Ganong's Review of Medical Physiology, 23rd Edition

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464 SECTION V Gastrointestinal Physiology intake of 1 g/kg body weight to supply the eight nutritionally essential amino acids and other amino acids is desirable. The source of the protein is also important. Grade I proteins, the animal proteins of meat, fish, dairy products, and eggs, contain amino acids in approximately the proportions required for protein synthesis and other uses. Some of the plant proteins are also grade I, but most are grade II because they supply different proportions of amino acid and some lack one or more of the essential amino acids. Protein needs can be met with a mixture of grade II proteins, but the intake must be large because of the amino acid wastage. Fat is the most compact form of food, since it supplies 9.3 kcal/g. However, often it is also the most expensive. Indeed, internationally there is a reasonably good positive correlation between fat intake and standard of living. In the past, Western diets have contained large amounts (100 g/d or more). The evidence indicating that a high unsaturated/saturated fat ratio in the diet is of value in the prevention of atherosclerosis and the current interest in preventing obesity may change this. In Central and South American Indian communities where corn (carbohydrate) is the dietary staple, adults live without ill effects for years on a very low fat intake. Therefore, provided that the needs for essential fatty acids are met, a low-fat intake does not seem to be harmful, and a diet low in saturated fats is desirable. Carbohydrate is the cheapest source of calories and provides 50% or more of the calories in most diets. In the average middle-class American diet, approximately 50% of the calories come from carbohydrate, 15% from protein, and 35% from fat. When calculating dietary needs, it is usual to meet the protein requirement first and then split the remaining calories between fat and carbohydrate, depending on taste, income, and other factors. For example, a 65-kg man who is moderately active needs about 2800 kcal/d. He should eat at least 65 g of protein daily, supplying 267 (65 × 4.1) kcal. Some of this should be grade I protein. A reasonable figure for fat intake is 50 to 60 g. The rest of the caloric requirement can be met by supplying carbohydrate. MINERAL REQUIREMENTS A number of minerals must be ingested daily for the maintenance of health. Besides those for which recommended daily dietary allowances have been set, a variety of different trace elements should be included. Trace elements are defined as elements found in tissues in minute amounts. Those believed to be essential for life, at least in experimental animals, are listed in Table 27–3. In humans, iron deficiency causes anemia. Cobalt is part of the vitamin B 12 molecule, and vitamin B 12 deficiency leads to megaloblastic anemia (see Chapter 32). Iodine deficiency causes thyroid disorders (see Chapter 20). Zinc deficiency causes skin ulcers, depressed immune responses, and hypogonadal dwarfism. Copper deficiency causes anemia and changes in ossification. Chromium deficiency causes insulin resistance. Fluorine deficiency increases the incidence of dental caries. Conversely, some minerals can be toxic when present in the body in excess. For example, severe iron overload causes hemo- TABLE 27–3 Trace elements believed essential for life. Arsenic Manganese Chromium Molybdenum Cobalt Nickel Copper Selenium Fluorine Silicon Iodine Vanadium Iron Zinc chromatosis, copper excess causes brain damage (Wilson disease), and aluminum poisoning in patients with renal failure who are receiving dialysis treatment causes a rapidly progressive dementia that resembles Alzheimer disease (see Chapter 19). Sodium and potassium are also essential minerals, but listing them is academic, because it is very difficult to prepare a sodium-free or potassium-free diet. A low-salt diet is well tolerated for prolonged periods because of the compensatory mechanisms that conserve Na + . VITAMINS Vitamins were discovered when it was observed that diets adequate in calories, essential amino acids, fats, and minerals failed to maintain health. The term vitamin has now come to refer to any organic dietary constituent necessary for life, health, and growth that does not function by supplying energy. Because there are minor differences in metabolism between mammalian species, some substances are vitamins in one species and not in another. The sources and functions of the major vitamins in humans are listed in Table 27–4. Most vitamins have important functions in intermediary metabolism or the special metabolism of the various organ systems. Those that are water-soluble (vitamin B complex, vitamin C) are easily absorbed, but the fat-soluble vitamins (vitamins A, D, E, and K) are poorly absorbed in the absence of bile or pancreatic lipase. Some dietary fat intake is necessary for their absorption, and in obstructive jaundice or disease of the exocrine pancreas, deficiencies of the fat-soluble vitamins can develop even if their intake is adequate. Vitamin A and vitamin D are bound to transfer proteins in the circulation. The α-tocopherol form of vitamin E is normally bound to chylomicrons. In the liver, it is transferred to very low density lipoprotein (VLDL) and distributed to tissues by an α-tocopherol transfer protein. When this protein is abnormal due to mutation of its gene in humans, there is cellular deficiency of vitamin E and the development of a condition resembling Friedreich ataxia. Two Na + -dependent L-ascorbic acid transporters have recently been isolated. One is found in the kidneys, intestines, and liver, and the other in the brain and eyes.
