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Chapter 4 Proteins Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 1 Key Concepts • Food proteins provide the amino acids necessary for building and maintaining body tissue. • Protein balance, both within the body and in the diet, is essential to life and health. • The quality of a protein food and its ability to meet the body’s needs are determined by the composition of amino acids. Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 2 Amino Acids: Basic Building Material • Each protein is composed of hundreds of amino acids • Amino acids form unique chain sequences to form specific proteins; the sequencing determines the function of the completed protein • When protein foods are eaten, proteins are broken down into amino acids • Amino acids are reassembled in the body to form a variety of proteins Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 3 The Nature of Proteins Proteins are relatively large, complex molecules May be subject to mutations or malformations 4 Dietary Importance • Amino acids – Named for chemical structure – Basic structure of carbon, hydrogen, and oxygen – Primary source of nitrogen in diet – Some contain small amounts of valuable minerals Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights 5 Classes of Amino Acids • Indispensable amino acids – Body cannot manufacture in sufficient quantity so it is essential to eat them • Dispensable amino acids – Amino acids that the body can synthesize from indispensable amino acids • Conditionally indispensable amino acids – Normally synthesized but some health conditions may require dietary intake Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 6 A Healthy Balance • Protein balance – Catabolism: breakdown – Anabolism: synthesis • Nitrogen balance (intake = excretion) – Positive nitrogen balance: body stores more than it excretes, storing and using protein instead of excreting it (periods of rapid growth or recovering from illness – Negative nitrogen balance: body takes in less than it excretes, losing more that it can build up (malnutrition or illness) Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 7 Functions of Protein • • • • • • Tissue building & repair Energy Water and PH balance Metabolism and transportation Body defense system Hormones, immunoglobulins, muscles, albumin, & antibodies are all proteins Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 8 Tissue Building • Fundamental structural material of every cell • Comprises bulk of: – Muscles – Internal organs – Brain and Nerves – Blood plasma • Protein repairs worn-out, wasted, or damaged tissue Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 9 Energy • May provide body fuel if the supply of carbohydrate and fat is insufficient for needs • Body’s third choice for energy production • Less efficient Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 10 Water Balance • Plasma proteins (especially albumin) attract water, resulting in maintenance of normal circulation volume • Proteins have a unique structure to act as buffering agents in pH balance Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 11 Metabolism & Transport • Enzymes – Digestive enzymes: amylases, lipases, proteases • Transport agents – Lipoproteins – Hemoglobin • Hormones – Insulin and glucagon – Thyroxine, calcitonin Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 12 Body Defense • Immune system defends against disease and infection – White blood cells – Antibodies Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 13 Key Concepts • Food proteins provide the amino acids necessary for building, maintaining and repairing body tissue. • Protein balance, both within the body and in the diet, is essential to life and health. • The quality of a protein food and its ability to meet the body’s needs are determined by the composition of amino acids. Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 14 Food Sources of Protein • Complete proteins – Meat, fish, poultry, seafood – Soy • Incomplete proteins – Plant-origin foods • Grains and Legumes • Nuts and Seeds • Fruits and vegetables • Complementary proteins -matching plant foods so that the amino acids missing in one food are supplied by another 15 Vegetarian Diets • Must combine foods to cover all amino acid needs (makes a complete protein) • Types of vegetarian diets – Lacto-ovo vegetarian – Lacto-vegetarian – Ovo-vegetarian – Vegan Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 16 Lacto-Ovo Vegetarian Diet Pyramid Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 17 Vegetarian Diets • Health benefits and risks – Less saturated fat and cholesterol – More fruits, vegetables, whole grains, other healthful foods – Lower BMI and risk of obesity – Lower rate of cardiovascular disease – Lower risk of renal disease – Lower risk of type 2 diabetes 18 Digestion of Proteins • Mouth – mechanical, no digestion • Stomach: enzymatic breakdown of protein by proenzymes – Pepsinogen & Hydrochloric acid = Pepsin – Rennin • Small intestine – Pancreatic secretions • Trypsin, chymotrypsin, carboxypeptidase – Intestinal secretions • Aminopeptidase, dipeptidase Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 19 Summary of Protein Digestion 20 Body Needs for Protein • Tissue growth and repair • Dietary protein quality is important – – – – Chemical score (CS) Biological value (BV) Net protein utilization (NPU) Protein efficiency ratio (PER) • Additional needs are caused by illness, disease and skin injury like decubiti or burns Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 21 Dietary Deficiency or Excess • Protein energy malnutrition – Kwashiorkor – Marasmus • Excess protein – Usually also means excess fat intake – Protein displaces other healthy foods in diet – Extra burden on kidneys to rid body of nitrogen waste Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 22 Dietary Guides • Recommended Dietary Allowances (RDAs) – Relate to age, sex, weight – Highest at birth and slowly decline into adulthood – Men and women: 0.8 g/kg of desirable weight – 10% to 35% of total caloric intake from protein (children and adults) Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 23 Calculating Protein Needs, cont’d • DRI: – Assumes individual consumes 2200 kcal/day; based on recommendation of 10% to 35% of total kilocalories from protein – 2200 kcal x 0.1- 0.35 = 220-770 kcal/day from protein – 220-770 kcal 4 kcal/g = 55-192.5 g/day of protein 24 Summary • Protein provides the body with its primary tissuebuilding units, amino acids • *20 common amino acids • Nine of the 20 amino acids are indispensable in the diet • Body can manufacture the remaining 11 amino acids • Complete proteins are foods that supply all the indispensable amino acids Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 25 Summary, cont’d • Complete proteins are usually of animal origin and soy • Plant foods are considered incomplete proteins because they lack one or more of the indispensable amino acids (with the exception of soy) • Vegetarian diets can be strict with only plant proteins (vegan), whereas other variations include dairy, egg, and sometimes fish Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 26 Summary, cont’d • Adequate dietary protein and a reserve “pool” of amino acids help maintain overall protein balance • A powerful digestive team of six protein-splitting enzymes acts to break down the protein into amino acids for vital tissue-building tasks • Protein requirements are influenced by growth needs and nature of diet in terms of protein quality and energy intake Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 27 Summary, cont’d • Clinical influences on protein include fever, disease, surgery, or other trauma to the body • Protein needs are calculated based on RDA standards related to age, sex, and weight, & is set at 0.8 g of high quality protein per kilogram of body weight/ day • Adjustments for protein intake are required for: – infants and pregnant and breastfeeding women – individuals following a vegan diet – critical illness Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 28 Williams' Basic Nutrition & Diet Therapy 14th Edition Chapter 5 Digestion, Absorption, Transport and Metabolism 29 Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. Key Concepts of Digestion, Absorption and Metabolism • Through a balanced system of mechanical and chemical digestion, food is broken down into smaller substances and the nutrients are released for biologic use. • Special organ structures and functions conduct these tasks through the successive parts of the overall system. Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 30 Digestion • Basic principles – Principle of change • The body cannot use food as it is eaten. • Food must be changed into simpler substances to be absorbed and then used by cells to sustain life. – Principle of wholeness • The parts of the digestive process comprise a continuous whole. • Food components travel through the gastrointestinal (GI) system until they are delivered to cells or excreted. 31 Mechanical Digestion • GI motility: Beginning in the mouth, muscles and nerves in the tract coordinate their actions to provide motility, an automatic response to the presence of food. • Muscles – Muscle tone/tonic contraction: Ensures continuous passage of the food mass and valve control along the GI tract – Periodic muscle contraction and relaxation: Rhythmic waves that mix the food mass and move it forward Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 32 Mechanical Digestion, cont’d • Nerves – Specific nerves regulate muscle action along the GI tract – The intramural nerve plexus is the network of nerves in the GI wall extending from the esophagus to the anus Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 33 Chemical Digestion • Digestive enzymes: Break down nutrients • Hydrochloric acid and buffer ions: Produce the correct pH necessary for enzyme activity • Mucus: Lubricates and protects the GI tract tissues; helps mix the food mass • Water : Carries and circulate the products of digestion through the tract and into the tissues • Electrolytes: absorbed, transported and play a role in homeostasis • Bile: Emulsifies fat into smaller pieces to assist fat enzymes Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 34 Digestion in the Mouth and Esophagus • Mechanical digestion – Mastication breaks down food orally. – Food is swallowed and passes down esophagus. – Muscles at tongue base facilitate the swallowing process. – Gastroesophageal sphincter at stomach entrance relaxes, allowing food to enter, then constricts to retain food. Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 35 Digestion in the Mouth and Esophagus, cont’d • Chemical digestion – Salivary glands secrete fluid containing salivary amylase or ptyalin. – Salivary glands also secrete a mucous material to lubricate and bind food particles, facilitating the swallowing of the food bolus. – Secretions from the mucous glands in the esophagus help ease movement of food toward the stomach. Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 36 Digestion in the Stomach 37 Digestion in the Stomach • Mechanical digestion – Under sphincter control, the food enters the upper portion of the stomach as individual bolus masses. – Stomach muscles knead, store, mix, and propel the food mass forward. – By the time the food mass reaches the lower portion of the stomach, it is a semi-liquid acid/food mix called chyme. – Chyme is released slowly into the first section of the small intestine (duodenum) by the pyloric valve. Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 38 Digestion in the Stomach, cont’d • Chemical digestion: three types of gastric secretions – Hydrochloric acid: Parietal cells in the stomach lining secrete acid to promote gastric enzyme activity. – Mucus: Secretions protect the stomach lining from the erosive effect of the acid and also bind and mix the food mass and help move it along. – Pepsinogen is secreted by stomach cells and activated by acid to become pepsin, a protein-splitting enzyme. Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 39 Digestion in the Small Intestine • Mechanical digestion – Peristaltic waves slowly push food mass forward. – Pendular movements from small local muscles sweep food back and forth stirring chime at the mucosal surface – Segmentation rings chop food mass into successive soft lumps and mix them with secretions. – Longitudinal rotation rolls food in a spiral motion, exposing new surfaces for absorption. – Surface villi motions stir and mix chyme Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 40 Digestion in Small Intestine, cont’d • Chemical digestion – Pancreatic enzymes • Carbohydrate: Pancreatic amylase converts starch to maltose and sucrose. • Protein: Trypsin and chymotrypsin split large protein molecules into small peptide fragments and eventually into single amino acids. • Fat: Pancreatic lipase converts fat to glycerides and fatty acids. Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 41 Digestion in the Small Intestine, cont’d • Small Intestine enzymes – Carbohydrate: Disaccharidases convert disaccharides into monosaccharides. – Protein: Enterokinase activates trypsinogen from the pancreas to become trypsin; amino peptidase removes end amino acids from polypeptides; dipeptidase splits dipeptides into amino acids. – Fat: Intestinal lipase splits fat into glycerides and fatty acids. (Triglycerides= 1 glyceride + 3 fatty acids) Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 42 Digestion in the Small Intestine, cont’d • Mucus: Large quantities of mucus protect the mucosal lining from irritation and erosion. – Bile: Emulsifying agent produced by the liver and stored in the gallbladder aids fat digestion and absorption. – Hormones • Secretin- produced by mucosal cells in response to acid in the duodenum releasing the alkaline pancreatic enzymes • Cholecystokinin-secreted by intestinal cells when fat enters the duodenum to release bile Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 43 Digestion in the Small Intestine (cont’d) 44 Factors Influencing GI Tract Secretions • • • • Nervous control Conditioned reflexes Oral reflexes Physical contact Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 45 Absorption and Transport • At this point in the process: – Carbohydrates: reduced to simple sugars (glucose, fructose, galactose) – Fats: changed into fatty acids and glycerides – Proteins: changed into single amino acids – Vitamins and minerals: liberated from food 46 Bioavailability • Bioavailability refers to: – Amount of nutrient present in the GI tract – Competition between nutrients for absorption – Form in which the nutrient is present • All nutrients present in a food are not absorbed because of differing bioavailability. – This is considered when determining dietary intake standards Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 47 Absorption in the Small Intestine • Three absorbing structures – Mucosal folds: Surface of small intestine piles into folds – Villi: Small, finger-like projections cover the mucosal folds, increasing the area of exposed intestinal surface – Microvilli: Smaller projections cover each villi (look like bristles on a