Glycogen is the body's main store of glucose. Held in the liver and the muscles, it supplies energy that is ready to use between meals and during exercise. Understanding how it works helps you eat better, train better and recover better.
This article answers the questions people ask most: what glycogen is, where it is stored, how it turns back into glucose, and how to keep your reserves topped up day to day.
What is glycogen? A simple definition
Glycogen is a large molecule made of thousands of glucose units linked together. It is the form the body uses to stockpile sugar. When you eat a meal rich in carbohydrates, some of the glucose passes into the blood. The surplus is then stored as glycogen, to be used later.
What does “glycogen” mean? The shape of the molecule
The word comes from Greek: “glykys” (sweet) and “gennan” (to produce). So glycogen literally “produces” sugar. Its structure is branched, like the limbs of a tree. That architecture lets enzymes release glucose very quickly when it is needed — a real advantage when effort comes on suddenly.
Glucose or glycogen: what is the difference?
Glucose is a simple sugar that circulates in the blood and feeds cells directly. Glycogen is its storage form. Put simply: glucose is the cash in your pocket, glycogen is the savings account. The body moves between the two constantly, depending on how much energy it needs.
Where is glycogen found in the body?
Storage happens almost entirely in two organs: the liver and the muscles. Hence muscle glycogen and liver glycogen, two reserves with clearly separate jobs.
Liver glycogen: the blood sugar regulator
The liver holds roughly 80 to 120 grams of glycogen. Liver glycogen keeps blood sugar steady, especially overnight and between meals. It can be released into the bloodstream to feed the brain and the vital organs. After 12 to 24 hours without food, this reserve is close to empty.
Muscle glycogen: fuel for training
Muscles store 300 to 500 grams of glycogen, depending on muscle mass and training level. Unlike the liver, muscle keeps its glucose to itself: muscle glycogen only fuels the muscle it sits in. This is the store that drains during a hard lifting session or a long endurance event.
A limited reserve compared with body fat
All told, these stores add up to around 2,000 calories, about a day of activity. Body fat, by comparison, holds tens of thousands of calories as lipids. Glycogen is a fast reserve, but a short one; fat is slow, but almost bottomless.
Glycogen metabolism: storage and breakdown
Glycogen metabolism runs on two opposing routes, controlled by specific hormones and enzymes. Understanding the mechanism makes it easier to handle your food and your recovery.
Synthesis: the role of insulin and glycogen synthase
After a meal, blood sugar rises and the pancreas releases insulin. That hormone opens the cells to glucose and switches on a key enzyme, glycogen synthase, which links glucose units together. This is glycogenesis: building new reserves. It is especially active in the two hours that follow exercise.
How does glycogen turn back into glucose?
In the other direction, when blood sugar falls, glucagon and adrenaline activate glycogen phosphorylase. This enzyme cuts the molecule apart and releases glucose that can be used straight away. This is glycogen breakdown, also called glycogenolysis. Inside the cell, that glucose is then burned to produce energy, with or without oxygen depending on the type of effort.
What happens when the stores are empty?
Once glycogen stores run out, the body switches fuel. It draws on the lipids in body fat and converts certain amino acids into glucose. That switch explains the famous “marathon wall”, which usually shows up after about 90 minutes of sustained effort. Emptying your stores completely can bring on sudden fatigue, dizziness and a sharp drop in performance.
How do you increase your glycogen stores?
Good news: your energy reserves rebuild naturally through food. A few simple rules are enough to get storage right.
Build your diet on quality carbohydrates
Diet is the foundation. To refill your stores, you need carbohydrates at every meal:
- Whole grains and starches: rice, pasta, bread, oats, sweet potato;
- Legumes: lentils, chickpeas, kidney beans;
- Fruit: banana, grapes, dried fruit, all rich in natural sugars;
- Fluids: every gram of glycogen holds on to around 3 grams of water.
Allow 24 to 48 hours for a full refill after a very long session, and sometimes several days after a marathon.
Glycogen before sport: setting up your effort
Before a race, plenty of athletes carb-load: they push to 8 or 10 grams of carbohydrate per kilogram of body weight over the two or three days beforehand. On the day itself, an easily digested meal three hours before the start tops up the liver stores. To plan these loading phases without getting it wrong, working with a personal trainer saves valuable time.
Make the most of the window after training
The first few hours after exercise are the ones that count. The synthesis enzymes are highly active then, even with little insulin. Taking in around 1 gram of carbohydrate per kilogram within the hour, ideally alongside some protein, speeds up resynthesis considerably. That is the best moment for your recovery snack.
Glycogen and weight loss: what is the link?
Low-carb diets, like keto, empty the stores fast. The result: two or three kilos off the scale in the first few days. Careful, though, that is mostly the water bound to glycogen, not fat. Real weight loss starts later, when the body begins drawing steadily on body fat. From that point on, lipids become the dominant energy source. A personalized training program helps you get past that stage without losing muscle.
Practical questions: pharmacy, MRI, medical tests
Why is Glycogène Plus no longer sold in pharmacies?
Glycogène Plus was a supplement built on vitamins and amino acids, used against short-term fatigue. It has been discontinued, which is why you can no longer find it in pharmacies. Your pharmacist can suggest alternatives with similar reviews: magnesium, B vitamins or anti-fatigue complexes.
Glycogen and MRI: why a diet before certain scans?
Before certain imaging tests, such as a PET scan or a cardiac MRI, patients are sometimes asked to follow a low-carb diet and then fast. The aim is to empty the stores so that glucose uptake by the tissues does not interfere with the images. Always follow the exact instructions from your imaging center.