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Why Is Protein Important?
• Constantly assemble, break down, and
use proteins
– Replace cells
– Produce antibodies to fight infections
– Assist in water balance, nutrient transport,
and muscle contractions
– Diets replace what is being used
– Excess
• Make energy
• Store as fat
Amino Acids Are Building Blocks of
Protein
• Sequences of 20 different amino acids aa
• Nonessential (dispensable) amino acids
– Body can produce
• Essential (indispensable) amino acids
– Must be taken in via food
• Conditionally essential amino acids
– Essential during infancy, disease or trauma
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When the body experiences periods of extreme
trauma or stress, nonessential amino acids become
conditionally essential, or indispensable.
The physiological demands of recovery may result in
the need for these nonessential amino acids to be
greater than the body’s ability to produce them.
As a result, the body’s supply of nonessential amino
acids must be provided by the diet. Arginine,
glutamine, and cystine are examples of conditionally
essential amino acids that have been combined with
other nutrients in supplements formulated for wound
healing. by Mary Ellen Posthauer RD, CD, LD
Amino Acids Are the Building Blocks
of Protein
• Identified by side chains
– Carbon, carboxylic acid, amino acid, and
side group
– Side group gives AA its identity
AA the Building Blocks of Protein
• Protein structure: unique three-dimensional
shapes and functions
– Amino acids sequence
• Peptide bonds
–Dipeptide
–Oligopeptide
–Polypeptide
– Protein shape
• Sequence of AA determines shape
• Shape of protein determines function
Synthesis of Proteins
• Amino acids are linked by peptide bonds to
form proteins
• Synthesis of protein determined through gene
expression
• DNA transcription phase
– DNA code transferred from the nucleus to the
cytosol via messenger RNA (mRNA)
• mRNA translation phase
– tRNA and ribosomes
• DNA-coded instructions determine shape,
and thus function of proteins
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Amino Acids Are the Building Blocks
of Protein
• Protein denaturation:
Destabilizing a protein’s
shape
– Disrupts shape
– Caused by acidic or alkalinity,
high temperature, alcohol,
oxidation, and agitation
Functions of Body Protein
• Structural and mechanical
functions
– Collagen
– Keratin
– Muscle
• Immune functions
– Antibodies
Functions of Body Protein
• Enzymes
– Catalyze chemical reactions
• Hormones
– Regulate body processes
• Acid-base balance
– Proteins as buffer
Functions of Body Protein
• Transport functions
– Channels and pumps
– Albumin, Lipoproteins
• Fluid balance
– Albumin & edema
• Source of energy and glucose
– Carbs and fat are proteinsparing
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Protein Digestion and Absorption
• Cooking denatures protein
• Stomach
– Hydrochloric acid denatures
– Pepsin begins enzymatic digestion
• Small intestine
– Stimulate release of pancreatic proteases (trypsin,
chymotrypsin, carboxypeptidases)
• Amino acids absorbed into the portal vein
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Protein Digestion and Absorption
• Amino acid and peptide absorption
– Travel via portal vein to liver  released
into general circulation
• Undigested protein
– Feces
– Diseases that cause problems with dig
• Celiac disease
• Cystic fibrosis
Proteins in the Body
• The AA pool
– Available AA
• Protein
synthesis;
produce glucose
• Protein turnover
– Constant recycling
of protein
Proteins in the Body
• Nitrogen balance
– Nitrogen intake vs. nitrogen output
• Nitrogen equilibrium
– Nitrogen intake = nitrogen output
– Healthy adults
• Positive nitrogen balance
– Nitrogen intake > nitrogen output
– Growth; recovery from illness
• Negative nitrogen balance
– Nitrogen intake < nitrogen output
– Injury and illness
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Proteins in the Diet
• Recommended protein intake
– Adult RDA = 0.8 gram per kilogram body
weight
– Infants, children, and teens
• Infants have highest needs relative to body
weight
• Physical stress
– Infection, burns, fevers, surgery, and intensive
weight training
• Consumption in the United States
– Generally within on higher than recommended
range
Proteins in the Diet
• Complete Proteins
– Adequate amounts of all the essential amino acids
– Animal proteins, except gelatin
• Incomplete Proteins
– Inadequate amounts of 1 or more of the essential amino
acids
– Plant proteins, except soybeans
• Complementary Proteins
– Combining plant proteins to compensate for limiting amino
acids
Photos courtesy of the USDA
Proteins in the Diet
• Protein quality
– Two incomplete proteins
= complete protein
– Complementary proteins
Photos (top) Courtesy of USDA (middle) Photo by
Keith Weller. Courtesy of USDA. (bottom) Photo by
Scott Bauer. Courtesy of USDA.
