NUTRITION for a Changing World Jamie Pope, Steven Nizielski, and Alison McCook NUTRITION for a Changing World FIRST EDITION Chapter 8 Proteins How Much of a Good Thing Do We Need? © 2016 by W. H. Freeman and Company & Scientific American

Chapter 8 Objectives Discuss at least four functions of protein in the body Distinguish between essential and nonessential amino acids Describe the primary steps in protein synthesis and what determines the shape of a protein Explain how denaturation may alter protein function Summarize protein digestion and absorption Identify the recommended dietary allowances and Acceptable Macronutrient Distribution Ranges of protein for adults Explain protein turnover and how amino acids may be used for energy Understand protein deficiency diseases, and identify areas in the world where this condition is prevalent

Could a Protein-Rich Diet Hurt More Than Your Budget? In the United States, most of us eat plenty of protein to meet our needs Do athletes like Michael Phelps need more protein?

Role of Protein in the Body Structural material Muscle Bone Hair Skin Fingernails Critical functions Enzymes Hormones Antibodies Fluid balance pH balance Transporters

Protein Needs Recommended Dietary Allowance (RDA) 0.8 g/kg of body weight for adults Acceptable Macronutrient Distribution Range (AMDR) 10% to 35% of total kcal Example: the RDA for a 154-pound adult 154 lb ÷ 2.2 lb/kg = 70 kg 70 kg x 0.8 g/kg = 56 g of protein/day Protein needs are relatively high during growth and developments periods, such as infancy and pregnancy, but adults with a healthy body weight need just 0.8 g/kg of body weight. The average male consumes 100 g of protein/day, and the average female consumes 70 g/day. Thus, most people consume well above the RDA.

Protein Structure Building blocks of protein are amino acids Like carbohydrates and fats, protein is composed of carbon, hydrogen, and oxygen, but another key molecule that distinguishes protein from other macronutrients is nitrogen, which is supplied by amino acids. Amino acids contain an amino group (NH2), an acid group (COOH), and a variable side chain.

Protein Structure Amino acids linked together by peptide bonds Dipeptides have 2 amino acids Tripeptides have 3 amino acids Polypeptides have many amino acids

Amino Acids in the Body We all need 20 different amino acids to make the necessary proteins. Nine are essential amino acids and must be obtained in the diet. The other 11 are nonessential (or disposable) amino acids and can be manufactured by the body.

Protein Synthesis Transcription Translation Nearly 22,000 different proteins are produced in the body in a two-step process of protein synthesis. The first step is transcription, during which the deoxyribonucleic acid (DNA) provides instructions that are read and made into a messenger ribonucleic acid (mRNA). The second step is translation, during which the mRNA is read by ribosomes to determine the order of the amino acids in the growing protein chain.

Functional Protein After translation, amino acid chains fold into a particular shape Shape determines the function of the protein Once the amino acids are put into sequence by the ribosomes and tRNA, following our genetic instructions, they will begin to fold into three-dimensional shapes due to the unit nature and position of the side chains on each amino acid. This final shape determines the function of each protein.

Protein Denaturation Denaturation Caused by heat, light, change in pH, alcohol, or motion Denaturation of protein affects it’s ability to function but is a necessary step in digestion, where acid and pepsin in the stomach cut proteins into smaller peptides.

Protein Digestion and Absorption Stomach Acid Pepsin Intestine Pancreatic proteases Absorption

Protein Turnover Amino acids needed for building and maintaining proteins in the body come from our diet and from proteins already in the body being broken down and reassembled. We build protein to support necessary functions in the body, so consuming more protein will not increase the amount of protein made in the body. Instead, extra protein is used for energy or stored as fat.

Fate of an Amino Acid Proteins in the body Metabolized as a source of energy Synthesized into glucose or fat When amino acids are used for energy or made into glucose or fat, the nitrogen (or amino group) must be removed and disposed of; otherwise it would accumulate in the body as ammonia, which is toxic.

Metabolism of Amino Acids Remove amino group Liver converts ammonia to urea Kidney excretes urea Remaining carbon skeleton Made into energy, glucose, or fat The amino group on amino acids is only retained in the body when it is used to synthesize proteins.

