If your body runs out of other sources of fuel, it will start to use muscle tissue for energy. This is not a normal condition, and your body will only start to use muscle tissue for energy under extreme conditions, such as if you are very sick, severely malnourished or not consuming enough calories over an extended period of time to support normal body functions.
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Every cell in your body needs energy to perform normal body functions such as moving, breathing, maintaining your heartbeat and healing damaged tissue. Normally, carbohydrates from your diet supply the types of sugar your body uses as its main source of energy. To get enough sugar from your diet to supply your body with the energy it needs, approximately half of your daily calories should come from carbohydrates. Your body is also able to produce energy from proteins and fat, but carbohydrates are considered the best source of energy because sugar is the only fuel your brain cells and certain other body cells can use.
During digestion, your body breaks down carbohydrates into simple sugars that are converted to glucose, or blood sugar. Glucose travels in your blood to every cell in your body, where it is used to manufacture energy. If you consume more sugar than your body needs for immediate energy, some of the excess is converted into glycogen, a type of sugar that is stored in your muscle tissue. If your body needs glucose, and no sugar is coming in from your diet, glycogen is released from your muscles and broken down to supply enough glucose for energy to last about half a day.
Muscle tissue is made up mostly of protein, which, in turn, is made up of amino acids. Normally, your body does not use protein to produce energy. If you run out of glycogen stores, however, and no glucose is available, your body will then break down its own muscle tissue to release amino acids. These amino acids are sent to your liver, where they are converted to glucose in a process called gluconeogenesis. When your body starts to use muscle tissue for energy, you lose muscle mass.
Normally, the body will only start to use muscle tissue for energy in the case of starvation. If you have an eating disorder such as anorexia nervosa or engage in long-term fasting, however, or if you are sick with a chronic disease, such as cancer, and you are unable to consume a balanced diet that contains sufficient calories, your metabolism changes. That means the rate at which you burn calories for fuel changes. Over time, as your body chemistry is altered and you continue to be undernourished, you may lose muscle because your body resorts to breaking down its own tissue in search of fuel.