Tryptophan
Methionine
(Cysteine)
Valine
Threonine
Total
protein
needs
Phenylalanine
(Tyrosine)
Methionine
Threonine
Phenylalanine
Isoleucine
Leucine
Histidine
(Infants)
Arginine
(Infants)
Beans
and other
legumes
Valine
Leucine
Lysine
(a) Essential amino acids
Tryptophan
Corn and
Isoleucine
other grains
Lysine
(b) Vegetarian diets providing the eight
essential amino acids for humans
Figure 24.2
Evaluating protein in the Diet
• Biological Value (BV)
– Egg white is highest
– Provides all the indispensable amino acids at
amounts the body needs
• Protein Efficiency Ratio (PER)
– Provides enough other amino acids to serve as
nitrogen sources for making dispensable amino
acids
– Protein digestibility-corrected amino acid score
(PDCAAS) - Is easy to digest
Proteins in the Diet
• Protein and AA additives
– Structure, texture, and taste
of foods
• Protein and AA
supplements
– Used for a variety of
reasons: dieters, athletes,
certain diseases
– Risks unknown
Photo © Jones and Bartlett Publishers
The Pros and Cons
of Vegetarian Eating
• Types of vegetarian diets
– Lacto-ovo vegetarian
• Includes milk, eggs
– Vegan
• Plant foods only
– Fruitarians
• Eat only raw fruit, nuts, and green foliage
© AbleStock
The Pros and Cons
of Vegetarian Eating
– Semi-vegetarian
• Avoid red meat; eat some chicken and
fish
– Zen macrobiotic diet
• Mostly vegans; stress whole grains and
locally grown food
The Pros and Cons
of Vegetarian Eating
• Health benefits of vegetarian diets
– Less fat, saturated fat, and cholesterol intake
– More magnesium and folate
– More antioxidants
– More fiber and phytochemicals
• Reduces risk for heart disease, obesity,
hypertension, and cancer
The Pros and Cons
of Vegetarian Eating
• Health risks of vegetarian diets
– Vegan diets may be low in some nutrients
• Calcium, iron, zinc, vitamin D, and
vitamin B12
– Higher intake of phytates
– More restrictive food choices = less
nutrients
– Careful planning needed for growth and
overall health
The Pros and Cons
of Vegetarian Eating
• Dietary recommendations
– Choose a variety of foods
– Choose whole, unrefined foods
– Choose a variety of fruits and vegetables
– Choose lower-fat dairy products and eggs in
moderation
– Consume a regular source of vitamins B12 and
D
• Fortified foods or supplements
The Health Effects
of Too Little Protein
• Dietary protein = indispensible amino
acids
• Protein deficiency occurs when energy
and/or protein intake is inadequate
• Protein-energy malnutrition (PEM)
– Kwashiorkor
• Edema and other signs
• Between ages of 18 and 24 months
• Associated with extreme poverty
The Health Effects
of Too Little Protein
• Protein-energy malnutrition (PEM)
– Marasmus
• Chronic PEM
• Infants and 6- to 18-month-old children
• Adults with cancer or starvation
– Nutritional rehabilitation
• Gradual and careful refeeding
The Health Effects
of Too Much Protein
The Health Effects
of Too Much Protein
• Strains kidney function
– Excretes products of protein breakdown
• Mineral losses
– Link between high-protein diet and
osteoporosis
• Obesity
– Correlation with body fat
The Health Effects
of Too Much Protein
• Heart disease
– Increased saturated fat and cholesterol
intake
• Cancer
– Increased risk for certain types of cancer
Food Protein Allergies
• Food proteins (allergens) cause an
immune response, creating an allergic
reaction
– IgE most commonly produced
– anaphylaxix
• 8 foods account for 90% of all food
allergies
– Peanuts, tree nuts, milk, eggs, fish, shellfish,
soy, and wheat
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