Stories occasionally appear in the news of people surviving surprising lengths of time without food. Many of these stories are hard to believe because the effects of fasting are so readily apparent after just a few hours. They also drive the question of how long a person can live without access to food.
Under traditional circumstances, our bodies break food down into glucose, which provides energy to the body. After eight to 12 hours without food, the body burns through its glucose reserves and begins to convert glycogen from the liver and muscles into glucose. After two or three days of fasting, the liver synthesizes ketone bodies for fuel by breaking down fat stores. Eventually, all cells in the body break down proteins into amino acids, which the liver then turns into glucose. If fasting continues, the body preserves lean body tissue and tries to balance its fat consumption. Once there is no more fat, the body returns to metabolizing muscle because it is the only remaining energy source.
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