Ch 6 lecture notes/outline PDF

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Proteins and Amino Acids Chapter 6 Nutrition: Science and Applications, 3e 6.1 Protein in Our Food – Our typical protein intake is about 90 grams daily with little change over the past 30 years. 6.1.1 Sources of Protein in Our Diet - In the United States, about two-thirds of the dietary protein comes from animal sources. 6.1.2 Nutrients that Accompany Protein Sources - Recommendations for a healthy diet suggest that our diets be based on whole grains, vegetables, and fruits, and provide smaller amounts of lean meats and low-fat dairy products. - Animal sources of protein are high in iron, zinc, and calcium - Plant sources of protein are rich in fiber, phytochemicals, and monounsaturated and polyunsaturated fats. 6.2 Protein Molecules – Proteins are comprised of amino acids which contain a central carbon atom bound to a hydrogen atom, an amino group (which contains nitrogen), an acid group, and a side chain that will vary to give the different 20 amino acids. 6.2.1 Amino Acid Structure - The 9 amino acids the body is unable to make in sufficient amounts are “essential” or “indispensable” amino acids and must be consumed in the diet. - The 11 nonessential or dispensable amino acids can be made by the body through the process of transamination, in which an amino acid from one amino acid is transferred to a carbon-containing molecule to form a different amino acid. 6.2.2 Protein Structure - To form proteins, amino acids are linked together by peptide bonds between the acid group of one amino acid and the nitrogen atom of the next amino acid. 6.2.2.1 Why Proteins Have Three-Dimensional Shapes - The chemical properties, such as its charge and attraction to water, of the amino acids in the polypeptide chain cause it to fold, contributing to the three-dimensional structure of the protein. 6.2.2.2 How Protein Structure Contributes to Function - The shape of the protein determines its function with changes to its structure changing the functionality of the protein.

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- “Denaturation” is the alteration of a protein’s three-dimensional structure; with denaturation of a protein, the “normal” functions ceases 6.3 Protein in the Digestive Tract – Protein enters the digestive tract from food, from digestive secretions, and from sloughed gastrointestinal cells. 6.3.1 Protein Digestion - The chemical digestion of protein begins in the stomach and where hydrochloric acid denatures proteins, activates pepsin. - Most protein digestion occurs in the small intestine where polypeptides are broken into down by pancreatic protein-digesting enzymes (trypsin and chymotrypsin), and by protein-digesting enzymes in the microvilli of the small intestine. 6.3.2 Amino Acid Absorption - Amino acids and di- and tri-peptides enter the mucosal cells of the small intestine using one of several active transport systems. - Amino acids pass from the mucosal cell into the blood and travel to the liver. 6.3.3 Why Undigested Protein Can Cause Food Allergies - Undigested protein fragments that are absorbed can trigger a food allergy. - The eight most common food allergens are the milk, eggs, peanuts, tree nuts, wheat, soy, fish, and shellfish. 6.4 Amino Acid Functions in the Body – Amino acids available for use in the body is referred to as the amino acid pool. These amino acids come from the diet and from the breakdown of body proteins. 6.4.1 Proteins Are Continually Broken Down and Resynthesized - Protein turnover is the continuous synthesis and breakdown of body proteins, which is necessary for normal growth and maintenance of body tissues. 6.4.2 How Amino Acids Are Used to Synthesize Proteins - DNA in the nucleus of cells contains the information needed to make body proteins. - In transcription, this information is copied into a molecule of mRNA, which carries it to the cytosol. - In translation, tRNA translates the mRNA code into a sequence of amino acids. 6.4.2.1 Why Genes Are Regulated - Gene expression is the process whereby the information coded in a gene is used to produce a protein, and is based on the need for the given protein. 10.1.16

