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DTN2PNU Principles of Human Nutrition
Lecture: Proteins
Lecture Prepared by: Dr Regina Belski & Associate Professor
Catherine Itsiopoulos
Department of Dietetics and Human Nutrition
latrobe.edu.au
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Today
• Structure and digestion of protein and amino acids
• Requirements for protein synthesis
• Sources of protein in food
̶
protein classification
• Protein deficiency
̶
Risk in vegetarian diets
• Protein excess/ toxicity
• Estimating protein requirements
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Required readings
• Whitney, E., Rolfes, SR, Crowe, T., Cameron-Smith, D.
& Walsh, A. (2011). Understanding Nutrition:
Australia and New Zealand Edition. South
Melbourne, Australia: Cengage Learning Australia.
• CHAPTER 6
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Proteins
•
Proteins are made from:
•
20 common different amino acids (9 are essential)
•
Each amino acid has an amino group, an acid group, a
hydrogen atom, and a side group.
•
It is the side group that makes each amino acid unique.
•
The sequence of amino acids in each protein determines
its unique shape and function, and hence role
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Amino Acid structure
Side group is unique to each AA
“R”
H
N
H
C
H
COOH
Carboxylic acid
group
Amine
group NH2
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Amino Acids
 Essential
̶
Essential amino acids must be supplied by the foods
people consume.
 Conditionally essential
̶
Conditionally essential amino acids refer to amino acids
that are normally nonessential but essential under certain
conditions.
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Essential and non essential amino acids
Essential
Non essential
 histidine











 isoleucine
 leucine
 lysine
 methionine
 phenylalanine
 threonine
 trytophan
alanine
arginine
asparagine
aspartic acid
cysteine
glutamic acid
glutamine
glycine
proline
serine
tyrosine
 valine
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Proteins
 Protein functions may include:
̶
Carriage and storage of materials.
̶
Providing strength
̶
Some require minerals for activation
 Protein denaturation is the uncoiling of protein that changes its
ability to function effectively
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Protein Metabolism
 Protein turnover and the amino acid pool
̶
Protein turnover refers to the continual making and breaking down of
protein.
̶
The Amino acid pool refers to the supply of amino acids that are
available.
 Amino acids from outside the body eg. food are called
exogenous
 Amino acids made within the body are called endogenous
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Protein synthesis & degradation
• Body constituents are in a dynamic state- proteins are constantly
being synthesised and degraded
• In healthy adults about 4g protein /kg BW are synthesised each day
• In newborn infants 12 g protein/kg BW/d decreasing to 6g/kg by 1
yo
• Carefully matched to size, shape, body composition
• Energy required for both protein synthesis and degradation 4kJ/g
• Biochemical cost of Peptide bond formation = 15-20% of REE
• FULL ENERGY COSTS OF MAINTAINING PROTEIN TURNOVER SYSTEM
= 33% REE
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Protein in Foods
•
Eating foods of high-quality protein is the best assurance
to get all the essential amino acids.
•
Complementary proteins can also supply all the essential
amino acids.
•
A diet inadequate in any of the essential amino acids
limits protein synthesis.
•
The quality of protein is measured by its amino acid
content, digestibility, and ability to support growth.
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Food sources of protein
Animal (high quality protein) - g protein / 100g food
 milk
3
 meat
15-30
 eggs
12
Plant (low quality protein)
 legumes
18
 breakfast cereal
10
 bread
8
 starchy vegetables
2
 rice
2
No significant amounts of protein
 fruit, fat
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Protein in Foods
 Protein Digestibility will depend on the food source
̶
o animal proteins are 90-99% absorbed
o plant proteins are 70-90% absorbed
o soy and legumes are 90% absorbed
It is important to note that other foods consumed at the
same time can change the digestibility.
