Whether you're training for an event, trying to increase your muscle mass or just want to get in better shape, your body relies on three main nutrients for fuel.
Because your body can break down carbohydrates, fat and muscle for energy, understanding the order of nutrient breakdown can help you save or build muscle mass and increase fat loss.
Your body burns fat before muscle, but only turns to burning fat after you've used up all of your glucose.
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're 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 _glycogen_, which is glucose stored in your muscles and liver.
Glycogen stores vary from person to person, but are typically depleted within 24 hours, at which point 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 9 calories per gram, fat is an efficient fuel source.
Additionally, your body metabolically prefers to preserve lean body mass and, when possible, breaks down fat stores for fuel. Only when your fat stores are extremely low or depleted does your body then have to break down protein.
When glucose and fat stores are depleted, your body will turn to muscle to break it 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.
Under normal conditions when you're eating on a regular basis, your body doesn't use muscle for energy. Typically, protein is used for fuel only when you're in starvation mode.
Because you need muscle tissue to survive and move, the natural tendency of metabolism is to spare muscle tissue and break down carbohydrates and fats first.
While your body avoids breaking down muscle tissue, an inadequate diet prevents muscle gains. Although you may be working out and trying to gain muscle mass, if you're not following a diet that supplies sufficient protein and calories, your body won't build muscle cells, and you may even notice muscle loss.
If you're not eating enough protein, the muscle damage that occurs during exercise can't be fixed, and muscle size or strength may decrease. Without adequate replenishment, you may think your body is breaking down muscle, when instead, your cells lack the nutrients needed to repair and rebuild.