VISUALIZING NUTRITION
CANADIAN EDITION
Mary B. Grosvenor • Lori A. Smolin • Diana Bedoya
Chapter 6:
Proteins & Amino Acids
CHAPTER 6: PROTEINS & AMINO ACIDS
LEARNING OBJECTIVES
At the end of this chapter, you should be able to:
• Describe the structure of proteins and amino acids and
explain their role in health and disease
• Plan a diet to meet protein recommendations
THINK about this – then share within
a PAIR – then SHARE with the class
• What do you know about proteins and amino acids?
• What are benefits of eating plant foods over animal
foods?
Sources of protein
• Animal: meat, eggs, dairy products
• High in saturated fat and cholesterol; provide B vitamins and
some absorbable minerals
• Plant: grains, nuts, legumes
• Low in saturated fat and cholesterol; high in fiber,
phytochemicals, and unsaturated fat
• Legumes: starchy plant seeds producing bean pods, including
peas, peanuts, beans, soybeans, lentils
Sources of protein
a. Animal products are
high in protein, iron,
zinc, and calcium but
add saturated fat and
cholesterol to the diet.
Sources of protein
Michael Newman/Photo Edit
b. Plant sources of
protein are rich in fibre,
phytochemicals, and
monounsaturated and
polyunsaturated fats.
Amino acids
• Amino acids: building blocks of proteins
• Carbon, hydrogen, amino group (contains nitrogen), acid
group, and a unique side chain
• 20 different side chains make 20 different amino acids
Amino acids link to form proteins
• We obtain amino acids from eating plant and animal
foods.
• Our bodies reassemble the amino acid building blocks
into our body proteins.
Building blocks
Match the building blocks with the nutrient:
____ complex carbohydrates
a) amino acids
____ triglycerides
b) fatty acids and
glycerol
____ proteins
c) monosaccharides
Essential vs. non-essential amino
acids
• Essential/indispensable: Cannot be made in the body
so “essential” to eat them
• If one is missing, body proteins are broken down to make new
proteins
• Non-essential/dispensable: Body can make them from
essential amino acids so not essential to eat them
Essential vs. non-essential amino
acids
b. Twenty amino acids are
commonly found in
proteins. The table shown
here lists them based on
whether they are essential
or nonessential and
indicates those that are
conditionally essential.
Essential vs. non-essential amino
acids
Food A
Food B
Essential vs. non-essential amino
acids
Can you use only the amino acids
from Food A to make this protein?
Essential vs. non-essential amino
acids
Can you use only the amino acids
from Food A to make this protein?
The yellow triangle is a nonessential amino acid
Essential vs. non-essential amino
acids
Can you use only the amino acids
from Food A to make this protein?
The blue square is a
limiting amino acid
Essential vs. non-essential amino
acids
Can you use only the amino acids
from Food A to make this protein?
Essential vs. non-essential amino
acids
Can you use only the amino acids
from Food B to make this protein?
Essential vs. non-essential amino
acids
Can you use only the amino acids
from Foods A and B to make this protein?
Essential vs. non-essential amino
acids
Did you use all of the amino acids
from Foods A and B to make this protein?
It not, what did the body do with the extras?
Essential vs. non-essential amino
acids
Foods A and B are complementary: each supplies
some amino acids for the full set of needed amino
acids
Essential vs. non-essential amino
acids
Essential amino acids
Cannot be made
Need to be eaten
Non-essential amino acids
Can be made from others
Conditionally essential amino acids
• Under certain conditions some of the nonessential
amino acids cannot be made in the body
• Example: in phenylketonuria (PKU) phenylalanine cannot be
converted to tyrosine thus tyrosine, a nonessential amino acid
in healthy individuals, becomes an essential amino acid in
people with PKU
Protein structure
• Peptide bonds: join the acidic group of one amino acid
with the amino group of another amino acid
• Polypeptides: a chain of amino acids linked by peptide
bonds
• Proteins: made of one or more polypeptide chains
folded into specific three-dimensional structures
Amino acids link to form proteins
Protein structure vs. function
• Shape of proteins determines their functions
• If protein shape is changed, then protein function may change
Protein denaturation
• Denaturation: change in a protein’s threedimensional shape
• Occurs with:
• Heat from cooking
• Acidity (example: low pH in the stomach)
• Mechanical agitation
Protein denaturation
© Can Stock Photo Inc./ glorcza
When an egg is cooked, the heat
denatures the protein. The protein
in a raw egg white forms a clear,
viscous liquid, but when cooking
denatures it, the egg white
becomes white and firm and
cannot be restored to its original
form. Other factors that denature
the proteins in food include
mechanical agitation, such as
when cream is whipped to make
whipped cream, and the addition
of acid, such as when milk is
curdled by the addition of an acid
such as lemon juice.