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Classifying food protein
Complete protein (animal food sources)
 High quality
Incomplete (plant food sources)
 Low quality
Protein complementation
 The phenomenon of one food supplementing low levels of
amino acids in another
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Protein Complementation
Legumes
Ileu
X
Lys
X
Grains
Both
X
X
Met
Trp
X
X
X
X
The phenomenon (or practice) of one food supplementing low
levels of amino acids in another to ensure all essential amino
acids are consumed
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Classifying Dietary Proteins
• Protein Quality = Digestibility + AA Composition
• High Quality Protein = complete AA profile and 90-99%
digestibility (e.g. animal food sources)
• Low Quality Protein = incomplete AA profile and 70-90%
digestibility (e.g. plant food sources)
• Limiting AAs = amino acids that are either missing or low
concentration in low quality proteins (e.g. lysine, methionine,
tryptophan, threonine)
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Measurement of protein quality
Biological value (BV)
 % of nitrogen retained by the body
 >70% high BV
Net protein utilisation (NPU)
 the amount of protein actually available to the body
̶
considers digestibility
High Quality Proteins (~100% digestibility) BV = NPU
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Protein Denaturation
• Proteins exposed to heat, acid or other conditions undergo
denaturation
• Examples:
̶
Hardening of egg white when cooked
̶
Stiffening of egg whites when whipped
̶
Curdling of milk when exposed to acid (as in the stomach)
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Vegetarian diets
• Fruitarian- plant foods only, fruit, nuts, seeds, honey and vegetable oil
• Macrobiotic - plant food only, unprocessed, unrefined cereal and condiments
such as misu
• Vegan - excludes all animal foods, dairy foods and eggs, includes only plant
foods and plant products
• Lacto-vegetarian - as for vegan but includes milk and milk products
• Lacto-ovo-vegetarian - as for vegan, but includes milk, milk products and eggs
• ‘New vegetarian’- some animal products, unrefined, unprocessed, organic,
free-range
• Quasi-vegetarian
̶
Excludes red meat most of the time but included chicken and fish
• Pescetarian
̶
Excludes meat but includes fish
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Burke and Deakin (1994, 366)
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Possible nutritional concerns in vegetarian
diets
 Energy
 Protein
 Possibly Low fat
 Iron
 Calcium
 Vitamin B12
 Riboflavin
 Vitamin B6
 Vitamin D
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Protein and Health
• Protein deficiency and excesses can be harmful to
health.
• Protein deficiencies arise from protein-deficient
diets and energy-deficient diets.
̶
This is a worldwide malnutrition problem, especially for
young children.
• High-protein diets have been implicated in several
chronic diseases (mixed associations): heart disease,
cancer, osteoporosis, ?kidney disease
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Factors affecting individual protein
requirements
• Body size
• Age
• Gender
• Nutritional status
• Stress, infection
• Type and duration of exercise
• Pregnancy
• Lactation
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Protein- Recommended Intake
RDI protein
 0.75g/kg for adult women
 0.84g/kg for adult men
 Around 1g/kg for pregnant and breast-feeding women, and for
men and women over 70 years.
 So, for example, a 75kg adult male would need 63g of protein
per day.
Other, more specific for growing children
Nutrient Reference Values for Australia and New Zealand
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Safe Upper Level of intake
• No Upper level of intake was set as there are insufficient data. However, a
Upper level of intake of 25% protein as energy is recommended for which
the rationale is provided in the 'Chronic disease' section of the NRV
document.
• Rationale: Humans consume widely varying amounts of proteins. Although
some adverse effects have been reported with moderate to high levels of
supplementation, the risk of adverse effects from foods consumed as part
of everyday diets is very low. This consideration, together with the limited
data available, makes it impossible to set an upper limit in terms of grams
per day. However caution is needed. Intakes of individual amino acids that
may be consumed as supplements should not exceed those normally found
in the diet.
• Source: http://www.nrv.gov.au/nutrients/protein.htm
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Summary: Protein
• Made up of amino acids (AA)- digestion breaks down whole
proteins into AA, dipeptides or small oligopeptides
• Absorbed in small intestine- energy-dependent process with
specific transporters
• Intestine has great capacity for protein absorption as in
addition to dietary protein, daily there are additional within
body secretions of protein into the bowel - extra 70-300g/d
• Many good food sources but both quantity and quality (AA
content and bioavailability) are important
• Energy= 17kJ/g
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References
• Whitney, E., Rolfes, SR, Crowe, T., Cameron-Smith, D. & Walsh, A. (2011).
Understanding Nutrition: Australia and New Zealand Edition. South
Melbourne, Australia: Cengage Learning Australia.
• Mann JI. Optimizing the plant- based diet. Asia Pacific H Clin Nutr 2000; 9
(Suppl): S60-64.
• Solomons NW. Plant based diets are traditional in developing countries.
21st century challenge for better nutrition and health. Asia Pacific J Clin
Nutr 2000; 9(Suppl):S42-54.
• http://www.nrv.gov.au/nutrients/protein.htm (accessed
22/03/13)
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Image Sources
The images used in this presentation are from www.office.com,
or the presenters own, unless otherwise attributed.
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Thank you
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