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6.4.2.2 What Are Limiting Amino Acids? - The limiting amino acid is the essential amino acid that is available in the lowest concentration in relation to the body’s need. 6.4.3 Synthesis of Nonprotein Molecules - Amino acids are used to make non-protein molecules that contain nitrogen, such as DNA, RNA, and neurotransmitters. 6.4.4 How Amino Acids Provide Energy - In order for the body to use amino acids as an energy source, the nitrogen-containing amino group must be removed (deamination) and converted into urea which goes to the kidneys for removal. - The remaining carbon skeleton can be broken down to produce ATP or used to make glucose or fatty acids. 6.4.4.1 When Energy Intake is Low - When energy is deficient, body proteins, such as enzymes and muscle proteins, are broken down into amino acids that can then be used to generate ATP or synthesize glucose. 6.4.4.2 When Protein Intake Exceeds Need – protein is not stored as protein. If the diet is adequate in energy and high in protein, the amino acids from the excess protein are deaminated and used to produce ATP or converted into acetyl-CoA to synthesize fatty acids for storage (i.e., eating protein in excess of need can increase body weight). 6.5 Functions of Body Protein – Proteins provide structure and facilitate body activities. 6.5.1 How Proteins Provide Structure - Proteins provide structure at the cellular level as an integral component of cell membranes. 6.5.2 How Proteins Facilitate and Regulate Body Processes - Proteins are the molecules that do most of the body’s work. 6.5.2.1 Enzymes and Protein Hormones - Enzymes are protein molecules that speed up metabolic reactions but are not used up or destroyed in the process. - Hormones are chemical messengers that are secreted into the blood and act on target cells in other parts of the body. 6.5.2.2 Transport Proteins - Proteins transport substances into and out of individual cells and throughout the body. 6.5.2.3 How Proteins Protect Us - Skin and antibodies are proteins that protect us from foreign substances. 10.1.16

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6.5.2.4 Contractile Proteins - During muscle contraction, the proteins actin and myosin slide past each other, causing the muscle fibers to shorten. 6.5.2.5 Proteins That Regulate Fluid Balance - If protein levels in the blood fall too low, water leaks out of the blood vessels and accumulates in the tissues, causing swelling known as edema. 6.5.2.6 Proteins That Regulate Acid-Base Balance - Proteins both in the blood and within the cells act as buffers by attracting or releasing a hydrogen ion to raise or lower the pH. 6.6 Protein, Amino Acids, and Health – A diet adequate in protein is essential to health. 6.6.1 Protein Deficiency - Protein-energy malnutrition (PEM) is a concern, primarily in developing countries, and is characterized by wasting and an increased susceptibility to infection. 6.6.1.1 Kwashiorkor - Kwashiorkor is a deficiency of protein in the presence of adequate energy that is more commonly seen in children. Characterized by a swollen belly, which is edema, due to problems with the fluid balance function of protein. 6.6.1.2 Marasmus - Marasmus is a condition in which the individual isn’t consuming enough energy which leads to severe body wasting. 6.6.2 Protein Excess- Adequate protein intake is absolutely essential to life, but too much protein has been hypothesized to affect the health of the kidneys and bones. 6.6.2.1 Hydration and Kidney Function - High-protein diets increase the production of urea and other waste products that must be excreted in the urine and therefore can increase water losses. 6.6.2.2 Bone Health - When calcium intake is adequate, adequate protein intake helps to strengthen bone. Some studies have shown that high intake of animal “flesh” (meat/ poultry/ seafood) can increase calcium loss in the urine, which could result in less calcium for bone (note: your need for calcium is lower with a diet low in animal flesh; e.g. a vegetarian diet) 6.6.2.3 Kidney Stones - The increase in urinary calcium excretion associated with high-protein diets may intake the risk of kidney stones in the urinary tract. 6.6.2.4 Heart Disease and Cancer Risk – diets high in red meat have been related to an increase in the risk of heart disease and cancer. This 10.1.16