TABLE 27–4 Vitamins essential or probably essential to human nutrition. a CHAPTER 27 Digestion, Absorption, & Nutritional Principles 465 Vitamin Action Deficiency Symptoms Sources Chemistry A (A 1 , A 2 ) Constituents of visual pigments (see Chapter 12: Vision); necessary for fetal development and for cell development throughout life B complex Thiamin (vitamin B 1 ) Riboflavin (vitamin B 2 ) Cofactor in decarboxylations Constituent of flavoproteins Niacin Constituent of NAD + and NADP + Pyridoxine (vitamin B 6 ) Pantothenic acid Forms prosthetic group of certain decarboxylases and transaminases. Converted in body into pyridoxal phosphate and pyridoxamine phosphate Night blindness, dry skin Yellow vegetables and fruit Beriberi, neuritis Liver, unrefined cereal grains Glossitis, cheilosis Liver, milk Pellagra Yeast, lean meat, liver Convulsions, hyperirritability Constituent of CoA Dermatitis, enteritis, alopecia, adrenal insufficiency Biotin Catalyzes CO 2 “fixation” (in fatty acid synthesis, etc) Folates (folic acid) and related compounds Cyanocobalamin (vitamin B 12 ) Coenzymes for “1-carbon” transfer; involved in methylating reactions Coenzyme in amino acid metabolism. Stimulates erythropoiesis C Maintains prosthetic metal ions in their reduced form; scavenges free radicals Yeast, wheat, corn, liver Eggs, liver, yeast Dermatitis, enteritis Egg yolk, liver, tomatoes Sprue, anemia. Neural tube defects in children born to folate-deficient women Pernicious anemia (see Chapter 26: Overview of Gastrointestinal Function & Regulation) Leafy green vegetables Liver, meat, eggs, milk Scurvy Citrus fruits, leafy green vegetables H3C CH3 CH3 C H2C C (CH CH C CH) 2 CH2OH H 2C CH 3 H 3C H 3C HO H 3C N N C H 2 C C N C NH 2 N H C C H CH 3 C C COOH CH 2OH CH 2 Vitamin A 1 alcohol (retinol). + N S CH 3 CH 2CH 2OH CH 2(CHOH) 3 CH 2OH N N C CH 2OH H CH3 H O HO C C C C N H CH3 OH H H O C N N H H C C H C N C O S CH H2C (CH2) 4COOH HOOC CH 2 CH 2 CHNH COOH O C Folic acid C O N H Can be synthesized in body from tryptophan. CH 2CH 2COOH NH CH 2 OH NH 2 Complex of four substituted pyrrole rings around a cobalt atom (see Chapter 26: Overview of Gastrointestinal Function & Regulation) CH 2OH HO C H C O CO H C C OH OH H Ascorbic acid (synthesized in most mammals except guinea pigs and primates, including humans). N N N N (continued)
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Copyright © 2010 by The McGraw-Hil
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iv Key Features of the 23rd Edition
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About the Authors KIM E. BARRETT Ki
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viii CONTENTS 27. Digestion, Absorp
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2 SECTION I Cellular & Molecular Ba
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10 SECTION I Cellular & Molecular B
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80 SECTION II Physiology of Nerve &
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100 SECTION II Physiology of Nerve
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168 SECTION III Central & Periphera
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302 SECTION IV Endocrine & Reproduc
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514 SECTION VI Cardiovascular Physi
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588 SECTION VII Respiratory Physiol
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640 SECTION VIII Renal Physiology T
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650 SECTION VIII Renal Physiology G
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654 SECTION VIII Renal Physiology I
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660 SECTION VIII Renal Physiology C
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672 SECTION VIII Renal Physiology A
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676 SECTION VIII Renal Physiology E
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680 SECTION VIII Renal Physiology I
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682 SECTION VIII Renal Physiology C
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688 Answers to Multiple Choice Ques
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690 INDEX Angiotensin III, 671 Angi
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692 INDEX Cannabinoids, 145, 179 Ca
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694 INDEX Climbing fibers, 255 Clin
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696 INDEX Endocrine functions of pa
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698 INDEX Flaccid paralysis, 244 Fl
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700 INDEX Hearing (continued) affer
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702 INDEX Intrinsic system, 532 Int
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704 INDEX Monoamines (continued) hi
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708 INDEX Proprioceptors, 632-633 P
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710 INDEX Respiratory compensation,
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712 INDEX Supratentorial lesions, 2
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714 INDEX Vestibular system, 213-21
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Ganong's Review of Medical Physiology, 23rd Edition

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