Perspectives in Nutrition, 8th Edition
Chapter 7 Outline: Proteins
After studying this chapter, you will be able to:
1. Define essential and nonessential amino acids and explain why adequate amounts of each of the
essential amino acids are required for protein synthesis.
2. Distinguish between high-quality and low-quality proteins and list sources of each.
3. Describe how 2 low-quality proteins can be complementary to each other to provide the required
amounts of essential amino acids.
4. Explain the methods used to measure the protein quality of foods, including assessment of
biological value.
5. List the factors that influence protein needs. Calculate the RDA for protein for a healthy adult
with a given body weight.
6. Explain positive nitrogen balance, negative nitrogen balance, and nitrogen equilibrium and list the
conditions under which they occur.
7. Describe how protein is digested and absorbed in the body.
8. List the primary functions of protein in the body.
9. Describe how protein-energy malnutrition can eventually lead to disease in the body.
10. Describe the symptoms and treatment of food allergies.
11. Develop a vegetarian diet plan that meets the body's protein needs.
7.1
Structure of Proteins
A.
Amino acids
1.
Composed of carbon, hydrogen, oxygen, and nitrogen (some also contain sulfur;
see Figure 7-2)
a.
Amino group
b.
Carboxyl group
c.
Hydrogen molecule
d.
R group (side chain) differentiates one amino acid from another
2.
Building blocks of proteins
3.
20 amino acids are required for human function at all life stages
a.
Non-essential amino acids can be synthesized by the body and do not
need to be obtained in the diet (11)
B.
C.
b.
Essential amino acids cannot be synthesized by the body and must be
obtained in the diet (9)
c.
Conditionally essential amino acids may become essential during
infancy, disease, or trauma
i.
Tyrosine for people with PKU
ii.
Glutamine and arginine for trauma patients
Synthesis of Nonessential Amino Acids (see Figure 7-3)
1.
Transamination: transfer of an amino group from 1 amino acid to a carbon
skeleton to form a new amino acid
2.
Deamination: loss of amino group from an amino acid
a.
Amino group is usually incorporated into urea by liver and excreted by
kidneys
b.
Remaining carbon skeleton can be used for fuel or synthesized into other
compounds
Amino Acid Composition: Complete and Incomplete Proteins
1.
Complete (high-quality) proteins contain sufficient amounts of all 9 essential
amino acids to support protein synthesis in the body (e.g., all animal proteins,
with the exception of gelatin)
2.
Incomplete (low-quality) proteins are lacking at least 1 of the 9 essential amino
acids (e.g., all plant proteins, with the exception of soy protein)
3.
Cells require a pool of amino acids for synthesis of body proteins
4.
Limiting amino acid: absence of 1 essential amino acid restricts protein synthesis
5.
a.
Legumes: methionine, tryptophan
b.
Nuts and seeds: lysine
c.
Grains: lysine
Complementary proteins: combination of various sources of low-quality proteins
to supply all 9 essential amino acids
a.
Legumes + grains, nuts, or seeds
b.
Mixed diets generally provide high-quality protein
7.2
c.
Complementary proteins need not be consumed at same meal, but can be
balanced throughout the day
d.
Adding a small amount of high-quality protein to low-quality protein
provides a full complement of essential amino acids
Sources of Protein
A.
General
1.
B.
Dietary sources (65 - 100 g/d)
a.
Typical American diets supply 70% of protein from animal sources
b.
Worldwide diets supply only 35% of protein from animal sources
c.
Plants can provide ample dietary protein with benefits of fiber, vitamins,
minerals, and phytochemicals, while avoiding cholesterol and saturated
fat
2.
Recycling of body proteins (250 - 300 g/d)
3.
Tips to increase intake of plant sources of protein
a.
Try veggie burger
b.
Top salads with nuts
c.
Add nuts to bread, muffin, or pancake batter
d.
Consume edamame or roasted soy beans for snacks
e.
Use peanut butter instead of butter or cream cheese
f.
Use soy milk
g.
Substitute beans for meat in tacos
h.
Make stir-fry with tofu, cashews, and vegetables
Evaluation of Food Protein Quality
1.
General
a.
Indicate ability of food protein to support body growth and maintenance
b.
Digestibility
i.
Animal proteins: 90 - 100%
ii.
2.
3.
4.
5.
Plant proteins: 70%
c.
Amino acid composition
d.
Applies only to conditions in which protein intakes are equal to or less
than protein required for essential amino acids
e.
