Dietary Protein intake Flashcards
Amino Acid structure: composition?
• AA contain Carbon, Hydrogen and Oxygen (as do CHO and Fat), but AA also contain Nitrogen.
Amino Acid structure: Structure?
- Carboxyl group
- Amino group
- Variable group (side chain)
- Each AA’s has a different side chain
protein structure/classification: Dipeptide and polypeptide?
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)
protein structure/classification: proteins?
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)
Amino Acids:
• 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
Amino Acids: Non-essential?
Alanine
Arginine
Asparagine
Aspartate
Cysteine
Glutamate
Glutamine
Glycine
Proline
Serine
Tyrosine
Amino Acids: Essential?
Isoleucine (BCAA)
Leucine (BCAA)
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine (BCAA)
Histidine
Roles of protein?
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
Protein digestion: stomach?
• Gastric Juice – HCl (hydrochloric acid) (via parietal cells) denatures the proteins and pepsin (via
chief cells) degrades proteins to large polypeptides and AA
Protein digestion: Small intestine?
• 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
protein digestion model:
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.
Determining protein requirement: Nitrogen balance, intake and excretion?
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
Determining protein requirement: ISSUES?
• This method is flawed due to:
1. Implausibly high nitrogen
retention on high protein
diets
- Increased economy of
nitrogen use on low protein
intakes - Estimated (not measured)
dermal/faecal losses - Duration of measurement
Nitrogen balance: approach 1
average= 0.605g/kg
EAR: 0.60g/kg
RNI: 0.75g/kg
Nitrogen balance: approach 2?
median requirement (for balance): 104.6 (101 to 110) mg N/kg/d
EAR: 0.66g/kg
RNI: 0.83g/kg
Protein Quality: Complete Proteins, Incomplete Proteins and Vegetarians?
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)
Protein Quality: Anti-nutritional factors?
• 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
Effects of Excess protein?
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.
Dietary protein intake: assumptions?
• 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.
Dietary protein intake: DRV and current UK intake?
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%)
