Exam 10: Weight Management

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The body's first adjustment to fasting is to use the liver's glycogen for needed fuel. (The glycogen in muscles is reserved for the muscles' own use-and they do use it. The liver's glycogen, remember, is the body's source of blood glucose to fuel brain and nerve activities. Ordinarily, the brain and nerves can use no other fuel, but after about a day without food, the primary supply is gone. Where, then, does the body turn to keep the nervous system going? Whatever it has to do, it will do, for the nervous system runs the body, and when it stops, the body dies.
An obvious alternative source of energy is the abundant fat stores that most people carry. At first, this fat is of no use to the nervous system. The muscles and other organs can use fat as fuel, but the nervous system ordinarily cannot. Nor can the body convert this fat to glucose, because it possesses no enzymes to do so. It does, however, possess enzymes to convert protein to glucose.
When the fast continues, the body turns to its own lean tissues to provide the necessary supply of glucose. One reason people lose weight so dramatically within the first three days of a fast is that they are devouring their own protein tissues as fuel. Because protein contains only half as many calories per pound as fat, it disappears twice as fast. Also, with each pound of body protein, three or four pounds of associated water are lost. This same process accounts for the rapid weight loss seen in the early stages of a low-carbohydrate diet.
If the body were to continue to consume itself at this rate, death would ensue within about 10 days. After all, the liver, the heart and skeletal muscles, the lung tissue, and the blood -all vital tissues-are being burned as fuel. (In fact, fasting or starving people remain alive only until their body fat is gone or until half their lean tissue is gone, whichever comes first. But now the body plays its last trump card: It begins converting fat stores into a form it can use to help feed the nervous system and so forestall the end. This is known as ketosis.