The red cell and the brain have an absolute requirement for blood glucose for energy metabolism. These cells consume about 80% of the 200 g of glucose consumed in the body per day. There is only about 10 g of glucose in the plasma and extracellular fluid volume, so that blood glucose must be replenished constantly. Otherwise, hypoglycemia develops and compromises brain function, leading to confusion and disorientation and possibly life-threatening coma at blood glucose concentrations below 2.5 mmol/L (45 mg/dL). We absorb glucose from our intestines for only 2-3 h following a carbohydrate-containing meal, so there must be a mechanism for maintenance of blood glucose between meals.
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| Glycogen, a polysaccharide storage form of glucose, is our first line of defense against declining blood glucose concentration. During and immediately following a meal, glucose is converted in liver into glycogen (a process known as glycogenesis). Hepatic glycogen is gradually degraded between meals, by the pathway of glycogenolysis, releasing glucose to maintain blood glucose concentration. However, total hepatic glycogen stores are barely sufficient for maintenance of blood glucose concentration during a 12-h fast.
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| During sleep, when we are not eating, there is a gradual shift from glycogenolysis to de novo synthesis of glucose, also an hepatic pathway, known as gluconeogenesis (Fig. 12.1). Gluconeogenesis is essential for survival during fasting or starvation, when glycogen stores are negligible. The liver uses amino acids from muscle protein as the primary precursor of glucose, but also makes use of lactate from glycolysis and glycerol from fat catabolism. Fatty acids, mobilized from adipose tissue triglyceride stores, provide the energy for gluconeogenesis.
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| Glycogen is also stored in muscle, but this glycogen is not available for maintenance of blood glucose. Glucose, derived in part from glycogen, especially during bursts of physical activity, is essential for muscle energy metabolism, even though muscle relies primarily on fats as a source of energy. The tissue concentration of glycogen is higher in liver than in muscle, but because of the relative masses of muscle and liver, the majority of glycogen in the body is stored in muscle (Table 12.1).
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| This chapter describes the pathways of biosynthesis (glycogenesis) and mobilization (glycogenolysis) of glycogen in liver and muscle and the pathway of gluconeogenesis in liver.
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