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Chapter 6 Proteins and Amino Acids Proteins in our Food • Animal foods contain the most concentrated sources of proteins in the diet. o Also provide B vitamins and readily absorbable minerals such as iron, zinc and calcium. o Low in fiber. o High in saturated fat. • Legumes: starchy seeds of plants that produce bean pods. o Include: peas, peanuts, beans, soybeans and lentils. o Also an important source of protein. o Provide most, but not all B vitamins. o Also supply iron, zinc and calcium in less absorbable f orms. o Excellent sources of fiber, phytochemicals and unsaturated fats. • Amino acids: the building blocks of proteins. o Each contains an amino group, an acid group and a unique side chain. The Structure of Amino Acids and Proteins Amino Acid Structure • Amino acids consist of: a carbon atom bound to a hydrogen atom; o An amino group which contains nitrogen; o A side chain: varies in size and structure; gives the amino acids their unique properties. • Essential amino acids: amino acids that cannot be synthesi zed by the body in sufficient amounts to meet its needs. o Must be included in the diet. o Also called indispensable amino acids. o Nine amino acids needed by the adult human body. • Nonessential amino acids: can be made by the body. o Also called dispensable amino acids. o Eleven amino acids are considered dispensable. • Conditionally essential amino acids: some of the nonessential amino acids cannot be synthesized in sufficient amounts to meet needs. o Phenylketonuria (PKU): an inherited disease where ph en ylalanine cann ot be converted into tyrosine. • What a Scienti st Sees: Phenylketonuria (PKU) inherited disease where phenylalanine is not metabolized correctly, leading to a buildup of phenylketones. o In infants and young children this will interfere with brain development. o In pregnant woman can cause birth defects. o To prevent the buildup of phenylketones, individuals must consume a special diet. o The artificial sweetener aspartame is a source of phenylalanine. Protein Structure • Peptide bonds: links amino acids together. Amino Acid and Protein Structure • Dipeptide: two amino acids linked together. • Tripeptide: three amino acids linked together. • Polypeptide: a chain of amino acids linked by peptide bonds that is part of the structure of a protein. o The order and chemical properties of the amino acids determine its final shape. • Denaturation: alternation of a protein’s three -dimensional structure. o Examples: digestion or cooking. Protein Digestion and Absorption • Proteins must be digested before their amino acid s can be absorbed into the body. • Mechanical digestion begins in the mouth, chewing breaks down the food • Chemical digestion begins in the acid environment of the stomach. • Pepsin: the protein digesting enzyme; breaks some of the peptide bonds leaving shorter polypeptides. • Most digestion occurs in the small intestine; polypeptides are broken into smaller peptides and amino acids. o Utilizing protein digesting enzymes from the pancreas and small intestine. o Single amino acids, dipeptides and tripe ptides ar e absorbed into the mucosal cells. • Amino acids enter the body by crossing from the lumen of the small intestine into the mucosal cells and then into the blood. o Process involves energy -requiring amino acid transport systems. o Amino acids may compete with one another for absorption. ▪ This is not a problem with foods because they contain a variety of amino acids, none of which occurs in excess. ▪ Could be a problem with amino acid supplements. Protein Synthesis and Functions • Amino acids are used to: o Make other ni trogen -containing molecules, including neurotransmitters; o The units that make up DNA and RNA; o The skin pigment melanin; o The vitamin niacin. o Creatine, which is used to fuel muscle contraction; o Histamine, which causes blood vessels to dilate. o Can provide energy or synthesize glucose or fatty acids. • Amino acid pool: all the amino acids in body tissues and flui ds that are available for use in the body. Synthesizing Proteins • Gene: a length of DNA that contains the information needed to synthesize a polypeptide chain. Regulating protein synthesis • The types of proteins made and when they are made is regulated by turning on and off the genes that code each protein. • Expressed: when a gene is turned on. Limiting amino acids • A shortage of one