Glycogen is a substance your body makes to store energy from the food you eat. Think of it as your body's backup battery. When you eat a meal with carbohydrates like bread, pasta, or fruit, your body converts some of that food into glucose (a type of sugar) for immediate energy. Any extra glucose gets transformed into glycogen and stored mainly in your liver and muscles, ready to be used later when you need it.

This storage system matters most during physical activity. When you're running around at recess or playing soccer, your muscles break down their glycogen stores to fuel your movement. Athletes talk about “hitting the wall” during long races because they've used up their glycogen and suddenly feel exhausted. That's why marathon runners often eat pasta the night before a big race: they're loading up their glycogen stores.

Your liver's glycogen serves a different purpose. It releases glucose into your bloodstream between meals to keep your blood sugar steady, which is why you can think clearly even hours after breakfast. Without glycogen, your body would need constant refueling, like a car that can't store any gas in its tank.