Glucose from dietary carbohydrates

Very low density lipoprotein (VLDL) is produced in the liver, mainly from dietary carbohydrate. Lipogenesis is an insulin-stimulated process through which glucose is converted to fatty acids, which are subsequently esterified to glycerol to form the triacylglycerols that are packaged in VLDL and secreted from the liver. Thus, chylomicrons primarily transport dietary lipids, and VLDL transports endogenously synthesized lipids. 

Mannose, the C-2 epimer of glucose (see p. 84), is an important component of glycoproteins (see p. 164). Hexokinase phosphorylates mannose, producing mannose 6-phosphate, which, in turn, is (reversibly) isomerized to fructose 6-phosphate by phosphoman-nose isomerase. [Note There is little mannose in dietary carbohydrates. Most intracellular mannose is synthesized from fructose, or is preexisting mannose produced by the degradation of structural carbohydrates and salvaged by hexokinase.]... 

Carbohydrate nutrition is different in nature from the nutrition of amino acids, fats, vitamins, and minerals. Humans have no dietary requirement for any specific type of carbohydrate. All carbohydrates of the body can be synthesized from dietary glucose or fructose, the major sugars in the diet. The body stores sugar in the form of glycogen, a polysaccharide composed of glucose units. 

Glycosidase inhibitors (Table 1.9) delay the digestion of dietary carbohydrate in the form of starch and sucrose into monosaccharides, which can be absorbed from the small intestine. By delaying absorption, these agents lower postprandial blood glucose and insulin levels and are used for this purpose in the treatment of type 2 diabetes. 

Although fatty acid synthesis occurs within the cytoplasm of most animal cells, liver is the major site for this process. (Recall, for example, that liver produces VLDL. See p. 349.) Fatty acids are synthesized when the diet is low in fat and/or high in carbohydrate or protein. Most fatty acids are synthesized from dietary glucose. As discussed, glucose is converted to pyruvate in the cytoplasm. After entering the mitochondrion, pyruvate is converted to acetyl-CoA, which condenses with oxaloacetate, a citric acid cycle intermediate, to form citrate. When mitochondrial citrate levels are sufficiently high (i.e., cellular energy requirements are low), citrate enters the cytoplasm, where it is cleaved to form acetyl-CoA and oxaloacetate. The net reaction for the synthesis of palmitic acid from acetyl-CoA is as follows ... 

More than 40% of the calories in the typical diet in the United States are obtained from starch, sucrose, and lactose. These dietary carbohydrates are converted to glucose, galactose, and fructose in the digestive tract (Fig. 1). Monosaccharides are absorbed from the intestine, enter the blood, and travel to the tissues where they are metabolized. 

Most foods derived from animals, such as meat or fish, contain very little carbohydrate except for small amounts of glycogen (which has a structure similar to amylopectin) and glycolipids. The major dietary carbohydrate of animal origin is lactose, a disaccharide composed of glucose and galactose found exclusively in milk and milk products (see Fig. 27.1). 

Conn (1940) showed that reduction of dietary carbohydrate in normal subjects and diabetics causes profound impairment of glucose tolerance, which reverts to normal when carbohydrate intake is increased again. His main purpose was to preach caution in the interpretation of glucose tolerance tests unless the previous diet had been standardized for several days and contained a sufficient quantity of carbohydrate. Amazingly, this simple point has been overlooked by physicians prescribing dietary treatment for their diabetics. There were a few reports of benefit from an increase in available carbohydrate intake (Kempner et al, 1958 Ernest et al., 1962), but it is only in the last 10 yr that any real interest in this effect has been shown. Even then, results were overshadowed by preoccupation with fiber, so that the possible benefits of increased intake of available carbohydrate have received much less attention. 

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