This is three separate meals: one breakfast, one lunch, one dinner.
After completing Unit 15 and its interactive learning activities, you will be able to:
Recall the basic functions, structures,and food sources of protein.
Summarize the effects of amino acid supplements and protein on muscle mass and strength.
protein Chemical substance in foods made up of chains of amino acids.
hormone A substance, usually a protein or steroid (a cholesterol-derived chemical), produced by one tissue and conveyed by the bloodstream to another.Hormones affect the body’s metabolic processes such as glucose utilization and fat deposition. immunoproteins Blood proteins such as antibodies that play a role in the func- tioning of the immune system (the body’s disease defense system). Antibodies attack
Illustration 15.1 The protein perception.
Protein ·Recall the basic functions, structures, and food sources of protein.
The term protein is derived from the Greek word protos,meaning “hrst”The derivation indicates_____importance ascribed to this substance when it was first recognized.Protein is an essential structural component of all living matter and is involved in virtually every biological process that occurs in cells.Protein has a very positive image (Illustration 15.1). The perception is so positive that you don’t have to talk about the importance of protein people are already convinced of it.
Key Nutrition Concepts
Content covered in Unit 15 on protein and amino acids relates to the following key nutrition concepts:
·Foods provide energy (calories),nutrients,and other substances needed for growth and health.
·Adequacy,variety,and balance are key characteristics of healthful diets.
Nearly all people in the United States get enough protein in their diets.2 The aver- age intake of protein by adults in the United States is 98 grams per day, approximately twice the recommended daily allowance (RDA) for men (56 grams) and for women of (46 grams). Approximately 16% of total calories in the average U.S. adult diet are supplied by protein.3
High-protein intakes are generally accompanied by high-fat and low-fiber intakes. That’s because foods high in protein, such as hamburger, cheese, nuts, and eggs, tend to be high in fat and contain little or no fiber. Even lean meats provide a considerable propor- tion of their total calories as fat (Illustration 15.2).
Functions of Protein
Proteins perform structural and functional roles in the body (Table 15.1). They are an integral structural component of skeletal muscle, bone, connective tissues (skin, collagen, and cartilage), organs (such as the heart, liver, and kidneys), red blood cells and hemo- globin, hair,and fingernails. Proteins are the basic substance that make up thousands of enzymes in the human body; they are a major component of hormones such as insulin and growth hormone, and serve as other substances that perform important biological functions.Tissue maintenance and the repair of organs and tissues damaged from illness or injury are functions of different types of protein. Albumin, a protein made by the liver, is the blood’s “tramp steamer” It attaches to and transports fatty acids, calcium, and other substances through the circulatory system to cells throughout the body.4 Protein serves as an energy source at the level of four calories per gram.
The body of a 154-pound man contains approximately 24 pounds of protein.Nearly half of the protein is found in muscle; the rest is prèsent in the skin, collagen,blood, enzymes, and immunoproteins; and in organs such as the heart, liver, and intestines; and other body parts.All protein in the body is continually being turned over, or broken down and rebuilt. This process helps maintain protein tissues in optimal condition so they continue to function normally. The process of protein turnover utilizes roughly nine ounces of protein each day. Yet, we consume only two to three ounces of protein daily. Most of the protein used for maintenance is recycled from muscle and other protein tis- sues being turned over.Proteins play key roles in the repair of body tissues by serving as substances-such as fibrin-that help blood clot (Illustration 15.3) and by replacing tissue proteins damaged by illness or injury.5
Protein serves as a source of energy in healthy people,but not nearly to the extent that carbohydrates and fats usually do. Protein is unlike carbohydrate and fat in that it contains nitrogen and does not have a storage form in the body.In order to_____protein for energy,amino acids that make up proteins must first be stripped of their nitrogen.The free nitrogen can be used as a component of protein formation within the body,or, if pres
15-2 UNIT 15
1 The primary function of protein is to provide energy. True/False
2“Nonessential amino acids” are not required for normal body processes. Only “essential amino acids” are. True/False
3 High-protein diets and amino acid supplements by themselves increase muscle mass and strength. True/False
Answers can be found at the end of the unit.