Concept check
• Which chemical element is found in protein but not in
carbohydrate or lipid?
• What determines whether an amino acid must be
consumed in the diet?
• What determines the shape of a protein?
• How does denaturation affect protein function?
Protein digestion
• Proteins have to be broken down to tripeptides,
dipeptides and amino acids, before they can be
absorbed into the mucosal cells of the small intestine
• Mechanical digestion of proteins starts in the mouth,
continues in the stomach and the small intestine
• Chemical digestion of proteins begins in the stomach but
most of it occurs in the small intestine
Protein absorption
• In the mucosal cells of the small intestine dipeptides
and tripeptides are broken down to single amino acids
• Amino acids are moved from the mucosal cells to
blood by the transporting system
• Since the transporting system has a limited capacity, amino
acid supplements may reduce the absorption of some
dietary amino acids
Protein digestion
Application
Insulin is a protein. Why do diabetics need to inject
insulin rather than take it orally?
Concept check
The purple amino acids are absorbed by the same
transport system as the green amino acids. If you
consume a sports drink that is supplemented with the
green amino acids what will happen to the absorption of
the purple ones.
a) nothing
b) the larger amount of green ones will limit
the absorption of the purple ones
c) the small amount of the purple ones will
be absorbed first
d) both will be absorbed equally because the
body needs them
Concept check
• Where does the chemical digestion of protein begin?
• Why might supplementing one amino acid reduce the
absorption of other amino acids?
Amino acid pool
• Amino acid pool: amino acids available for use in the
body
• From the diet or breakdown of body proteins
• Used to make other proteins or chemicals (for example: DNA,
RNA, histamine, etc.)
• Used to provide energy or make glucose or fatty acids
Amino acid pool
Amino acids enter the available pool from the diet and from the breakdown of
body proteins. Of the approximately 300 grams of protein synthesized by the
body each day, only about 100 grams are made from amino acids consumed in
the diet. The other 200 grams are produced by the recycling of amino acids from
protein broken down in the body. Amino acids in the pool can be used to
synthesize body proteins and other nitrogen-containing molecules, to provide
energy, or to synthesize glucose or fatty acids.
Protein synthesis
• Protein synthesis is regulated by genes
• Depending whether a gene is turned on (expressed) or
turned off, the protein is synthesized or its synthesis
stops
• Chemicals can turn genes on and off
• For example, high iron turns on the ferritin gene to make more
ferritin which is needed for iron storage
Limiting amino acid
• A shortage of just one essential amino acid can stop
the process of protein synthesis
• Limiting amino acid is the essential amino acid present in
short supply relative to body’s need
• Nonessential amino acids can be made in the body via
transamination
• Transamination: amino acid group from one amino acid is
transformed to carbon-containing molecule to form another
amino acid
Protein synthesis
Genes contain instructions for
proteins
Protein functions
•
•
•
•
•
•
•
•
•
Structure (e.g., skin, hair, muscle)
Speed up chemical reactions (enzymes)
Chemical signaling (e.g., hormones)
Transportation of substances (e.g., blood proteins)
Movement (e.g., muscles)
Immunity (e.g., antibodies)
Blood clotting
Fluid balance
Maintenance of pH
Protein functions
Structure: Collagen, which
is the most abundant protein
in the body, plays important
structural roles. It is the
major protein in ligaments,
which hold our bones
together, and in tendons,
which attach muscles to
bones, and it forms the
protein framework of bones
and teeth.
Protein functions
Catalyzing reactions: Enzymes, such as this one that
speeds up the breakdown of starch, are protein
molecules. All chemical reactions occurring within the
body require the help of enzymes. The specific structure
or shape of each enzyme allows it to interact with the
molecules in the specific reaction it accelerates. Without
enzymes, metabolic reactions would occur too slowly to
support life.
Protein functions
Transport: Proteins help
transport materials
throughout the body and
into and out of cells. The
protein hemoglobin, which
gives these red blood cells
their colour, shuttles oxygen
to body cells and carries
away carbon dioxide.
© Can Stock Photo Inc./Eraxion
© Can Stock Photo Inc./BVDV
Protein functions
Protection from disease:
Young women can now be immunized
against the human papilloma virus
(HPV). The vaccine contains a small
amount of dead or inactivated HPV
virus. It does not make a person sick,
but it does stimulate the immune
system to make proteins called
antibodies, which specifically attack
the virus and prevent HPV from being
contracted.
