Gluconeogenesis

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  • 17.1 Glucose Can Be Synthesized from Noncarbohydrate Precursors

  • 17.2 Gluconeogenesis and Glycolysis Are Reciprocally Regulated

  • 17.3 Metabolism in Context: Precursors Formed by Muscle Are Used by Other Organs

Fasting is a part of many cultures and religions, including those of the Teton Sioux. Fasting is believed to cleanse the body and soul and to foster spiritual awakening. Gluconeogenesis is an important metabolic pathway during times of fasting because it supplies glucose to the brain and red blood cells, tissues that depend on this vital fuel.

We now turn to the synthesis of glucose from noncarbohydrate precursors, a process called gluconeogenesis. Maintaining levels of glucose is important because the brain depends on glucose as its primary fuel and red blood cells use glucose as their only fuel, as stated earlier. The daily glucose requirement of the brain in a typical adult human being is about 120 g, which accounts for most of the 160 g of glucose needed daily by the whole body. The amount of glucose present in body fluids is about 20 g, and that readily available from glycogen, the storage form of glucose, is approximately 190 g. Thus, the direct glucose reserves are sufficient to meet glucose needs for about a day. Gluconeogenesis is especially important during a longer period of fasting or starvation.

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The major site of gluconeogenesis is the liver, with a small amount also taking place in the kidney. Little gluconeogenesis takes place in the brain, skeletal muscle, or heart muscle. Rather, gluconeogenesis in the liver and kidney helps to maintain the glucose concentration in the blood, from which it can be extracted by the brain and muscle to meet their metabolic demands.

In this chapter, we begin by examining the reactions that constitute the gluconeogenic pathway. We then investigate the reciprocal regulation of gluconeogenesis and glycolysis. The chapter ends with a look at how gluconeogenesis and glycolysis are coordinated between tissues.