levels can make the difference between reaching their limits and surpassing them.

The role of glycogen in athletic performance is multifaceted, influencing everything from sprint power to endurance capacity. Its importance cannot be overstated, particularly in sports requiring rapid bursts of energy or sustained effort over time. By taking a deep dive into how glycogen works and exploring how it can be optimized through diet and training, athletes can significantly improve their performance and recovery.

In this comprehensive guide, we will delve into the biochemistry of exercise, dismantle the myth of the “wall” and give you precise nutritional protocols to optimize your stores.

Understanding how our muscles work

The idea is not to delve into a course on anatomy or biomechanics, so let’s limit ourselves to what our muscles need to function efficiently. Muscle contraction requires energy, primarily in the form of ATP (adenosine triphosphate). Muscles obtain ATP through the breakdown of carbohydrates (glycogen), fats and, to a lesser extent, proteins. That will be enough for what follows.

What is glycogen?

Glycogen is the storage form of carbohydrates in animals (the equivalent of starch in plants). It is a glucose polymer, a complex and branched structure that allows large amounts of energy to be stored in a small space.

But be careful: glycogen is not stored “dry.” It is a hydrated fuel. For every gram of glycogen stored, your body simultaneously stores approximately 3 grams of water. This physiological characteristic is fundamental to understanding weight fluctuations in athletes (we will return to this).

Liver vs. Muscle: The duel between the reservoirs

There is not “one” glycogen store, but two distinct reservoirs with opposite functions: the liver store and the muscle store.

For your performance, understanding this distinction is crucial:

Characteristic	Liver Glycogen (The “Liver”)	Muscle Glycogen (The “Muscles”)
Role	Altruistic: Maintains blood glucose levels for the brain and organs.	Selfish: Used only for contraction of the muscle where it is stored.
Average Amount	~80g to 100g (varies depending on fasting).	~350g to 700g (depending on muscle mass).
Key Enzyme	Contains glucose-6-phosphatase.	Does not contain glucose-6-phosphatase.
Consequence	Can release glucose into the bloodstream.	Glucose is trapped in the muscle cell.

Why is this important? Muscle is "selfish." If your arms are loaded with glycogen but your legs are empty, your arms cannot transfer their energy to your legs. Skeletal muscle lacks the enzyme glucose-6-phosphatase needed to release sugar into the bloodstream. Once glucose enters the muscle, it is either burned there or stays there.

Conversely, the liver is the control tower. It ensures that your brain (which consumes ~120g of glucose/day) does not run out of fuel.

Glycogen, stored in the muscles and liver, is the main source of energy for the human body. It can be mobilized quickly and enables energy to be produced efficiently, but these stores are limited. In a well-trained athlete, they can provide energy for approximately 90 to 120 minutes of moderate- to high-intensity exercise. Like a car engine that consumes more electricity or fuel at high speed than at medium speed, glycogen is depleted more quickly when the effort is intense. When there is none left, you hit the wall and run out of energy! Note that glycogen stores are replenished very slowly, over 24 to 48 hours when the stores are depleted.

What are carbohydrates?

Carbohydrates are therefore a broad category of nutrients found in many foods and are an important source of quick energy for the body.

They are made up of sugar molecules and are classified into three main types:

• Monosaccharides (such as glucose and fructose)
• Disaccharides (such as sucrose and lactose)
• Polysaccharides (such as starch and cellulose).

Carbohydrates are consumed through food and metabolized by the liver and insulin to provide immediate energy or be stored as glycogen in the liver and muscles.

How can glycogen stores be replenished?

The strategy is to fuel early and regularly, providing the body with an external source of energy in the form of carbohydrates through solid and liquid food. This has several advantages:

• We preserve muscle and liver glycogen stores so they can be used later.
• We maintain a stable blood glucose level, which is key to avoiding performance declines and feelings of fatigue or hypoglycemia. During a short effort, the liver can release glucose to maintain blood sugar levels, but over a longer effort, if we do not fuel properly... the run will become more difficult.
• This allows for better management of nutrient absorption and digestion, thereby avoiding gastrointestinal discomfort that could occur if one waited until being in a state of advanced fatigue or physical stress to eat.
• Finally, this helps optimize recovery in relation to the time needed to replenish the stores mentioned above.