As protein intake increases, efficiency of protein use decreases because
excess amino acids cannot be readily stored and are primarily degraded
for use as energy
Biological Value (BV)
a.
Measure of how efficiently an absorbed food protein is converted to
body tissue protein
b.
Determined by comparing nitrogen retention in body to nitrogen content
of food protein
c.
Higher BVs result when food’s amino acid composition is similar to
body’s amino acid composition
d.
Egg white protein BV = 100
e.
Most animal proteins have high BV
Protein Efficiency Ratio (PER)
a.
Compares weight gained by growing laboratory animal consuming a
standardized amount of a given protein compared to weight gained by an
animal consuming a reference protein (e.g., casein)
b.
Animal proteins have higher PER than plant proteins
c.
Method is used in labeling of foods intended for infants
Chemical Score
a.
Amount of each amino acid in a gram of a given food protein is divided
by the ideal amount for each amino acid in a reference food protein (e.g.,
egg whites)
b.
Lowest (limiting) amino acid ratio is chemical score, range from 0 - 1.0
Protein Digestibility Corrected Amino Acid Score (PDCAAS)
a.
Most widely used measure of protein quality
b.
Multiply food’s chemical score by its digestibility
7.3
c.
Ranges from 0 - 1
d.
For nutrition labeling, protein content (% Daily Value) is reduced if
PDCAAS <1; rarely reported on food labels because PDCAAS is
expensive to determine
Recommended Intakes of Protein
A.
Protein balance
1.
Equilibrium: protein intake replaces protein losses from urine, feces, sweat, skin
cells, hair, and nails
2.
Negative protein balance: intake < losses, leading to depletion of blood proteins,
skeletal muscles, and organs (brain resists protein breakdown)
3.
4.
B.
a.
Inadequate diet
b.
Illness or injury
c.
Diseases (e.g., Cushing’s disease, AIDS) that increase protein breakdown
Positive protein balance: intake > losses; requires anabolic hormones (e.g.,
insulin, growth hormone, testosterone) in addition to adequate protein intake
a.
Growth
b.
Recovery from injury, trauma, or illness
Measured as nitrogen balance
a.
Nitrogen makes up 16% amino acid weight
b.
Nitrogen (g) x 6.25 = Protein (g)
c.
Measurement is only practical in hospital or research environments
d.
More convenient to estimate protein needs based on RDA
Dietary Reference Intake for Protein
1.
For most adults, RDA for protein = 0.8 g/kg/d based on healthy weight
2.
RDA does not address additional protein requirements for recovery from illness
or injury (0.8 - 2.0 g/kg) or for highly trained athletes (0.8 - 1.7 g/kg)
3.
Typical American diet supplies ample protein to meet RDA
4.
7.4
Excess protein cannot be stored as such; carbon skeletons are used for other
purposes or used to meet energy needs
Protein Digestion and Absorption
A.
Digestion (see figure 7-11)
1.
Begins with cooking, which denatures proteins and softens connective tissues in
meat
2.
Stomach
3.
B.
a.
HCl denatures proteins
b.
Pepsinogen  pepsin, which begins to break long polypeptide chains
into shorter peptides through hydrolysis reactions
Small intestine
a.
CCK released from intestinal walls stimulates pancreas
b.
Pancreas secretes enzymes to hydrolyze peptides and polypeptides into
amino acids
i.
Protease
ii.
Trypsin
iii.
Chymotrypsin
iv.
Carboxypeptidase
Absorption
1.
Short peptides and amino acids are actively absorbed into small intestinal cells
2.
Inside small intestinal cells, any short peptides are broken down to amino acids
by peptidase enzymes
3.
Amino acids enter portal vein and travel to liver
a.
Protein synthesis
b.
Energy needs
c.
Conversion to carbohydrate or fat
d.
Release into bloodstream for transport to other cells
4.
7.5
Except during infancy, intact proteins are usually not absorbed; delay
introduction of commonly allergenic foods to infants
Functions of Proteins
A.
Producing Vital Body Structures
1.
Only when adequate carbohydrate and fat are consumed can amino acids be used
for synthesis of body proteins
2.
Structural proteins make up 1/3 of body protein; provide matrix for bone, muscle,
and connective tissue
3.
B.
C.
Collagen
b.
Actin
c.
Myosin
During growth, proteins are synthesized; during malnutrition or disease, protein
synthesis declines and wasting occurs
Maintaining Fluid Balance
1.
