The first fuel that your body breaks down for energy is carbohydrates. After a meal, your body is in the “fed” state and preferentially breaks down carbohydrates since they are easily accessible and turned into energy. After your body has used up the carbohydrates from a meal or snack, your cells begin to break down glucose stored in your muscles and liver known as glycogen. Glycogen stores vary in each person, but are typically depleted within 24 hours, meaning your body has to begin breaking down other compounds for energy.
Fat Preferentially Metabolized
When glucose and glycogen are not available, your body preferentially breaks down fatty compounds known as triacylglycerols which are present in adipose or fat tissue. Because fat is a high-energy source with nine calories per gram, fat provides an efficient fuel source. Additionally, your body metabolically prefers to preserve lean body mass and, when possible, breaks down fat stores for fuel as much as possible. Only when your fat stores are extremely low or depleted does your body have to then break down protein.
When glucose and fat stores are depleted, your body will then turn to muscle to break down into individual amino acids for energy. Unlike carbohydrates and fat, your body does not store amino acids, which is why muscle breakdown is the only way to release amino acids for fuel. In typical conditions where you are eating on a regular basis, your body will not use muscle for energy. Typically, protein is used for fuel only in a starvation state. Because you need muscle tissue to survive and move, the natural tendency of metabolism is to spare muscle tissue and break down carbohydrates and fat first.