(e) xioin roast: 36%
(g) Chicken thigh, no skin: 47%
(d)Pork chop,lean: 48%
(h) Baked chicken breast, no skin: 19%
mu ntion 15.2 The fat content of three-ounce portions of “lean” meats. The percentage of calories from fat is indicated for each portion.(A three- ouncs portion of meat is about the size of a deck of cards.)Each portion of meat provides approximately 21 grams of protein.
ent in excess, it is largely excreted in urine. Excretion of nitrogen requires water, so high intake of protein increases water need. Amino acids missing their nitrogen component are converted to glucose or fat that then can be used to form energy. A small amount of pro- tein (1%) can be obtained from the liver and blood and used to cover occasional deficits in protein intake.6
Nitrogen Balance Nitrogen balance represents the balance between protein intake and protein utilization by the body. Protein intake is estimated as the nitrogen content of protein consumed. The nitrogen content of protein is estimated as 16% of the weight of protein consumed. Nitrogen excretion is assessed as the amount of nitrogen excreted in the form of urea. Nitrogen balance is measured as the difference between nitrogen intake and nitrogen excreted.22
· A person is considered to be in “nitrogen balance” when her or his intake of nitrogen equals nitrogen excreted. Example: A person consumes 10 grams of nitrogen and excretes 10 grams of nitrogen: 10 grams nitrogen – 10 grams nitrogen = 0 grams nitrogen. People who are in nitrogen balance are consuming as much protein as they are utilizing.
·A person who consumes less nitrogen than excreted is in “negative nitrogen balance” Example: A person consumes 10.2 grams nitrogen and excretes 14.0 grams: 10.2 grams
BSIP Agency/Index Stock Imagery/Jupiterimages
Table 15.1 Examples of functions
·Serves as a structural material in muscles,connective tissue, organs,and hemoglobin
·Maintains and repairs protein- containing tissues
·Serves as the basic component of enzymes, hormones, and other biologically important chemicals
·Serves as an energy source
·Helps maintain body fluid balance
·Helps maintain acid-base balance in body fluids
nitrogen balance The difference between nitrogen intake and excretion. It is assessed as the difference between nitrogen intake and nitrogen excreted.
urea Nitrogen released from the break- down of proteins for energy is largely excreted in the urine in the form of urea. It can be measured in urine, or in blood as blood urea nitrogen (BUN).
Illustration 15.3 Red blood cells enmeshed in fibrin in a color-enhanced microphotograph. Red blood cells and fibrin (which helps stop bleeding by caus- ing blood to clot) are made primarily from protein.
essential amino acids Amino acids that cannot be synthesized in adequate amounts by humans and therefore must be obtained from the diet. They are sometimes referred to as “indispensable amino acids.”
nonessential amino acids Amino acids______can be readily produced by humans from components of the diet. Also referred to as “dispensable amino acids.”
DNA(deoxyribonucleic acid) Genetic material contained in cells that directs the production of proteins in the body.
Illustration 15.4 The basic chemical structure of alanine, a nonessential amino acid.
A person is considered to be in “positive nitrogen balance” when his or her intake of
fever, and other illnesses.
nitrogen is higher than nitrogen excretion.Example:A person consumes 18 grams
of nitrogen and excretes 14.5 grams:18 grams nitrogen-14.5 grams nitrogen =+35
grams nitrogen.A positive nitrogen balance indicates that some of the protein com.
sumed is being retained and used to build up body protein tissues.Positive nitrogen
balance occurs during growth,pregnancy,breastfeeding,and recovery from illness or
Results of nitrogen balance studies help determine protein need and are sometimes
used clinically to adjust a person’s protein intake to meet protein need.22
The building blocks of protein are amino acids,which share the characteristic of contain.
ing nitrogen. Illustration 15.4 shows an example of the basic structure of an amino acid
and its nitrogen component. Twenty common amino acids (Table 15.2) form proteins,
and every protein in the body is composed of unique combinations of amino acids.Nine
of the twenty common amino acids are considered essential, and 11 are nonessential..
The essential amino acids are called essential because the body cannot produce them.
or produce enough of them, so they must be provided by the diet. Healthy individuals
can produce the nonessential amino acids, so we don’t require a dietary source of them.
Despite the labels, all 20 amino acids are required to build and maintain protein tissues.
Proteins in foods contain both essential and nonessential amino acids.
Amino Acids and Protein Structure The assembly of amino acids into proteins is directed by DNA, the genetic material within each cell. Some proteins are made of only a few amino acids, while other proteins contain over 25,000. The arrangement of amino acids determine whether a protein functions as an enzyme or a hormone, or it becomesa component of red blood cells, muscle, or other substance.