amino acid can stop the process of protein synthesis. o If the amino acid missing is a nonessential amino acid, it can be made and synthesis will continue. o If the amino acid missing is an essential amino acid, the body can break down its own protein to obtain it. o If an amino acid cannot be supplied, protein synthesis stops. • Transamination: the process by which an amino group from one amino acid is transferred to a carbon compound to form a new amino acid. • Limiting amino acid: the essential amino acid that is available in the lowest concentration relative to the body’s needs. Protein functions • Collagen: most abundant protein in the body. o Plays an important structural role: major protein in ligaments. o Forms protein structure of bones and teeth. • Enzymes: facilitates chemical reactions in the body. • Transport materials throughout the body and into and out of cells. • Help protect from disease. o Antibodies: part of the immune system. • Help body movement. o Actin and myosin in long muscles of arms and legs. o Contrac tions in the heart muscle . o Muscles of the digestive tract, blood vessels, and body glands. • Regulate fluid balance. o Edema: swelling caused by too little protein in the blood. Proteins Provide Structure and Regulation • Structural prot eins: skin, hair, ligaments, tendons , and bones . • Structure to individual cells: integral part of cell membrane, cytoplasm, and organelles. • Enzymes: speed up biochemical reactions. • Transport proteins that travel in the blood or help materials cross membrane s. • Part of the body’s defense -mechanisms: skin, antibodies. • Contractile properties: allow muscle movement to various parts of the body. • Hormones: regulate b iological processes. o Examples: insulin, growth hormone, and glucagon. • Help regulate fluid balance. • Regulate s body pH. Protein as a Source of Energy • If the diet does not provide enough energy to meet the body’s needs, body protein can be used to provide energy. o Example: starvation. • When excess amino acid s are consumed, they can be metabolized for energ y. o Extra amino acids can’t be stored. • When diet provides more calories than needed, amino acids can be converted to fatty acids which are stored as triglycerides thus contributing to weight gain . • Deamination: the removal of the amino group. Protein in Health and Disease Protein Deficiency • Protein -energy malnutrition (PEM): long -term consumption of insufficient amounts of energy and/or protein to meet the body’s needs. o Characterized by loss of muscle and fat tissue and increased susceptibility to infect ion. • Kwashiokor: form of protein -energy malnutrition in which only protein is deficient. o Characterized by a swollen belly, growth impairment. o Poor immune function, changes in hair color, and impaired nutrient absorption. o The word literally means: “the dise ase that the first child gets when a second child is born.” ▪ Children are weaned from breastmilk to a nutritionally inadequate diet. • Marasmus: form of protein -energy malnutrition in which a defic iency of energy in the diet caus es severe body wasting. o Charac terized by depletion of fat stores and wasting of muscle. o The word means “to waste away.” High Protein Diets and Health Debate: Is a high -protein diet safe and effective for weight loss? • Recent popularity of high -protein, low -carbohydrate diets for weight loss. • Little evidence that a high -protein diet would precipitate kidney disease in a healthy person. • Epidemiological studies suggest that diets rich in animal protein, high in sodium, and low in fluid contribute to the for mation of kidney stones. • High -protein diets are high in animal products; also high in saturated fat and low in fiber. Increases the risk of heart disease. • These diets are typically low in grains, vegetables and fruits. May be high in red meat and processed meat. This pattern is associated with an increased risk of cancer. Proteins and Food Allergies and Intolerances • Food allergy: an adverse immune response to a specific food protein. • Most c ommon food allergens: milk, eggs, peanuts, tree nuts, wheat, soy, f ish and shellfish. • Food intolerance or food sensitivity: an adverse reaction to food that does not involve the production of antibodies by the immune system. o Example: MSG complex or Chinese restaurant syndrome. • Celiac disease : a n autoimmune disease that ca uses damage to the villi in the small