If there is no more glycogen and no external carbohydrate intake, blood glucose levels fall, followed by a decrease in insulin. The body will still produce glucose to protect itself. Let us remember that the heart is a muscle and it is better for it to remain active.

•  Gluconeogenesis: In response to glycogen depletion, the body increases glucose production from other non-carbohydrate sources. This conversion can occur from lactate, glycerol (derived from lipids), and amino acids (resulting from protein breakdown). This can lead to a loss of muscle mass if the state is prolonged.

• Ketogenesis: If activity continues or in the event of prolonged fasting, the body begins to use fatty acids more significantly to produce energy. As fatty acid breakdown increases, the liver produces ketone bodies, which are used as an alternative energy source, particularly by the brain. This is the famous ketogenic or Keto diet based solely on fat.

In response to prolonged use of gluconeogenesis and ketogenesis, the body adapts by gradually reducing its basic energy needs and increasing its efficiency in using ketone bodies and fatty acids as its main energy sources. Some athletes who regularly train to deplete their glycogen stores may improve their ability to store glycogen and use fat as an energy source, a phenomenon known as "metabolic flexibility".

Some people think there is no need to consume carbohydrates because fat is sufficient. A simple rule of biochemistry states that fat only burns in the fire of carbohydrates. If there are no carbohydrates, the Krebs cycle does not run, and therefore the body cannot use fat. The only way to prove them right is to follow a suitable ketogenic diet.

The "Crossover Concept": managing your fuel

Your body is hybrid: it burns a mixture of fat (lipids) and sugars (glycogen).

• At low intensity (fundamental endurance): You mainly burn fat.

• At high intensity (>70% VO2max): The proportion of glycogen rises sharply.

That is why training at low intensity is vital: it teaches your body to spare its precious glycogen for difficult moments.

Carbohydrates to prioritize during post-exercise recovery

We often hear about the “anabolic window” the period during which muscles can absorb a greater amount of protein and thereby maintain anabolism, but above all promote greater muscle growth. It would therefore be necessary to consume protein within 30 minutes after exercise. Recent studies show that protein consumption can wait compared with carbohydrate consumption, especially for those who exercise every day.

Consume carbohydrates after exercise? Indeed, muscle glycogen stores become depleted, especially if the exercise was intense or prolonged. These stores are essential for your future performance, and replenishing them is slow (24 to 48 hours). So if they are not adequately replenished, the next workout will start with lower reserves. 

After exercise, the body is particularly receptive to carbohydrates, especially those with a high glycemic index, which accelerates their storage in the muscles. However, muscle protein synthesis remains stimulated for several hours after exercise, allowing greater flexibility regarding their consumption. Research shows that what matters most is having a sufficient total intake throughout the day. Distribute this intake evenly to stimulate muscle protein synthesis regularly (at least 0.3 g/kg of body weight/meal). Ensure a variety of essential amino acids in protein sources.

What is the glycemic index?

The glycemic index (GI) is a ranking system that measures the impact of carbohydrate-containing foods on blood glucose levels. This index compares the ability of a given amount of food to raise blood glucose levels with a reference, generally pure glucose or white bread, which have a glycemic index of 100. However, the glycemic index does not take into account the amount of food consumed, so it is necessary to look further down at the glycemic load.

Here are the main points to understand about the glycemic index:

How the glycemic index works

Measurement: The GI measures the speed at which the carbohydrates in a food are converted into glucose and absorbed into the bloodstream. High-GI foods are rapidly digested and absorbed, causing a rapid and significant rise in blood glucose levels.

