Glucose Consumption

figure 7-1 Schematic diagram describing the utilization and production of glucose by a normal man in a postabsorptive state (i.e., not actively ingesting a meal). The values for glucose uptake represent the amounts in grams consumed per day; in the case of muscle this refers to the resting state. Glucose output from the liver derives from glycogenolysis; thus, approximately 25% of the liver glycogen is released as glucose and 75% is retained by the liver. As starvation extends beyond 12 hr, glycogen stores are depleted and the contribution from gluconeogenesis increases. Abbreviations: RBC, red blood cell; WBC, white blood cell; AA, amino acid. [Reproduced with permission from the author and publisher of Felig, P. (1979). Starvation. In "Endocrinology" (L. J. DeGroot et al, eds.), Vol. 3, pp. 1927-1940. Grune & Stratton, New York.]

figure 7-1 Schematic diagram describing the utilization and production of glucose by a normal man in a postabsorptive state (i.e., not actively ingesting a meal). The values for glucose uptake represent the amounts in grams consumed per day; in the case of muscle this refers to the resting state. Glucose output from the liver derives from glycogenolysis; thus, approximately 25% of the liver glycogen is released as glucose and 75% is retained by the liver. As starvation extends beyond 12 hr, glycogen stores are depleted and the contribution from gluconeogenesis increases. Abbreviations: RBC, red blood cell; WBC, white blood cell; AA, amino acid. [Reproduced with permission from the author and publisher of Felig, P. (1979). Starvation. In "Endocrinology" (L. J. DeGroot et al, eds.), Vol. 3, pp. 1927-1940. Grune & Stratton, New York.]

of the daily energy intake should be derived from protein, fat, and carbohydrate, respectively. Each food substance has its own characteristic energy yield (see Table 7-4), which reflects the relative extent of reduction of the carbon atoms of that food component.

The unit of metabolic energy historically utilized by nutritionists is the kilocalorie (1000 cal); 1.0 kcal represents the energy required to raise the temperature of 1000 g of water from 15.5 to 16.5°C. However, increasingly nutritionists are expressing energy in terms of joules; 1.0 kcal is equivalent to 4.185 kj.

In addition to kilocalories or kilojoules, the diet must supply, on a regular basis, adequate amounts of the various essential dietary constituents; these include water and fat-soluble vitamins, the essential unsaturated fatty acids, the essential amino acids, and both bulk and trace minerals.

The basal metabolic rate (BMR) is defined as the energy expenditure required to maintain those cellular processes that are essential for the continuing activities of the organism; these include the metabolic activity of the heart, lungs, kidneys, and other vital organs.

The BMR of an adult 70-kg male is 35-40 kcal / m2 body surface/hr. The comparable value for a woman of the same age is 29-36 kcal/m2 body surface.

Not all of the caloric content of dietary constituents is available to support the daily kilocalorie requirements. When food is eaten there is a rise in total body oxygen consumption; this is known as the dietary-induced thermogenesis (DIT) of foodstuffs. Although a detailed molecular explanation of the process of DIT has not been given, in essence it represents energy that is spent in the metabolic processing of food, which is therefore lost or not available to support other bodily energy requirements. The approximate DIT is ~30% for protein, 0% for carbohydrate, and 4% for lipid, each in terms of the theoretical energy value of the food component ingested.

The dietary energy requirements necessary for normal bodily functions vary with age and sex; the recommended values are tabulated in Table 7-5. Of course, the energy intake must be increased appropriately during intervals of prolonged exercise.

II. ANATOMICAL, MORPHOLOGICAL, AND PHYSIOLOGICAL RELATIONSHIPS

A. Introduction

The hepatopancreatic complex, along with the gall bladder, is responsible for the integrated digestion and subsequent processing of most dietary nutrients. The complex series of steps and the components of the digestive system are summarized in Table 8-3. The pancreas is both an exocrine and an endocrine gland. The exocrine pancreas biosynthesizes and secretes the major digestive enzymes, for example, the proteases (chymotrypsin and trypsin), amylase, and lipase, as well as bicarbonate, while the endocrine tissue of the pancreas produces the peptide hormones insulin, glucagon, pancreatic polypeptide, somatostatin, and small amounts of other peptide hormones (see Table 7-6).

Figure 7-2 illustrates the gross anatomical features of the human pancreas. Anatomically, the pancreas has distinct dorsal and ventral lobes. The pancreas can be divided into lobules that contain the exocrine acinar glands and the endocrine islets of Langerhans. The islet cells are separated from the surrounding acinar cells by a thin layer of reticular tissue. The endocrine protein of the pancreas is only 1-2% of the weight of the gland. Figure 7-3 schematically illustrates the relationship between the number and distribution of the insulin- and the glucagon-secreting cells in typical rat and human pancreatic islets.

TABLE 7-5 Median Heights and Weights and Recommended Energy Intake0
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