Carbohydrate loading starts about 3 weeks before an event with glycogen content more than doubling in athletes who follow a regimented high carbohydrate program

This is not true. Carb loading is done the week before the high-endurance activity (marathon for example). A carbohydrate loading diet is diet with high carbohydrates .  The carbohydrates are the body’s main energy source. The goal is  increasing the amount of fuel stored in you muscles in order to improve athletic performance and endurance.