Comparison: Foods with a GI of 70 or higher are considered to have a high GI, those with a GI between 56 and 69 have a medium GI, and those with a GI of 55 or lower have a low GI.

Importance of the glycemic index

• Diabetes control : Understanding and selecting foods based on their glycemic index can help manage diabetes by controlling blood sugar spikes.
• Weight management : Low-GI foods can help control appetite and prolong feelings of fullness, which may be beneficial for weight management.
• Cardiovascular health : A diet based on low-GI foods can help reduce the risk of cardiovascular disease and other chronic conditions.

During physical exercise : Knowing the glycemic index of your energy bars, gels, or fruit purées allows you to adjust your intake according to your needs. Choose high-glycemic-index carbohydrates (such as energy gels or sports drinks) for rapid energy release before a challenge such as climbing a mountain pass by bike or at the end of a marathon. 

Factors affecting the glycemic index

• Preparation and processing : Prolonged cooking, fine grinding, and other processing methods can increase a food’s GI.
• Composition : Foods rich in fiber, protein, or fat tend to have a lower GI because these nutrients slow the digestion and absorption of carbohydrates.
• Fruit ripeness : Riper fruits generally have a higher GI than less ripe fruits.
  

Limitations of the glycemic index

Individual variability : The glycemic response to a food can vary from one person to another depending on many factors, including differences in digestion and metabolism.

Combined foods : The GI of a food does not take into account other foods consumed at the same time, which can alter the overall glycemic response.

The glycemic index is a useful tool for choosing foods that can help maintain stable blood sugar levels, particularly for people with blood glucose regulation disorders, such as type 2 diabetes. However, it is also important to consider other aspects of nutrition, such as overall nutrient balance and the calorie density of foods.

What is glycemic load?

Glycemic load is essential for preventing overweight and diabetes. It measures the impact of a food on blood glucose levels. 

Glycemic load is calculated based on a food’s GI and the amount of carbohydrates in a serving. Although glycemic load provides more reliable results than GI, it is a less practical concept to use on a daily basis because you need to know the carbohydrate content of every food.

GL = (GI x amount of carbohydrates in the food serving (g))/100

A daily glycemic load (the sum of the GLs of all foods consumed) is considered low if it is below 80. Conversely, it is high if it exceeds 120.

What are the sources of carbohydrates for athletes?

Sports nutrition brands offer numerous sources of carbohydrates in different forms to replenish our carbohydrate stores during physical exercise, both in training and competition. Your choice should depend on the type and intensity of the activity, the number of calories provided, and above all, how practical the product is for the sport in question.

Cereal & fruit bars:

Energy bars are essential for athletes, adventurers, and anyone with an active lifestyle, as a snack. Designed to nourish and provide a quick energy boost, these compact, easy-to-carry bars are packed with nutritious ingredients such as whole grains, nuts, dried fruit, protein, and, of course, carbohydrates.

Since they need to be chewed, they should be used in sports where this does not interfere with breathing and where it is possible to use your hands. Apart from road running and high-level cycling, most other sports are compatible.

It is worth noting that solid foods take between 20 and 30 minutes to digest. An energy bar is therefore not an immediate source of energy, but rather a slow-release energy product that is ideal for endurance events such as trail running or hiking.

Energy gels:

Energy gels are essential for athletes and endurance enthusiasts who need a quick energy boost during exercise. Concentrated and easy to consume, even while on the move, these gels are formulated to provide a rapid dose of simple carbohydrates, often enriched with electrolytes and sometimes vitamins or caffeine for an additional boost. Their viscous texture allows for rapid digestion and immediate absorption, thereby avoiding the discomfort of cramps or heaviness that solid foods can cause. Perfect for long-distance races, marathons, and cyclists, energy gels offer a practical way to boost performance without slowing down.

It is advisable to choose gels with a fairly fluid texture to facilitate absorption, because during exercise your mouth can easily become very dry. This will prevent you from having to drink too much afterward to wash them down, thereby reducing the risk of bloating.

