When your body runs low on fuel, it draws first on sugar polymers called glycogen in the liver and muscles, its primary stockpile for stored energy. Soon thereafter it starts to burn fat, and if deprivation by fasting continues for a more prolonged time, your body may in turn begin to break down nonessential proteins for energy.
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Your body can burn through the glycogen stored in your liver over the first 18 to 24 hours of fasting. As glycogen stores run low, your body will resort to fat for energy, breaking down the fat molecules to supply your liver and muscles with energy. The resulting fatty acids cannot cross the blood-brain barrier, however, so your liver converts some of the fatty acids to compounds called ketone bodies, which serve as fuel for your brain.
Protein is the last source of stored energy available to your body, and thus the fuel of last resort. Under ordinary circumstances, the rate at which proteins are synthesized is equal to the rate at which they are degraded. During prolonged fasting, by contrast, your liver takes proteins, breaks them down to amino acids and then removes the amine groups, creating urea in the process. The urea is excreted, while the other compounds produced by protein breakdown are fed into the citric acid cycle to make glucose for the brain and other tissues.
Your liver begins to respond to the effects of prolonged fasting by breaking down nonessential proteins. During the early stages of fasting, fat is the primary source of fuel, although protein is metabolized also. Only when fat reserves are exhausted does the body begin to cannibalize essential proteins from liver and heart muscle tissue; at this point, continued fasting can end in death by cardiac arrest. During shorter periods of fasting, however, essential proteins remain unaffected and are not used for fuel.
Fasting can be divided into several periods. During the first six to 24 hours, glycogen is the primary source of energy. Over the next eight days, fat becomes the primary fuel and the rate of protein metabolism also increases. Protein constitutes about 15 percent of the body's stored energy, in contrast to the much larger portion represented by fat. Since protein breakdown is signaled by increased urea production, levels of nitrogen excretion can serve as one indicator of the rate at which proteins are metabolized.