Protein functions
© Can Stock Photo Inc./gregepperson
Movement: The proteins actin
and myosin in the arm and leg
muscles of this rock-climber are
able to slide past each other to
contract the muscles. A similar
process causes contraction in
the heart muscle and in the
muscles of the digestive tract,
blood vessels, and body glands
Protein functions
Fluid balance: Blood proteins
contribute to the number of dissolved
particles in the blood. Since proteins are
charged particles, they help attract water to
keep it in the blood. If protein levels in the
blood fall too low, osmosis causes fluid to
leak out of the blood vessels and
accumulate in the tissues, causing swelling
known as edema, shown here. Proteins also
regulate fluid balance because some are
membrane transporters, which pump
dissolved substances from one side of a
membrane to the other.
Functions of amino acids
•
Amino acids from the diet are used to make proteins
and nitrogen-containing molecules
•
Extra amino acids cannot be stored as proteins
•
Extra amino acids are used for energy or stored as
fat
Using proteins for energy
•
When we do not consume enough calories, body
proteins are broken down and amino acids used to
provide energy
•
Deamination removes amino group (NH2)
•
•
Produces urea which is excreted in urine
Carbon, hydrogen, and oxygen from amino acids can
be converted to glucose and can be broken down to
produce ATP
Producing ATP from amino acids
Protein deficiency
•
Protein-energy malnutrition: loss of fat and muscle
and decreased immunity from long-term protein and
calorie deficiencies
•
Types:
•
•
Kwashiorkor: pure protein deficiency
Marasmus: overall protein-energy deficiency
Protein deficiency: Kwashiorkor
Kwashiorkor, seen in this child from Haiti, is characterized by
a swollen belly, which results from fluid accumulating in the
abdomen and fat accumulating in the liver. Growth is
impaired, but because energy intake is not necessarily low,
the child may not appear extremely thin. The lack of protein
also causes poor immune function and an increase in
infections, changes in hair colour, and impaired nutrient
absorption.
Protein deficiency: Marasmus
Marasmus, seen in this Somalian boy, is
characterized by depletion of fat stores and wasting of
muscle. It has devastating effects on infants because most
brain growth takes place in the first year of life;
malnutrition in the first year causes decreases in
intelligence and learning ability that persist throughout the
individual’s life.
Concept check
• Why does protein synthesis stop when the supply of an
amino acid is limited?
• How does the body know in what order to assemble
the amino acids when making a protein?
• When is protein used as an energy source?
High-protein diets
•
Excess dietary amino acids can be deaminated and
converted to fatty acids and stored as triglycerides in
adipose tissue
•
Increased urea output
•
•
•
•
Increased demands on the kidneys
Increased loss of water from the body
Possible increased loss of calcium
Increased risk of kidney stones
High-protein diets
Often:
• high in animal proteins with high saturated fat and
cholesterol and low in fiber
• low in grains, fruits, and vegetables
• high in calories and fat
• Increased risk of heart disease, cancer, obesity,
diverticulosis (outpouching of the colon wall) and
diabetes mellitus
Diverticulosis
CNRI/ Science Source
simononly
Concept check
•
What are consequences of low or high protein diets?
Food allergies
• Food allergy: an adverse immune response to a
specific food product
Protein allergies
•
•
•
When a protein is absorbed intact, the immune
system can recognize it as an antigen and start
cellular reactions
The second time immune system detects that same
allergen (allergy-causing antigen), there is an allergic
reaction
Major food allergens: milk, eggs, peanuts, tree nuts,
fish, shellfish, soy, and wheat
Food sensitivities
•
Food intolerance or food sensitivity: an adverse
reaction to food; usually does not involve production
of antibodies
•
Triggers various symptoms after consumption
•
Examples:
•
•
MSG symptom complex or Chinese restaurant syndrome
Gluten intolerance or Celiac disease
•
Ingestion of gluten in rye, wheat or barley triggers an
autoimmune reaction against villi in the small intestines
Food allergy labelling
Protein balance
• Protein intake recommendations are based on nitrogen
balance
• Nitrogen balance: the amount of nitrogen consumed
versus nitrogen excreted
Nitrogen balance
•
Nitrogen balance: nitrogen intake equals nitrogen
loss
•
•
Maintenance of body protein and weight
Negative nitrogen balance: more nitrogen lost than
consumed
•
•
From illness, injury, or decreased consumption
Positive nitrogen balance: more nitrogen consumed
than lost
•
During growth, pregnancy, or weight training
Nitrogen balance
Nitrogen balance
Negative nitrogen
balance
Positive nitrogen
balance
Recommended daily allowance
(RDA):
•
0.8 g/kilogram of body weight for adults
•
•
•
•
With more weight, more protein is needed for maintenance
and repair
70 kg (154 lb) adult = 56 g of protein/day
Typical Canadian adult consumes
less than 80 g of protein/day
Higher needs in infants, children, during pregnancy
and lactation (breast feeding), after injury, and in
athletes
•
Pregnant/lactating women: add 25 g of protein/day
Recommended protein intake
Protein intake in athletes
Endurance athletes, like triathlete Simon
Whitfield, need more protein than others
because some protein is used for energy
and to maintain blood glucose during
endurance events, such as triathlons.
