A normal person who is eating three meals a day and snacking between meals gets almost all of his or her energy from the glucose that carbohydrates provide. What happens if you stop eating, however? For example, what if you are lost in the woods, or you are purposefully fasting? What does your body do for energy? Your body goes through several phases in its attempt to keep you alive in the absence of food.
The first line of defense against starvation is the liver. The liver stores glucose by converting it to glycogen. It holds perhaps a 12-hour supply of glucose in its glycogen. Once you finish digesting all of the carbohydrates that you last ate, the liver starts converting its stored glycogen back into glucose and releases it to maintain glucose in the blood. Lipolysis also starts breaking down fat in the fat cells and releasing fatty acids into the bloodstream. Tissues that do not need to use glucose for energy (for example, muscle cells) start burning the fatty acids. This reduces the glucose demand so that nerve cells get the glucose.
Once the liver runs out of glycogen, the liver converts to a process called gluconeogenesis. Gluconeogenesis turns amino acids into glucose (see this article for more on gluconeogenesis).
The liver then begins producing ketone bodies from fatty acids being made available in the blood by lipolysis. Brain and nerve cells convert over from being pure consumers of glucose to partial consumers of ketone bodies for energy (see this article for information on ketone body metabolism).
Some of these alternative metabolic processes are actually used on a regular basis. For example, Eskimos eating a traditional Eskimo diet have virtually no carbohydrates on the menu. You may have also read about several recent weight-loss programs that try to take advantage of ketone metabolism to "burn fat" (this article offers a thorough description of the "ketogenic diet" as used in medicine, and this article talks about the "fad diets" that utilize the ketone effect). When you hear about these diets you will now have a better idea of what they're about!
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