Nitrogen Balance Nin − Nout Nin Nout Protein intake Urine and feces Sweat and other secretions Skin, hair, and nails Nitrogen balance reflects whether the body is gaining, losing, or maintaining nitrogen (and thus protein).

Positive Nitrogen Balance Nin > Nout Positive nitrogen balance means retaining nitrogen or protein, which occurs during growth and pregnancy.

Nitrogen Equilibrium Nin = Nout

Negative Nitrogen Balance Nin < Nout

Do Athletes Need More Protein? What affects the success of an athlete like Michael Phelps? Genes, training, and maybe diet Athletes might need more protein Optimal protein intake for performance advantage 1.2 to 1.7 g/kg body weight

Special Protein Needs of Older Adults Adults 50 to 65 years of age 0.7 to 0.8 g/kg of body weight/day is beneficial Adults over 65 years of age 1.2 g/kg of body weight/day is beneficial For adults 50 to 65 years of age, 0.7 to 0.8 g/kg of body weight/day is beneficial. Eating this amount of protein improves overall mortality, helps prevent cancer, and possibly decreases diabetes For adults over 65 years of age, 1.2 g/ kg of body weight/day is beneficial. It reduces loss of lean body mass, improves functionality, and reduces the risk of disability and death.

Typical Protein Intake For adults 50 to 65 years of age, 0.7 to 0.8 g/kg of body weight/day is beneficial. Eating this amount of protein improves overall mortality, helps prevent cancer, and possibly decreases diabetes For adults over 65 years of age, 1.2 g/ kg of body weight/day is beneficial. It reduces loss of lean body mass, improves functionality, and reduces the risk of disability and death.

Sources of Dietary Protein

Protein Quality Complete proteins Incomplete proteins Limiting amino acid Complementary protein Complete proteins provide all nine essential amino acids in the proper proportions for the body. Incomplete proteins are of lower protein quality, as they lack or supply low amounts of one or more essential amino acid. Without the availability of amino acids, protein synthesis is disrupted or limited. The amino acid in the shortest supply is referred to as the limiting amino acid. By consuming two or more incomplete protein sources with different limiting amino acids, like beans and rice, you can make a complementary protein.

Choose Your Protein Wisely Eating complementary proteins can give you all the correct amino acids to meet your proteins needs, with less saturated fat.

Protein Deficiency

Protein Deficiency Marasmus Protein Energy Malnutrition (PEM)

Protein Deficiency Locations where malnutrition is common

High-Protein Diets May help initial weight loss Little benefit in maintaining weight loss Type of protein important High animal protein High in saturated fat, cholesterol, and sodium Low in fiber, phytochemicals, and certain vitamins Connected to increased kidney stones, diabetes, cancer, and heart disease Eat more fish, poultry, nuts, and beans Reduce risk of diabetes, cancer, and heart disease

Summary Protein has many critical roles in the body’s structure and processes, including catalyzing chemical reactions (enzymes), regulating body functions (hormones), and transporting substances in the blood; also has central roles in immunity, fluid balance, and blood clotting Proteins are composed of carbon, hydrogen, and oxygen but also contain nitrogen, which is supplied through amino acids, the building blocks of protein Protein needs are determined by sex, life stage, and other factors; RDA for adults is 0.8 g/kg/day, and AMDR for protein is 10% to 35% of total kcal

Summary (Cont’d) Proteins are complex structures synthesized by linking 20 different amino acids into chains of varying sequence and length; nine of these amino acids are considered essential because they must be supplied through the diet, and remaining 11 nonessential amino acids can be manufactured by the body Sequences of DNA called genes provide the instructions for the synthesis of every protein in the body; this is a two-step process that begins in the nucleus with gene transcription and is completed in the cytoplasm with translation The overall shape of a protein molecule determines its function and how it interacts with other molecules The process of denaturation can alter shape and function of proteins

Summary (Cont’d) The digestion of proteins begins in the stomach, where proteins are denatured and fragmented, and is completed within the mucosal cells of the small intestine Proteins in the body are constantly being broken down into amino acids and reassembled in a process called protein turnover Nitrogen balance is a reflection of protein intake versus protein breakdown Depending on the proportion of each of the essential amino acids present (protein quality), foods can be classified as complete or incomplete proteins Inadequate or excess protein intake can have health and metabolic consequences