Albumin and globulin remain in bloodstream and counteract the force of blood
pressure that forces body fluids into interstitial spaces
2.
With inadequate protein intake, blood protein levels decline and excessive fluid
collects in tissues (edema)
Contributing to Acid-Base Balance
1.
2.
D.
a.
pH describes acid-base balance
a.
low pH is more acidic
b.
high pH is more alkaline
Proteins act as buffers: maintain acid-base balance by accepting and releasing
hydrogen ions as needed
Forming Hormones, Enzymes, and Neurotransmitters
1.
Hormones: chemical messengers, regulate body functions
2.
Enzymes: facilitate chemical reactions
3.
Neurotransmitters: chemical messengers
E.
F.
7.6
b.
Norepinephrine synthesized from tyrosine
c.
Serotonin synthesized from tryptophan
1.
Antibodies bind to antigens (foreign proteins) and prevent their attack on target
cells
2.
Without sufficient protein, immune system is compromised (anergy) and ability
to fight infection is reduced
Transporting Nutrients
2.
H.
Dopamine synthesized from tyrosine
Contributing to Immune Function
1.
G.
a.
Carry nutrients through bloodstream
a.
Hemoglobin carries oxygen
b.
Lipoproteins transport lipid molecules
Allow nutrients to cross cell membranes
a.
Retinol binding protein: carrier for vitamin A
b.
Transferrin: carrier of iron
c.
Ferritin: storage of iron
d.
Ceruloplasmin: carrier for copper
Forming Glucose
1.
If carbohydrate intake is inadequate to maintain blood glucose levels, liver and
kidneys make glucose from amino acids in body tissues (gluconeogenesis)
2.
Chronic use of amino acids for gluconeogenesis leads to cachexia (muscle
wasting)
Providing Energy
1.
Under most conditions, body uses fats and carbohydrates for energy
2.
Proteins are a costly source of energy
Health Concerns Related to Protein Intake
A.
Protein-Energy Malnutrition
1.
General
a.
Protein deficiency usually occurs in combination with deficiencies of
other nutrients
b.
Most common in developing areas of world
c.
Most devastating for children
i.
Poor growth
ii.
Diarrhea
iii.
Infections
iv.
Diseases
v.
Early death
d.
2.
High-risk populations
i.
Poverty
ii.
Isolation
iii.
Substance abuse
iv.
Anorexia nervosa
v.
Debilitating diseases (e.g., AIDS or cancer)
vi.
Hospitalized patients, due to poor health, low dietary intakes,
and increased protein needs
Kwashiorkor: marginal calorie intake with deficient protein intake
a.
“The disease that the first child gets when the new child comes” due to
abrupt shift from breast milk to starchy roots and gruels
b.
Bulk of diet limits energy and protein intake
c.
Increased protein needs due to parasites and other infections
d.
Characteristics
i.
Edema
ii.
Mild to moderate weight loss
iii.
Growth impairment
iv.
Fatty liver
v.
Apathy
vi.
Diarrhea
vii.
Listlessness
viii.
Infections
ix.
3.
B.
Withdrawal from environment
Marasmus: chronic protein and energy deficiency
a.
“To waste away”
b.
Usually occurs in infants who are not breastfed or are weaned early;
formula is improperly diluted, use of unsafe water supplies
c.
Most common in large cities of developing countries
d.
Characteristics
i.
Extreme weight loss
ii.
Muscle and fat wasting (skin and bones appearance)
iii.
Impaired growth
iv.
Poor cognitive development
High-Protein Diets
1.
Diets supplying >35% energy from protein
2.
Overburden kidneys’ capacity to excrete nitrogen wastes (greatest concern for
those with impaired kidney function)
3.
Inadequate fluid intake may increase the risk of dehydration
4.
High intake of animal proteins may present several problems
a.
Low fiber intake
b.
Low vitamin C, E, and folate intake
c.
Low magnesium and potassium intake
d.
Low phytochemical intake
7.7
e.
High saturated fat and cholesterol intake
f.
May increase risk for cardiovascular disease
g.
High intakes of red meat or cured meats may increase risk for certain
types of cancer
h.
Increase urinary calcium losses, influencing risk of osteoporosis; most
likely a concern for those with marginal calcium intakes
i.
Protein and amino acid supplementation by athletes may result in amino
acid imbalances and toxicity risk
Food Protein Allergies
A.
B.
