Glycogen

Glycogen is the form in which your body stores carbohydrates: long chains of glucose, kept in the liver and muscles. The liver keeps about 80 to 120 grams on hand and releases sugar from it into the blood so that the brain and organs stay supplied between meals and overnight. Depending on how well trained they are, the muscles store 300 to 500 grams – exclusively for their own use during exercise. Together, that's around 1,600 to 2,400 kcal, which are used up in roughly a day without carbohydrates or during a long endurance run.

For the scales, one property is crucial: every gram of glycogen is stored with roughly three grams of water. Full stores can therefore weigh up to two kilos. If you eat low-carb, you empty them within a few days and lose these kilos quickly – that's the famous instant success of low-carb diets. It's real, but it isn't fat: at the first pasta night, the kilos come back just as fast. Once you know that, you can read the scales more calmly.

For sport, full stores are an advantage. During intense exercise, carbohydrates are the fastest fuel; when the muscle stores are empty, you “hit the wall” – marathon runners know the feeling from around kilometre 30. After training, the body refills them within 24 to 48 hours, faster if you eat carbohydrates in the first few hours afterwards. For easy everyday training, a normal diet is perfectly sufficient.

Glycogen also explains why insulin and muscles work together so well: trained muscles have larger stores that are emptied more often, and they take up sugar from the blood more readily – one reason why exercise stabilises blood sugar so effectively.

In children, the stores work the same way but are smaller; that's why, for sport and school, they need regular meals with complex carbohydrates such as wholegrain bread, oats or fruit rather than quick sugar rushes.