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Why Is Protein Important? • Constantly assemble, break down, and use proteins – Replace cells – Produce antibodies to fight infections – Assist in water balance, nutrient transport, and muscle contractions – Diets replace what is being used – Excess • Make energy • Store as fat Amino Acids Are Building Blocks of Protein • Sequences of 20 different amino acids aa • Nonessential (dispensable) amino acids – Body can produce • Essential (indispensable) amino acids – Must be taken in via food • Conditionally essential amino acids – Essential during infancy, disease or trauma 3 When the body experiences periods of extreme trauma or stress, nonessential amino acids become conditionally essential, or indispensable. The physiological demands of recovery may result in the need for these nonessential amino acids to be greater than the body’s ability to produce them. As a result, the body’s supply of nonessential amino acids must be provided by the diet. Arginine, glutamine, and cystine are examples of conditionally essential amino acids that have been combined with other nutrients in supplements formulated for wound healing. by Mary Ellen Posthauer RD, CD, LD Amino Acids Are the Building Blocks of Protein • Identified by side chains – Carbon, carboxylic acid, amino acid, and side group – Side group gives AA its identity AA the Building Blocks of Protein • Protein structure: unique three-dimensional shapes and functions – Amino acids sequence • Peptide bonds –Dipeptide –Oligopeptide –Polypeptide – Protein shape • Sequence of AA determines shape • Shape of protein determines function Synthesis of Proteins • Amino acids are linked by peptide bonds to form proteins • Synthesis of protein determined through gene expression • DNA transcription phase – DNA code transferred from the nucleus to the cytosol via messenger RNA (mRNA) • mRNA translation phase – tRNA and ribosomes • DNA-coded instructions determine shape, and thus function of proteins 9 10 Amino Acids Are the Building Blocks of Protein • Protein denaturation: Destabilizing a protein’s shape – Disrupts shape – Caused by acidic or alkalinity, high temperature, alcohol, oxidation, and agitation Functions of Body Protein • Structural and mechanical functions – Collagen – Keratin – Muscle • Immune functions – Antibodies Functions of Body Protein • Enzymes – Catalyze chemical reactions • Hormones – Regulate body processes • Acid-base balance – Proteins as buffer Functions of Body Protein • Transport functions – Channels and pumps – Albumin, Lipoproteins • Fluid balance – Albumin & edema • Source of energy and glucose – Carbs and fat are proteinsparing 16 Protein Digestion and Absorption • Cooking denatures protein • Stomach – Hydrochloric acid denatures – Pepsin begins enzymatic digestion • Small intestine – Stimulate release of pancreatic proteases (trypsin, chymotrypsin, carboxypeptidases) • Amino acids absorbed into the portal vein 18 Protein Digestion and Absorption • Amino acid and peptide absorption – Travel via portal vein to liver released into general circulation • Undigested protein – Feces – Diseases that cause problems with dig • Celiac disease • Cystic fibrosis Proteins in the Body • The AA pool – Available AA • Protein synthesis; produce glucose • Protein turnover – Constant recycling of protein Proteins in the Body • Nitrogen balance – Nitrogen intake vs. nitrogen output • Nitrogen equilibrium – Nitrogen intake = nitrogen output – Healthy adults • Positive nitrogen balance – Nitrogen intake > nitrogen output – Growth; recovery from illness • Negative nitrogen balance – Nitrogen intake < nitrogen output – Injury and illness 25 Proteins in the Diet • Recommended protein intake – Adult RDA = 0.8 gram per kilogram body weight – Infants, children, and teens • Infants have highest needs relative to body weight • Physical stress – Infection, burns, fevers, surgery, and intensive weight training • Consumption in the United States – Generally within on higher than recommended range Proteins in the Diet • Complete Proteins – Adequate amounts of all the essential amino acids – Animal proteins, except gelatin • Incomplete Proteins – Inadequate amounts of 1 or more of the essential amino acids – Plant proteins, except soybeans • Complementary Proteins – Combining plant proteins to compensate for limiting amino acids Photos courtesy of the USDA Proteins in the Diet • Protein quality – Two incomplete proteins = complete protein – Complementary proteins Photos (top) Courtesy of USDA (middle) Photo by Keith Weller. Courtesy of USDA. (bottom) Photo by Scott Bauer. Courtesy of USDA. Tryptophan Methionine (Cysteine) Valine Threonine Total protein needs Phenylalanine (Tyrosine) Methionine Threonine Phenylalanine Isoleucine Leucine Histidine (Infants) Arginine (Infants) Beans and other legumes Valine Leucine Lysine (a) Essential amino acids Tryptophan Corn and Isoleucine other grains Lysine (b) Vegetarian diets providing the eight essential amino acids for humans Figure 24.2 Evaluating protein in the Diet • Biological Value (BV) – Egg white is highest – Provides all the indispensable amino acids at amounts the body needs • Protein Efficiency Ratio (PER) – Provides enough other amino acids to serve as nitrogen sources for making dispensable amino acids – Protein digestibility-corrected amino acid score (PDCAAS) - Is easy to digest Proteins in the Diet • Protein and AA additives – Structure, texture, and taste of foods • Protein and AA supplements – Used for a variety of reasons: dieters, athletes, certain diseases – Risks unknown Photo © Jones and Bartlett Publishers The Pros and Cons of Vegetarian Eating • Types of vegetarian diets – Lacto-ovo vegetarian • Includes milk, eggs – Vegan • Plant foods only – Fruitarians • Eat only raw fruit, nuts, and green foliage © AbleStock The Pros and Cons of Vegetarian Eating – Semi-vegetarian • Avoid red meat; eat some chicken and fish – Zen macrobiotic diet • Mostly vegans; stress whole grains and locally grown food The Pros and Cons of Vegetarian Eating • Health benefits of vegetarian diets – Less fat, saturated fat, and cholesterol intake – More magnesium and folate – More antioxidants – More fiber and phytochemicals • Reduces risk for heart disease, obesity, hypertension, and cancer The Pros and Cons of Vegetarian Eating • Health risks of vegetarian diets – Vegan diets may be low in some nutrients • Calcium, iron, zinc, vitamin D, and vitamin B12 – Higher intake of phytates – More restrictive food choices = less nutrients – Careful planning needed for growth and overall health The Pros and Cons of Vegetarian Eating • Dietary recommendations – Choose a variety of foods – Choose whole, unrefined foods – Choose a variety of fruits and vegetables – Choose lower-fat dairy products and eggs in moderation – Consume a regular source of vitamins B12 and D • Fortified foods or supplements The Health Effects of Too Little Protein • Dietary protein = indispensible amino acids • Protein deficiency occurs when energy and/or protein intake is inadequate • Protein-energy malnutrition (PEM) – Kwashiorkor • Edema and other signs • Between ages of 18 and 24 months • Associated with extreme poverty The Health Effects of Too Little Protein • Protein-energy malnutrition (PEM) – Marasmus • Chronic PEM • Infants and 6- to 18-month-old children • Adults with cancer or starvation – Nutritional rehabilitation • Gradual and careful refeeding The Health Effects of Too Much Protein The Health Effects of Too Much Protein • Strains kidney function – Excretes products of protein breakdown • Mineral losses – Link between high-protein diet and osteoporosis • Obesity – Correlation with body fat The Health Effects of Too Much Protein • Heart disease – Increased saturated fat and cholesterol intake • Cancer – Increased risk for certain types of cancer Food Protein Allergies • Food proteins (allergens) cause an immune response, creating an allergic reaction – IgE most commonly produced – anaphylaxix • 8 foods account for 90% of all food allergies – Peanuts, tree nuts, milk, eggs, fish, shellfish, soy, and wheat 45