intestine when the protein gluten is eaten. o Gluten is found in wheat, rye and barley. o Celiac Disease affects 3 million Americans, where as non -celiac gluten sensitivity affects 18 million. • Food allergy labeling: food label must indicate if it contains a common food allergen. o Often highlighted at end of list. ▪ Example: “Contains soy ingredients.” o May contain a warning if a product could be cross -contaminated by a common food allergen. ▪ Example: “Manufactur ed in a facility that processes p eanuts.” Meeting Protein Needs Balancing Protein Intake and Losses • Nitrogen balance: the amount of nitrogen consumed in the diet compared with the amount excreted over a period of time. • Nitrogen balance: nitrog en intake equals nitrogen output. • Negative nitrogen balance: nitrogen intake is less than nitrogen output. o Examples: injury, illness or diet low in protein or calories. • Positive nitrogen balance: nitrogen intake exceeds nitrogen output. o Examples: pregnancy or individuals increasing muscle mass by lifting weights. Recommended Protein Intake • Protein RDA for adults : 0.8 g/kilogram of body weight • Protein recommendations are higher for children, adolescents, and pregnant or lactating women. • Protein needs increase when the body is healing due to injury or illness. • Athletes need more protein. • Acceptable Macronutrient Distributi on Range: 10 -35% of total calories. Protein needs of athletes • Athletes: 1.2 to 2.0 g of protein/kilogram of body weight. • Because athletes also consume more calories, they should be able to meet increased protein needs with diet alone. Protein and amino acid supplements • Athletes use s upplements to boost total protein intake and to add individual amino acids. • Weak evidence to support many of the claims. • Consuming a large amount of one amino acid may interfere with the absorption of others. • Due to insufficient research, no ULs have been set for amino acids. Choosing Protein Wisely • Pr otein quality: a measure of how good the protein in a food is at providing the essential amino acids the body needs to synthesize proteins. • High -quality protein or complete dietary protein: foods of animal origin. • Incomplete dietary protein: plant proteins . o More difficult to digest and lower in one or more of the essential amino acids. Protein comple me ntation • The process of combining proteins from different sources so that they collectively provide the proportions of amino acids required to meet the body’s needs. • Grains, nuts, or seeds + Legumes = Complete protein o Plant proteins are limited in lysine, methionine, and cysteine. o Legumes are deficient in methionine and cystein e, but high in lysine. o Grains, nuts and seeds are deficient in lysine by high in methionine and cystein e. • Traditional diets take advantage of complementary plant proteins o India: lentils and rice or chickpeas and rice o Mexico and South America: rice and pinto or black beans o Middle East: hummus (chickpeas and sesame seeds) and bread o Unit ed States: bread and peanut butter MyP late and Dietary Guideline s recommendations • Choosing a variety of foods will provide enough protein and enough of each of the essential amino acids to meet the body’s needs. • Include both animal and plant sources of pr otein. • Food groups high in protein: Dairy and Protein . What Should I Eat? Protein Sources • Increase your plant protein. • Get protein without too much saturated fat. • Reduce your portions of animal proteins. Vegetarian Diets • Diets that include plant -based foods and eliminate s some or all foods of animal origin. o Semivegetarian: occasional red meat , fish and poultry, as well as dairy products and eggs. o Pescetarian: exclud es all animal flesh except fish. o Lacto -ovo vegetarian: excludes all animal fl esh but does include eggs and dairy products such as milk and cheese. o Lacto vegetarian: excludes animal flesh and eggs but does include dairy products. o Vegan: excludes all food of animal origin. • Benefits of vegetarian diets: o Can be a healthy, low -cost alt ernative to traditional American diet. o Lower body weight and reduced incidence of obesity and other chronic diseases. • Risks of vegetarian diets: o Poorly plan diets can cause nutrient deficiencies or excess . o Nutrients of concern: vitamin B12, calcium, vitami n D, iron, zinc, iodine, and omega -3 fatty acids. Thinking it Through: A Case Study on Choosing a Healthy Vege tarian Diet