C.
Some food proteins (allergens) cause hypersensitivity reactions and trigger immune
response
1.
Mild reactions: runny nose, sneezing, itching skin, hives, digestive upset
2.
Anaphylaxis: decreased blood pressure, respiratory distress, possibly death
Common food allergens
1.
Peanuts/tree nuts
2.
Milk products
3.
Soy products
4.
Wheat
5.
Eggs
6.
Fish/shellfish
Avoiding food allergens
1.
2.
For all ages
a.
Food labeling
b.
Know menu ingredients
c.
Sanitation of food preparation environment and serving dishes/utensils to
prevent cross-contamination with allergenic foods
To prevent development of allergies in infants and children
D.
7.8
a.
Maternal dietary restrictions during pregnancy (e.g., avoiding peanuts
and tree nuts during last three months of pregnancy) may play a role, but
likely insignificant
b.
Maternal dietary restriction during lactation
c.
Exclusively breastfeed or formula feed for first 6 months
d.
Delay introduction of cow’s milk until 1 year
e.
Delay introduction of egg whites until 2 years
f.
Delay introduction of peanuts, tree nuts, fish, and shellfish until 3 years
Statistics
1.
11 million Americans have food allergies
2.
Number of children with peanut allergies has doubled from 1997 to 2002
3.
80% of young children with food allergies outgrow them before 3 years of age
Vegetarian Diets
A.
B.
Statistics
1.
2.5% and 4% of adults in US and Canada, respectively, follow vegetarian diets
2.
20 - 25% of Americans eat at least 4 meatless meals/week
Rationale
1.
Religion
2.
Philosophy
3.
Ecology
a.
4.
Raising meat protein requires 40% of world’s grain production
Health
a.
Increased antioxidant nutrients
b.
Increased fiber
c.
Decreased saturated fat and cholesterol
d.
C.
D.
Many health agencies promote vegetarian diets as a way to reduce risk
for chronic diseases
Types of plant-based diets
1.
Vegan: only plant foods
2.
Lacto-vegetarians: exclude meat, poultry, eggs, and fish, but do consume milk
and milk products
3.
Lacto-ovo-vegetarians: exclude meat, poultry, and fish, but do consume milk
products and eggs
Vegetarian diets require careful planning to ensure high-BV protein (i.e., complementary
proteins) and other key micronutrients
1.
Riboflavin
2.
Vitamin D
3.
Vitamin B-12
4.
Calcium
5.
Iron
6.
Zinc
E.
Use of fortified breakfast cereal and other fortified foods may close any nutritional gaps
F.
Special Concerns for Infants and Children
1.
2.
With use of complementary proteins and good sources of problem nutrients,
nutritional requirements of vegetarian and vegan infants and children can be met
a.
Iron
b.
Vitamin B-12
c.
Vitamin D
d.
Zinc
e.
Calcium
Bulk of high-fiber diet may lead to early satiety, thus limiting energy intake
a.
May substitute some refined grains, fruit juices, and peeled fruit for high
fiber foods
b.
7.9
Include concentrated sources of energy
i.
Fortified soy milk
ii.
Nuts
iii.
Dried fruits
iv.
Avocados
Expert Perspective: Nutrition and Immunity
A.
B.
Innate (non-specific) immunity
1.
Present at birth
2.
First barrier against antigens
3.
General, non-specific response
4.
Components
a.
Physical barriers (e.g., skin, mucous membranes)
b.
Chemical secretions (e.g., HCl)
c.
Physiological barriers (e.g., fever)
d.
Phagocytic cells
Acquired (specific) immunity
1.
Initiated by recognition of specific antigen
2.
Develops throughout lifespan
3.
Adaptive immunity
4.
Bone marrow and thymus produce antibodies (immunoglobulins) and other
specialized immune cells to destroy antigens
C.
Breastfeeding is recommended for infants to transfer immune components (e.g.,
immunoglobulins and lactoferrin) from mother to infant
D.
Importance of nutrition for immune function
1.
Malnutrition leads to loss of antigen-producing tissue, decreased number and
effectiveness of antibodies, and a breakdown of physical barriers to antigens
2.
Severe PEM with accompanying micronutrient deficiencies, infections, and
diarrhea impair immunity
3.
Nutrients that increase immune protection during critical illness and trauma
(immunomodulators)
4.
a.
Arginine
b.
Glutamine
c.
EFAs
Maintaining optimal nutritional status supports immune function