Proteins produced vary in size and complexity based on their role in cellular pro- cesses. They are classified by their properties as having primary, secondary, tertiary, or quaternary structures (Illustration 15.5). Proteins with primary structure consist of a lin- ear arrangement of linked amino acids, whereas proteins with secondary structures have folded chains of amino acid. Some hormones and chemical messengers that initiate cel- lular processes have these structures.Tertiary structures represent the three-dimensional structure of more complex and larger proteins. These proteins have elaborate folding patterns such as found in insulin (Illustration 15.6).Proteins with quaternary structure are the largest and consist of multiple, linked chains of amino acids folded and formed in sud a way that they can perform specific functions. Hemoglobin, a component of red blood cells that transports oxygen to cells and removes carbon dioxide from cells, is an example of this type of protein structure.6
Table 15.2 Essential and nonessential amino acids
|Histidine Threoninesoleucine TryptophanLeucine ValineLysineMethioninePhenylalanine||AsparagineAlanineCysteineAspartic acidArginineGlt TyrosineGlutamineGlycineProlineSerine|
15-4 UNIT 15
Photo by Education Images/UIG via Getty lmages
Amino acids cannot be stored very long in the body, so we need a fresh supply of essential amino acids daily.This means we need to consume foods that provide a sufficient amount of all essential amino acids every
Complete Proteins Food sources of high-quality protein (meaning
they contain all the essential amino acids in the amount needed to sup-
port protein formation) are called complete proteins. Proteins in this
category include those found in animal products such as meat, milk, and
eggs (Illustration 15.7). Incomplete proteins are deficient in one or more
essential amino acids. Proteins in plants are “incomplete,” although soy-
beans are considered a complete source of protein for adults. ‘ (Soybeans
may not meet the essential amino acid requirements of young infants.) You
can complement the essential amino acid composition of plant sources of
Illustration 15.7 Animal sources of protein shown here supply “complete proteins.”
complete proteins Proteins that Contain all of the essential amino acids in amounts needed to support growth and tissue maintenance.
incomplete proteins Proteins that are deficient in one or more essential amino acids.
Illustration 15.8 Each of these com- binations of plant foods provides comple- mentary protein sources.
protein by combining them to form a “complete” source of protein. Illustration 15.8 shows
a few complementary plant food combinations that produce complete proteins.
Vegetarian Diets Diets consisting only of plant foods can provide an adequate amount of complete proteins. A key to success is eating a variety of complementary sources of protein each day. Vegetarian diets have been practiced for centuries by some religious and cultural groups, bearing testimony to their general adequacy and safety. (Unit 16 on vegetarian diets expands on this topic.)
Amino Acid Supplements Because amino acids occur naturally in foods, people may assume that amino acid supplements are harmless, no matter how much is taken (Illustra- tion 15.9). Researchers have known for decades, however, that high intakes of individual amino acids can harm health. High amounts may disrupt normal protein production by overwhelming cells with___surplus of some amino acids and a relative deficit of others.8 Excess consumption of the amino acid methionine, in particular, causes a host of prob- lems. Intake levels two to five times above the amount normally consumed from foods worsen the symptoms of schizophrenia, promote hardening of the arteries, impair fetal and infant development, and lead to nausea, vomiting, bad breath, and constipation.8,9 Adverse health effects associated with supplemental cysteine and phenylalanine have also been reported.8,10 Use of amino acid supplements should be supervised by a physician.
15-6 UNIT 15
A.Primary structure: the sequence of amino acids in a protein
B.Secondary structure: regions of alpha helix,beta strand, or random coil in a polypeptide chain
C. Tertiary structure: overall three-dimensional folding of a polypeptide chain
D. Quaternary structure: the arrangement of polypeptide chains in a protein that contains more than one chain
Illustration 15.5 The primary, secondary, tertiary, and quaternary structures of proteins. Source:Russell,Biology:The Dynamic Science,2nd ed.,chap.3,fig.3.18,p.58.
Illustration 15.6 Amino acid chains and folding of insulin, a tertiary protein.
The ability of proteins to support protein tissue construction in the body varies depending on their content of essential amino acids. How well dietary proteins support protein tissue construction is captured by tests of the protein’s “quality”
Proteins of high quality contain all the essential amino acids in the amounts needed to support protein tissue formation by the body. If any of the essential amino acids are missing in the diet, proteins are not formed-even those proteins that could be produced from available amino acids. Shutting off all protein formation for want of an amino acid or two may appear inefficient; but if the body did not cease all protein formation, cells would end up with an imbalanced assortment of proteins. This would seriously affect cell func- tions. When the required level of an essential amino acid is lacking, the remaining amino acids are primarily used for energy.