The human body stores approximately 400-500g of carbohydrate as glycogen (1600-2000 kcal). At moderate intensity, this buffers 90-120 minutes of riding before performance drops. Glycogen depletion (classic 'bonking') forces the body to run primarily on fat — slower to mobilise, so sustainable intensity falls 20-40%. Prevention: 60-90g of carbs per hour from minute 30 onward, matched to ride duration and intensity. Strategic glycogen depletion ('train low') is sometimes used to drive aerobic adaptations, but should not happen in races or hard-session training.