When are carbohydrates a reserve body fuel?

The body has a priority list when choosing among energy sources, in order to preserve homeostasis and the most important organs from malnutrition. Carbohydrates are utilized first, because they are the easiest to metabolize and are widely available. They are in circulation (as glucose) and stored or converted within the liver. Since they take the least amount of energy to use, they get first choice. Then the fats are used. Also widely available and require only transport out of adipose tissue with little energy used. Proteins are reserved as a last source of energy, since they require the most energy to derive energy from (nitrogen bonds are tough to break apart), and since use beyond that provided by dietary intake must be obtained my catabolic "muscle-wasting" processes.