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Carbohydrate synthesis and degradation play a vital role in cellular function by providing the energy required for most metabolic processes. The carbohydrates this chapter covers include three monosaccharides—glucose, galactose, and fructose. We also discuss the polysaccharide glycogen. The relevant biochemical pathways of these carbohydrates are shown in Figure 154-1.

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Glucose is the principal substrate of energy metabolism in humans. Metabolism of glucose generates adenosine triphosphate (ATP) via glycolysis (conversion of glucose or glycogen to pyruvate) or oxidative phosphorylation in the mitochondria (conversion of pyruvate to carbon dioxide and water), or both. A continuous source of glucose from dietary intake, gluconeogenesis (glucose made de novo from amino acids, primarily alanine), and degradation of glycogen maintains normal blood glucose levels. We obtain glucose in our diet by ingesting polysaccharides, primarily starch, and disaccharides, including lactose maltose and sucrose.

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Galactose and fructose are two other monosaccharides that can provide fuel for cellular metabolism; however, their role is much less significant than that of glucose. Galactose is derived from lactose (galactose + glucose), which is found primarily in milk and milk products. If necessary, galactose can be incorporated into glycogen via galactose-1-phosphate and glucose 1-phosphate, becoming a source of glucose. Galactose is also an important component for certain glycolipids, glycoproteins, and glycosaminoglycans. The two dietary sources of fructose are sucrose (fructose + glucose), a commonly used sweetener, and fructose, which is found in fruits, vegetables, and honey.

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The defects in glycogen metabolism typically cause an accumulation of glycogen in the tissues, hence the name glycogen storage disease (GSD). The defects in gluconeogenesis or in the glycolytic pathway, including galactose and fructose metabolism, do not usually result in an accumulation of glycogen in the tissues.

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Clinical manifestations of the various disorders of carbohydrate metabolism differ markedly. The symptoms range from relatively benign to lethal.

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Figure 154-1.
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Metabolic pathways related to glycogen storage diseases and to galactose and fructose disorders. Nonstandard abbreviations are as follows: GSa, active glycogen synthase; GSb, inactive glycogen synthase; Pa, active phosphorylase; Pb, inactive phosphorylase; PaP, phosphorylase a phosphatase; PbKa, active phosphorylase b kinase; PbKb, inactive phosphorylase b kinase; G, glycogenin, the primer protein for glycogen synthesis. (Modified from AR Beaudet. Glycogen storage diseases. In: Isselbacher KJ, et al, eds. Harrison’s Principles of Internal Medicine. 13th ed. New York: McGraw-Hill; 1994.)

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