The choice of packaging is also key. Some are easier to open than others, especially with one hand—cyclists will understand. Others are made of plastic, which is not in keeping with the times. Finally, and they are very rare, some are genuinely resealable, offering a double advantage.

• The gel can be consumed in several portions. This avoids a sudden intake of sugar into the body, which can have the opposite effect to the one expected, known as reactive hypoglycemia or “cotton legs” syndrome. To keep blood glucose levels stable, there is nothing better than taking a little gel every 15 minutes.

• A gel that does not reseal always retains a little product that eventually leaks and becomes sticky. When there is no trash can in the middle of nature, some inconsiderate athletes may be tempted to dispose of it on the spot. A resealable gel does not leak, so it can be kept until the next trash can.

Mulebar energy gels are resealable with a cap like a toothpaste tube, but they are also reusable—the only ones in the world. Simply rinse them with hot water after use and refill them with their eco-refill. It’s ecological and economical. A refill of 12 gels is 33% cheaper than 12 individually purchased tubes. 

Gummies:

Gummies are fairly recent and are intended to replace gels. They are small, gelatinous balls that simply need to melt in the mouth. In reality, they contain much less active ingredient than gels, which means you need to take more of them. Manufacturers state that they cannot put more than 8% active ingredient in a gummy. Some people tend to consume them like sweets, which quickly becomes expensive.

Energy fruit purées:

Energy fruit purées are a delicious and nutritious alternative to traditional energy bars and gels. These fruit purées, often enriched with cereals, seeds, or even vegetables, are designed to provide a quick, easily digestible release of energy. Their resealable pouch makes them convenient and quick to consume, ideal for athletes in the middle of intense effort. Rich in carbohydrates and sometimes boosted with vitamins and minerals, energy fruit purées provide not only an energy boost but also hydration and essential nutrients to support performance and recovery. They are an excellent option for those looking for a gentle and tasty source of energy, more convenient to consume than a bar because they are liquid, and more nourishing than a gel.

Sports drinks:

Sports drinks, also known as isotonic or athletic drinks, are specially formulated to support physical performance by providing rapidly assimilated carbohydrates and water. Generally available as a powder to dilute, they contain a balanced blend of carbohydrates, sodium, and other essential minerals, facilitating the rapid absorption of fluids and energy by the body. These drinks are ideal for athletes during extended training sessions or competitions, helping maintain fluid balance and prevent fatigue. Consuming them helps optimize performance and promote faster recovery, making them a preferred choice for athletes of all levels.

 

Dried fruit:

Dried fruit is a valuable ally for athletes, providing a concentrated and long-lasting source of energy during exercise. Rich in natural carbohydrates, it provides a rapid energy boost that is essential for supporting endurance and performance. In addition, dried fruit is an excellent source of minerals such as potassium and magnesium, which play a crucial role in preventing muscle cramps and improving recovery. Its small size and ease of transport make it particularly practical for long training sessions or competitions, where every second counts. By incorporating dried fruit into your sports diet, you benefit not only from a quick energy boost, but also from essential nutrients to optimize your performance and overall well-being.

Whatever form you choose, pay close attention to the ingredients listed on the packaging. Do not buy products containing ingredients that seem unnatural to you, or those containing sweeteners, preservatives, or other artificial flavorings. This will help you avoid potential digestive problems during exertion.

Finally, everyone will tell you that it is not recommended to consume a new product during a race without having tested it beforehand during training. 

The marathon “wall”: running out of fuel or hitting a mental block?

The famous “wall” marathon runners hit around the 30th–35th kilometer is not solely due to muscle fatigue. It is a double failure:

1. Peripheral Exhaustion: The muscle glycogen stores in your legs are empty. The muscle can no longer produce ATP (energy) at high intensity.

2. Central Fatigue (Neuroglycopenia): Your liver glycogen stores are depleted. Your blood glucose level threatens to drop. Your brain, detecting a vital danger to itself, sends signals of pain and extreme fatigue to force you to stop.

