A starchy substance made and stored by animals (especially in the liver and muscles) that can be quickly broken down into glucose for energy when needed.
From Greek 'glykys' (sweet) + '-gen' (producer). Named in the 1850s when scientists discovered this was how animals store sugar energy, parallel to how plants store starch.
Your muscles are literally packed with glycogen right now—it's your body's 'quick cash' for emergency energy, which is why sprinters can explode with power but marathon runners need to fuel differently!
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