brush) Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 48 Small Intestine (cont’d) 49 Absorption Processes • Simple diffusion: The force by which particles move outward in all directions—from areas of greater to lesser concentration. • Facilitated diffusion: Similar to simple diffusion but uses a protein channel to carry larger items. • Active transport: The force by which particles move from areas of lesser to greater concentration using a carrier to “ferry” particles. • Endocytosis: Penetration of larger materials by attaching to the cell membrane and being engulfed by the cell. • Pinocytosis: liquids are engulfed by a cell Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 50 Absorption in Large Intestine • Bacteria helps make Vitamin K • Water is taken up by the large intestine – Most water in chyme is absorbed in the first half of the colon – Only a small amount remains to form and eliminate feces • Dietary fiber is not digested – Contributes bulk to food mass – Helps form feces Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 51 Transport • Nutrients must be transported to cells • Vascular (blood circulatory) system – Veins and arteries – Transports waste, such as carbon dioxide and nitrogen, to lungs and kidneys for removal – Portal circulation carries nutrients to the liver which filters blood. • Lymphatic system – Route for fatty materials, which are not water soluble – Fat molecules pass into lymph capillaries near the villi called Lacteals Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 52 Metabolism • Mitochondria of cells is where metabolism takes place • Two metabolic processes – Catabolism: Breaking down of large substances into smaller units (e.g., breaking down a protein chain into amino acids) – Anabolism: Building of larger substances from smaller particles (e.g., building a complex protein from single amino acids) Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 53 Metabolism, cont’d • Metabolic processes ensure that the body has energy in the form of adenosine triphosphate (ATP). • Metabolism of glucose (4 kcal/kg) from carbohydrates yields less energy than metabolism of fat (9 kcal/kg). Still, glucose is the body’s primary source of energy. • Protein can be an energy source, but it is relatively inefficient. Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 54 Energy Storage: Glycogenesis • Glycogenesis: Anabolic process of converting extra glucose into glycogen • Glycogen is stored in the liver and muscles for quick energy to be used at a later time • Gluconeogensis: Making glucose from non carbohydrate sources Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 55 Energy Storage: Lipogenesis • When glycogen reserves are full, additional excess energy from carbohydrates, fat, or protein is stored as fat in adipose tissue • Lipogenesis: The building up of triglycerides for storage in adipose tissue Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 56 Errors in Digestion and Metabolism • Genetic defects: cell is missing enzyme controlling metabolism of a specific nutrient – Phenylketonuria (PKU) • Enzyme responsible for metabolizing essential amino acid phenylalanine is missing • Untreated, causes permanent mental retardation and CNS damage • With proper treatment, affected children may have normal and healthy lives Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights 57 Errors in Digestion and Metabolism (cont’d) • Galactosemia – Enzyme responsible for metabolizing galactose to glucose is missing – All sources of lactose must be eliminated from diet – Untreated, can cause brain and liver damage – Screening and treatment can enable normal life Copyright © 2013 Mosby, Inc., an imprint of Elsevier Inc. All rights 58 Lactose Intolerance • Most common disaccharidase deficiency • Lactase is not present in sufficient amounts, it is not absent • Causes abdominal cramping and diarrhea Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 59 Summary • Nutrients from food must be changed, released, regrouped, and rerouted into forms the body can use. • The activities of digestion, absorption, and transport ensure that key nutrients are delivered to the body’s cells so metabolic tasks can be completed. Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 60 Summary, cont’d • Mechanical digestion consists of spontaneous muscular activity responsible for initial mechanical breakdown and the movement of the food mass along the GI tract by the motion of peristalsis. • Chemical digestion involves the enzymatic action that breaks food down into smaller components and releases nutrients for absorption. Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 61 Summary, cont’d • Absorption involves the passage of food nutrients from the intestines into the mucosal lining of the intestinal wall. • Nutrients absorbed are transported throughout the body by the circulatory system. • Metabolism is the sum of the body processes that change food energy taken in (carbohydrate, protein, and fat) into various forms of energy Copyright © 2009, by Mosby, Inc. an affiliate of Elsevier, Inc. All rights reserved. 62