How much carbohydrate is recommended during exercise?

The recommended amount of carbohydrates during exercise varies according to the intensity and duration of the activity, but there are general guidelines that can help optimize performance and endurance. For long-duration efforts (more than one hour), it is recommended to consume between 30 and 60 grams of carbohydrates per hour. This intake helps maintain blood glucose levels and delay the depletion of muscle glycogen stores.

For very long efforts (more than 2.5 to 3 hours), such as marathons or triathlons, intake can be increased to 90 g or even 120 g of carbohydrates per hour, using multiple carbohydrate sources to promote better absorption and minimize the risk of gastrointestinal problems.

It is important to personalize your intake based on your body’s responses and your training experiences, as carbohydrate tolerance can vary considerably from person to person. It is also advisable to start consuming carbohydrates early during exercise and continue at regular intervals to maintain a steady energy supply.

Nutrition strategy: The loading protocol (carb loading)

Forget the old methods of aggressive “depletion.” Modern science recommends a gentler and more effective approach to maximize glycogen supercompensation.

J-7 to J-4: Preparation

• Training: Reduce the volume (tapering).

• Nutrition: Normal diet (5 to 7 g of carbohydrates per kg of body weight / day).

J-3 to J-1: The loading phase (The key to performance)

This is where you fill the tanks to the brim.

• Goal: Reach 8–10 g of carbohydrates / kg / day. For a 70 kg runner, that means up to 700 g of carbohydrates!

• What should you eat? Favor low- or moderate-glycemic-index carbohydrates (basmati rice, al dente pasta, sweet potatoes, quinoa) for sustained storage without sharp insulin spikes.

• Hydration: Drink 2 to 3 L of water. Without water, glycogen cannot be stored.

Race Day: The Glucose-to-Fructose Ratio

During exercise, your intestinal transporters become saturated. The glucose transporter (SGLT1) becomes saturated at 60 g/hour. To absorb more (necessary during an ultra), you need to use a second pathway: fructose (GLUT5). By combining glucose + fructose, you can absorb up to 90 g of carbohydrates per hour, reducing digestive issues and delaying fatigue. This is the foundation of modern energy gel formulations.

Why do I gain weight before my race? (The Glycogen–Water Link)

This is a common concern (and a frequently searched topic on Google). You follow your carbohydrate-loading diet, and the scales show +1.5 kg the day before the race. Panic? Absolutely not! This is an excellent sign.

As explained above, 1 gram of glycogen binds to approximately 3 grams of water. If you store an additional 500 g of glycogen through your diet, you mechanically store 1.5 kg of water. This is not fat. It is hydrated energy. This water will be released as you burn glycogen during the race, helping you stay hydrated from within.

 

In conclusion, glycogen plays a crucial role in sports performance, acting as a vital source of rapidly available energy to support the intensity and duration of physical effort. Proper management of glycogen stores through an appropriate diet and supplementation strategy can make a noticeable difference in achieving sporting goals, whether for regular training or competitions. By better understanding glycogen dynamics and optimizing its replenishment, athletes can not only improve their performance but also their ability to recover and prepare for future challenges.

 

FAQ: your frequently asked questions about glycogen

Does glycogen make you gain weight? No. Glycogen is a temporary energy reserve stored in the muscles and liver. If it is not used through physical activity and your stores become full, excess carbohydrates will then be converted into triglycerides (fat). But glycogen itself is not fat.

How long does it take to deplete your stores? On average, intense exercise (such as a half marathon or marathon at a good pace) will deplete your stores within 90 minutes to 2 hours. Beyond that, exogenous fuel (gels, bars) becomes essential to maintain intensity.

Which foods contain glycogen? Technically, almost none! Animal glycogen is rapidly broken down after slaughter. You do not “eat” glycogen; you eat carbohydrates (starch from starchy foods, fructose from fruit) that your body converts into glycogen through glycogenesis.