Endurance athletes may benefit from the
daily consumption of 1.2 to 1.4 g of
protein/kilogram of body weight.
Andy Lyons/Staff/Getty Images Sport
Strength athletes, such as
weight lifter Marie-Eve
Beauchemin-Nadeau, need
extra protein to provide the raw
materials needed for muscle
growth; 1.2 to 1.7 g
kilogram/day is recommended.
Laurence Griffiths/Staff/Getty Images Sport
Protein supplements
Calculate
•
•
•
100 pound adult
weight in kilograms equals weight in pounds x0.45
kg/lb
Protein RDA = 0.8 g/kilogram of body weight
How much protein does this person need?
Choosing correct protein intake
During the first year of life, growth is rapid, so a large amount of
protein is required per unit of body weight. As growth rate
slows, requirements per unit of body weight decrease but
continue to be greater than adult requirements until age 19.
What is the RDA for a 2-year-old child who weighs 14
kilograms?
a)14 g/day
b)15.4 g/day
c)11.2 g/day
d)21 g/day
Recommended protein intake
•
Acceptable Macronutrient Distribution Range for
protein:10% to 35% of calories
•
Protein quality: measure content of essential amino
acids compared to body needs
•
Protein digestibility corrected amino acid score
(PDCASS): most commonly used for determining
protein quality
Protein quality
High-quality, or complete dietary proteins
•
•
•
•
•
•
Contain all amino acids to meet body’s needs
Higher PDCASS
More easily digested
Animal proteins and soy
Incomplete dietary proteins
•
•
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Lower in one or more essential amino acids
Most plant proteins
Combine complementary proteins
Complementary proteins
• Protein complementation: combining proteins from
different sources to assure presence of all essential
amino acids
• Example: legumes with grains
Complementary proteins
The amino acids that are most often limited in plant proteins
are lysine (lys), methionine (met), and cysteine (cys). As
a general rule, legumes are deficient in methionine and
cysteine but high in lysine. Grains, nuts, and seeds are
deficient in lysine but high in methionine and cysteine.
Complementary proteins
Melanie Acevedo/FoodPix, Getty Images, Inc.
Many traditional diets take advantage
of complementary plant proteins, such
as lentils and rice or chickpeas and
rice in India, rice and beans in Mexico
and South America, hummus
(chickpeas and sesame seeds) in
them Middle East, and bread and
peanut butter (peanuts are a legume)
in Canada. Complementary proteins
do not need to be consumed in the
same meal, so eating an assortment
of plant foods throughout the day can
provide enough of all the essential
amino acids.
Canada Food Guide
Recommendations
• Choose variety of foods to meet body needs of the
essential amino acids
Choosing healthy protein sources
• Get protein without too much saturated fat
• Eat both animal and plant proteins
• Go with beans
What are some menu items
you could consume?
Calculate
Which foods provide the
cheapest source of protein?
Types of vegetarian diets
• Vegetarian diets: plant-based diets that eliminate some
or all animal foods
Vegetarian diets
• Reasons to be vegetarian:
•
•
•
•
•
Limited land and/or resources
Health
Religion
Personal ethics
Environmental concerns
• Benefits: lower cost and healthier
• Risks: amino acid, mineral, and B vitamin deficiency
Meeting dietary needs with a vegan
diet
Debate
Should you switch to soy?
Good sources of soy
Concept check
• How can nitrogen balance be maintained?
• What are benefits and risks of consuming a plantbased diet?
• How can vegetarians avoid nutrient deficiencies?
Think critically
Sickle cell anemia is an inherited disease caused by an
abnormality in the gene for the protein hemoglobin. It
causes red blood cells to take on a sickle shape.
• Sickle cell hemoglobin differs from normal hemoglobin by one
amino acid. Why might this difference change the shape of the
hemoglobin?
• Do you think sickle-shaped red blood cells can travel easily
through narrow capillaries?
• How might this disorder affect the ability to get oxygen to the
body’s cells?
What are similarities and differences
between:
•
•
•
•
Proteins and amino acids?
Plant and animal proteins?
Kwashiorkor and marasmus?
Vegetarian and vegan diets?
Applications
What advice could you give to a loved one about protein
consumption to decrease disease risk?
Checking student learning outcomes
• How do proteins contribute to health and disease?
• What advice would you give to a loved one about
protein consumption?
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