One gram of carbohydrates provides four calories of energy. The body can store a maximum of 15 grams of glycogen per kilogram of body weight (15 grams per 2.2 pounds). This would mean that a 175-pound athlete could store up to 1,200 grams of glycogen (4,800 calories).
What happens to excess glycogen, for example if the athlete consumed 5,000 calories? Is it all converted to fat?
Yes but also it’s a bit more complex than that. Glycogen can be stored in skeletal muscle tissue and the liver. Because the body needs a certain concentration of glucose at all times in the bloodstream, the liver slowly releases it between meals (the muscle is similar except it’s usage is dependent on how much you actually use said muscle). Fat cells are also living tissue, so they are dynamically storing and releasing energy in the form of fat as well, that gets converted into more usable forms of energy in the liver. An athletes body will also use up more energy, so they will be able to use up those “excess” calories faster/more efficiently