Harpers

480 / CHAPTER 44growing children are increasing the protein in the body,they have a proportionately greater requirement thanadults and should be in positive nitrogen balance. Evenso, the need is relatively small compared with the requirementfor protein turnover. In some countries, proteinintake may be inadequate to meet these requirements,resulting in stunting of growth.There Is a Loss of Body Protein inResponse to Trauma & InfectionOne of the metabolic reactions to major trauma, suchas a burn, a broken limb, or surgery, is an increase inthe net catabolism of tissue proteins. As much as 6–7%of the total body protein may be lost over 10 days. Prolongedbed rest results in considerable loss of proteinbecause of atrophy of muscles. Protein is catabolized asnormal, but without the stimulus of exercise it is notcompletely replaced. Lost protein is replaced duringconvalescence, when there is positive nitrogen balance.A normal diet is adequate to permit this replacement.The Requirement Is Not for Protein Itselfbut for Specific Amino AcidsNot all proteins are nutritionally equivalent. More ofsome than of others is needed to maintain nitrogenbalance because different proteins contain differentamounts of the various amino acids. The body’s requirementis for specific amino acids in the correctproportions to replace the body proteins. The aminoacids can be divided into two groups: essential andnonessential. There are nine essential or indispensableamino acids, which cannot be synthesized in the body:histidine, isoleucine, leucine, lysine, methionine, phenylalanine,threonine, tryptophan, and valine. If one ofthese is lacking or inadequate, then—regardless of thetotal intake of protein—it will not be possible to maintainnitrogen balance since there will not be enough ofthat amino acid for protein synthesis.Two amino acids—cysteine and tyrosine—can besynthesized in the body, but only from essential aminoacid precursors (cysteine from methionine and tyrosinefrom phenylalanine). The dietary intakes of cysteineand tyrosine thus affect the requirements for methionineand phenylalanine. The remaining 11 amino acidsin proteins are considered to be nonessential or dispensable,since they can be synthesized as long as there isenough total protein in the diet—ie, if one of theseamino acids is omitted from the diet, nitrogen balancecan still be maintained. However, only three aminoacids—alanine, aspartate, and glutamate—can be consideredto be truly dispensable; they are synthesizedfrom common metabolic intermediates (pyruvate, oxaloacetate,and α-ketoglutarate, respectively). The remainingamino acids are considered as nonessential, butunder some circumstances the requirement for themmay outstrip the organism’s capacity for synthesis.SUMMARY• Digestion involves hydrolyzing food molecules intosmaller molecules for absorption through thegastrointestinal epithelium. Polysaccharides areabsorbed as monosaccharides; triacylglycerols as2-monoacylglycerols, fatty acids, and glycerol; andproteins as amino acids.• Digestive disorders arise as a result of (1) enzyme deficiency,eg, lactase and sucrase; (2) malabsorption,eg, of glucose and galactose due to defects in theNa + -glucose cotransporter (SGLT 1); (3) absorptionof unhydrolyzed polypeptides, leading to immunologicresponses, eg, as in celiac disease; and (4) precipitationof cholesterol from bile as gallstones.• Besides water, the diet must provide metabolic fuels(carbohydrate and fat) for bodily growth and activity;protein for synthesis of tissue proteins; fiber forroughage; minerals for specific metabolic functions;certain polyunsaturated fatty acids of the n-3 and n-6families for eicosanoid synthesis and other functions;and vitamins, organic compounds needed in smallamounts for many varied essential functions.• Twenty different amino acids are required for proteinsynthesis, of which nine are essential in thehuman diet. The quantity of protein required is affectedby protein quality, energy intake, and physicalactivity.• Undernutrition occurs in two extreme forms: marasmusin adults and children and kwashiorkor in children.Overnutrition from excess energy intake is associatedwith diseases such as obesity, type 2 diabetesmellitus, atherosclerosis, cancer, and hypertension.REFERENCESBender DA, Bender AE: Nutrition: A Reference Handbook. OxfordUniv Press, 1997.Büller HA, Grand RJ: Lactose intolerance. Annu Rev Med1990;41:141.Fuller MF, Garlick PJ: Human amino acid requirements. AnnuRev Nutr 1994;14:217.Garrow JS, James WPT, Ralph A: Human Nutrition and Dietetics,10th ed. Churchill-Livingstone, 2000.National Academy of Sciences report on diet and health. Nutr Rev1989;47:142.Nielsen FH: Nutritional significance of the ultratrace elements.Nutr Rev 1988;46:337.