Protein Nutrition, Digestion, absorption Flashcards Preview

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Flashcards in Protein Nutrition, Digestion, absorption Deck (32)
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What determines the function of an amino acid

The variable 'R group'


What makes up an amino acid?

amino group, acid group, R group, H


Non-essential amino acids are?

we can make, either from essential aa or derive from other metabolic components


Essential aa are?

Required in our diet, we can't make ourselves


Roles of Proteins? (8)

-building material
-enzymes (digestive)
- immune function
-fluid balance
- Transporters
- AB
- Energy source


Conditionally Essential aa

When the thing that makes the aa has to be avoided, so nonessential aa become essential

Phenile pertenuria: have to avoid phenilalinine, can't make tyrosin


Bonds of proteins

peptide bonds (OH + H)


Polypeptides function due to?

Their sequencing of aa, determined by DNA/RNA

eg) haemoglobin (4pp bound)
if aa sequence altered, the protein can't function > Sickle cell anaemia


Main victims of Protein-energy malnutrition



what is the protein RDI? Total energy intake? Is this attainable?

M: 0.84g/kg/day
F: 0.75g/kg/day

12-25% of energy intake

This is very easy to achieve


Is taking protein supplements always necessary? What can this lead to

No. its very easy to achieve RDI, so you are getting excessive amounts of protein. Even when you are active, and your protein turnover is higher.
Potential links with CVD and renal stones?


High Quality Protein?

Provide enough of all the EAA
Meats do this
But vegans/vegetarians need to find other sources (supplements etc)


Difference in digestibility of proteins?

animal protein: 99%
plant protein: 70-90%
soy & legumes: 90%


Amino Acid composition means

Dietary protein must supply minimum
9EAA + N containing amino groups + energy

if there is not enough EAA synthesis limited


Recommended serving of protein?

1-2 per day of MIXED servings. By combing protein sources its easy to get EAA

Complementary proteins (provide SOME of the EAA)


Protein digestion in the mouth

mouth and salivary glands chew, crush and moisten protein-rich foods and mix them with saliva


Protein digestion in the stomach

Acid environment denatures proteins and actives pepsinogen > pepsin.
HCl: uncoats protein strands and activates stomach enzymes
Pepsin: cleaves proteins > smaller polypeptides and some free aa. Inhibits pepsinogen synthesis


Protein digestion in the SI

Degraded by a combo of pancreatic endopeptidases. (trypsin, chymotrypsin, elastase)
Polypeps >>>> smaller polypeptides and amino acids

Then pancreas enzymes (intestinal tri/di peptidases) of the surface of enterocytes hydrolyse these and they are absorbed

Peptides >>>> amino acids


What initiates protein digestion in the SI to occur. What happens next?

Trypsinogen >> Trypsin
by membrane -bound enteropeptidases
Trypsin (when high enough) has a negative feedback on trypsinogeen synthesis. It activates inactive enzymes such as chymotrypsinogen to their active form.

These active enzymes can now cleave off aa chains to make them absorbable.


Absorption of Proteins works by?

peptides/ aa absorbed through gut enterocytes, facilitated by either Na+ or H+ co-transporter.

A antiport in the BL membrane swaps Na+ for K+ and creates the necessary conc gradient

Larger peptides can be transcytosed and engulfed into membrane

venule > villi > portal vein > liver


Does digestion occur in the enterocytes?

Yes of di/tripeptides


Downside of using protein as energy?

Ammonia and urea.
These are toxic and need to be excreted


Nitrogen balance?

-Usually in nitrogen balance (protein/nitrogen intake = protein/nitrogen excretion)
-Nitrogen is taken into the body largely as aa.
-Majority excreted via urine as N products
-This is relatively stable over a wide range 50-300g


g Protein =

gN x 6.25


Negative Nitrogen balance? Factors that cause this?

Net loss of Nitrogen.
1) Decreased protein intake
2) starvation or decreased GI function
3) injury, trauma, burns (not enough protein> tissue damage leads to death)
4) illness or infection
5) some post-op conditions
6) many cancers
7) lactation

degradation > synthesis


Positive Nitrogen balance. Factors that produce this.

Net gain of Nitrogen (synthesis> degredation)
1) increased protein intake
2) growth
3) pregnancy
4) recovery from illness/ trauma

Only for a short period, body quickly excretes


PEM stands for

Protein-Energy Malnutrition
occurs in developing countries, especially children


Why is malnutrition so dangerous in children

Leaves them vulnerable to infection >> low food makes infection worse >> hard to get them too eat when infection develops death


Two main forms of PEM

1) Marasmus
2) Kwashiorkor


Marasmus (like anorexia nervosa)

-Severe deprivation or impaired absorption of protein, energy, vitamins, minerals.
-Slowly develops

- Leads to severe weight loss, muscle-wastage, no body-fat, growth