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could be related to a lower intake of plant foods, but one could also make the case that red meat intake could increase chronic disease rates. How red meat could be related to an increase in chronic disease:  High, dry heat for preparing results in the production of heterocyclic amines, which have been related to initiating atherosclerosis and cancer (mechanism is similar to nicotine)  Essential amino acids increase insulin secretion; excess insulin is related to an increase risk of CHD and some cancers  Red meat is the main dietary source of arachidonic acid – the 20 carbon omega 6 fatty acid  Contains high amounts of methionine – the metabolism of which can increase homocysteine levels which increase oxidation  High protein diets are related to higher body weight, which is related to all chronic diseases  High meat diets mean less plant products; higher intake of plant products means higher intake of phytonutrients, which will always decrease chronic disease risk 6.6.3 Protein and Amino Acid Intolerances - While some higher protein foods can cause food allergies, some individuals may also experience food intolerances. 6.6.3.1 Why People with Celiac Disease Must Avoid Gluten- Celiac disease is a disorder that causes damage to the intestines when the protein gluten is eaten. 6.6.3.2 Why Aspartame is Dangerous for People with Phenylketonuria (PKU)- PKU is an inherited disease in which the body cannot metabolize the amino acid phenylalanine. 6.7 Meeting Recommendations for Protein Intake – Protein consumed in the diet must supply amino acids to replace losses that occur during protein turnover, to repair damaged tissues, and to synthesize new body proteins for growth. 6.7.1 How Protein Requirements Are Determined- Protein requirements can be determined using nitrogen balance, which compares the amount of nitrogen consumed in the diet with the amount excreted. - A negative nitrogen balance occurs when the body breaks down more protein than it synthesizes. - A positive nitrogen balance occurs when total body protein increases, such as in times of growth. 6.7.2 Protein Recommendations - The RDA for protein for healthy adults is 0.8 g/kg of body weight per day. 10.1.16

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It is best to use ideal body weight to determine protein needs as most Americans have too much body fat, which leads to a higher weight. Protein needs are related to the amount of protein (muscle) in the body, not body fat - Healthy diets can include 10 to 35% of energy from protein. Diets on the lower end of total protein tend to be healthier diets as the protein is coming from plant sources. 6.7.2.1 How Protein Needs Change throughout the Life CycleGrowth, pregnancy, and lactation increase protein requirements. - Infants and children need more protein per kg of body weight than adults, even though they need less total protein. 6.7.2.2 Why Protein Needs Are Increased by Illness and InjuryThe body needs extra protein during times of illness and injury to help the body heal and rebuild. 6.7.2.3 How Exercise Affects Protein Needs - Certain types of physical activity may increase protein needs. 6.7.2.4 Do We Need Protein and Amino Acid Supplements?Protein supplements are not needed to meet the protein needs of healthy individuals. Remember: this industry is not well regulated and driven by marketing, not science. 6.7.3 Translating Recommendations into Healthy Diets - To meet protein needs, it is important to consider both the amount and the quality of the protein. - Protein quality is the measure of how efficiently a protein in the diet can be by used to make body. - Animal proteins are a higher quality protein versus plant proteins as animal proteins are complete proteins. ** sufficient dietary protein can easily be obtained without animal protein (i.e, vegan diets). 6.7.3.1 How Protein Quality is Measured- The protein digestibilitycorrected amino acid score is a measure of protein quality that reflects a protein’s digestibility as well as the proportions of amino acids it provides. 6.7.3.2 Complementary Proteins - Protein complementation is 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. 6.7.3.2.2 Using Food Labels to Make Healthy Choices- Food labels provide a ready source of information about the protein content of packaged

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foods; however, since the labeling of raw meats and fish is voluntary, many of the greatest sources of protein in the diet do not carry food labels. 6.7.3.3 How to Meet Nutrient Needs with a Vegetarian Diet- A vegetarian diet is a pattern of food intake that includes plant-based foods and eliminates some or all foods of animal origin. 6.7.3.3.1 What Are the Benefits of Vegetarian Diets? - Vegetarian diets are associated with a lower risk for obesity, diabetes, cardiovascular disease, high blood pressure, and some types of cancer. 6.7.3.3.2 Why Some Nutrients Are at Risk of Deficiency in Vegetarian Diets - Vegetarian diets can easily meet protein needs, but care must be taken to include enough iron and zinc in lacto-ovo vegetarian diets. - Well-planned vegan diets can provide adequate amounts of calcium, iron, and zinc, but must include supplements or fortified foods to meet the need for vitamin B12. 6.7.3.3.3 How to Plan Healthy Vegetarian Diets - Well-planned vegetarian diets can meet the nutritional needs of individuals throughout the life cycle.

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