Dietary Protein intake

Question 1

Amino Acid structure: composition?

Answer 1

• AA contain Carbon, Hydrogen and Oxygen (as do CHO and Fat), but AA also contain Nitrogen.

Question 2

Amino Acid structure: Structure?

Answer 2

• Carboxyl group
• Amino group
• Variable group (side chain)
• Each AA’s has a different side chain

Question 3

protein structure/classification: Dipeptide and polypeptide?

Answer 3

Dipeptide:
• Composed of 2 AA that are joined by a chemical bond, known as a peptide bond

Polypeptide:
• Longer chains of AA are known as polypeptides (>50 AA)

Question 4

protein structure/classification: proteins?

Answer 4

Proteins:
• Proteins consist of many polypeptide
chains

• Primary Structure – Determined by the
AA sequence

• Secondary Structure – Determined by
the interactions between groups (short)

• Tertiary Structure – Determined by
interactions between side chains (3D)

Question 5

Amino Acids:

Answer 5

• There are 20 AA within the body and these can be categorised as essential and non-essential.

• Essential AA cannot be synthesised by the body and therefore must be
consumed in the diet.

• Non-essential AA can be synthesised from other AA via transamination

Question 6

Amino Acids: Non-essential?

Answer 6

Alanine

Arginine

Asparagine

Aspartate

Cysteine

Glutamate

Glutamine

Glycine

Proline

Serine

Tyrosine

Question 7

Amino Acids: Essential?

Answer 7

Isoleucine (BCAA)

Leucine (BCAA)

Lysine

Methionine

Phenylalanine

Threonine

Tryptophan

Valine (BCAA)

Histidine

Question 8

Roles of protein?

Answer 8

STRUCTURE:
Collagen/Keratin
↓
Bone, skin, hair, nails, tendons, ligaments

TRANSPORT:
Haemoglobin/Cytochromes / Lipoproteins
↓
O2 & CO2 transport/ETC / Lipids

IMMUNE FUNCTION:
Antibodies
↓
Eliminates foreign pathogens

CONTRACTION:
Actin/Myosin
↓
Contractile proteins

METABOLIC REGULATION:
Enzymes & hormones
↓
Nutrient anabolism/catabolism

ACID-BASE BALANCE:
AA/proteins
↓
Buffering changes during  
exercise/disease

Question 9

Protein digestion: stomach?

Answer 9

• Gastric Juice – HCl (hydrochloric acid) (via parietal cells) denatures the proteins and pepsin (via
chief cells) degrades proteins to large polypeptides and AA

Question 10

Protein digestion: Small intestine?

Answer 10

• Pancreatic enzymes (incl trypsin and chymotrypsin) degrade polypeptides
to oligopeptides and AA

• Aminopeptidase degrade oligopeptides (6-8 AA) to peptides and AA

• AA and small peptides are
transported across the intestinal membrane.

• ~90% of dietary protein is absorbed from animal protein sources.
• Peptides are hydrolysed in the cytosol and thus only AA enter the circulation

Question 11

protein digestion model:

Answer 11

Dietary Protein (HCl)—> Denatured protein (Pepsin)—>Polypeptides ( Pancreatic
proteases)+ AA—> Oligopeptides (Aminopeptidase) + AA—> Tripeptides
& Dipeptides (PEPT1)
+ AA—> Tripeptides
Dipetides+AA—> AA=catabolism and anabolism of protein.

Question 12

Determining protein requirement: Nitrogen balance, intake and excretion?

Answer 12

NITROGEN BALANCE:
• Used to determine DRV
• Protein is ~16% nitrogen

Intake:
• Diet

Excretion
• Skin (<5%)
• Faeces (~5%)
• Urine (~90%)
• Urea (~85%)
• Ammonia
• Creatinine
• Some AA

Question 13

Determining protein requirement: ISSUES?

Answer 13

• This method is flawed due to:
1. Implausibly high nitrogen
retention on high protein
diets

2. Increased economy of
   nitrogen use on low protein
   intakes
3. Estimated (not measured)
   dermal/faecal losses
4. Duration of measurement

Question 14

Nitrogen balance: approach 1

Answer 14

average= 0.605g/kg

EAR: 0.60g/kg
RNI: 0.75g/kg

Question 15

Nitrogen balance: approach 2?

Answer 15

median requirement (for balance): 104.6 (101 to 110) mg N/kg/d

EAR: 0.66g/kg
RNI: 0.83g/kg

Question 16

Protein Quality: Complete Proteins, Incomplete Proteins and Vegetarians?

Answer 16

Complete Proteins:
• All EAA present

Incomplete Proteins:
• Plant proteins are typically deficient in one EAA (lysine, methionine)

Vegetarians:
• Digestibility is reduced (45-80%)
• Cell walls of plant proteins (inhibit digestion)
• Anti-nutritional factors (inhibit digestion)

Question 17

Protein Quality: Anti-nutritional factors?

Answer 17

• Trypsin Inhibitors
– A molecule that inhibits trypsin and chymotrypsin.
– Found in legumes (including soya beans), cereals, potatoes and tomatoes

• Tannins
– Water-soluble polyphenols that precipitate proteins
– Found in legumes, cereals, tea, wine.

• Phytic Acid
– Inhibits pepsin and trypsin
– Found in seeds, grains, nuts

Question 18

Effects of Excess protein?

Answer 18

Renal Disease:
• A high protein intake has been suggested to ↑ glomerular filtration rate & proteinuria

• But, elevated protein intakes are not detrimental in those without existing renal disease.

Bone Demineralisation:
• A high protein intake has been suggested to ↑ calcium excretion and consequently negatively impact bone health.

• But, elevated protein intakes are not detrimental to bone health and may even be beneficial due to elevations in IGF-1

Safe Upper Level (guidance):
• Whilst there is little evidence, a value greater than 1.5g/kg/d is not advised.

Question 19

Dietary protein intake: assumptions?

Answer 19

• Healthy adults consuming a mixed diet of high quality protein sources.

• Based upon nitrogen balance studies (eg
achieve balance (adults)), assuming protein
is 16% nitrogen

• For diets containing considerable amounts
of unrefined cereal grains and vegetables, a correction for digestibility of 85%
should be applied.

Question 20

Dietary protein intake: DRV and current UK intake?

Answer 20

DRV:
• EAR = 0.60g/kg/d
• RNI = 0.75g/kg/d
• Advice (UL) = 1.5g/kg/d

Current UK Intake:
• Men = 85.1g/d (16.4%)
• Women = 65.4g/d (16.6%)