1. 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

2. 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

3. 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?

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

5. 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.

6. 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.

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

8. 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.

9. 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.

10. 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.

11. 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.

12. Protein Digestion and Absorption
Stomach
Acid
Pepsin
Intestine
Pancreatic proteases
Absorption

13. 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.

14. 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.

15. 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.

16. 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).

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

18. Nitrogen Equilibrium
Nin = Nout

19. Negative Nitrogen Balance
Nin < Nout

20. 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

21. 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.

22. 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.

23. Sources of Dietary Protein

24. 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.

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

26. Protein Deficiency

27. Protein Deficiency
Marasmus
Protein Energy Malnutrition (PEM)

28. Protein Deficiency
Locations where malnutrition is common

29. 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

30. 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

31. 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

32. 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