1
Protein & Amino Acid Metabolism
Whole body metabolism
Nitrogen requirement
Recommended protein intake
Protein quality assessment
Essential amino acids
Amino acid catabolism
Disposal of amino nitrogen
Urea cycle
Interorgan nitrogen exchange
2
3
In adults, Nitrogen Equilibrium is a Dynamic Steady-State
T1/2 of proteins
minutes for Inducible Enzymes
months/years for Structural Proteins
Whole body turnover is ~ 3 weeks Daily protein synthesis is ~ 250 g
NOTE - this amount is 3-4 x our normal dietary intake
This implies RECYCLING being an important biological process
Concept of Nitrogen Balance
Input minus output
input is dietary
output is all sources of loss
Input minus output = ?
> 0 positive balance < 0 negative balance = 0 at equilibrium
Nitrogen balance is a macro-approach to investigate
body protein metabolism (protein turnover)
4
Balance Study
Estimate lowest protein intake to maintain N
equilibrium in healthy adults
Outcome of the studies
The amount of N needs to feed to subjects is ~30% higher
than that predicted by the Factorial Method!
Why such a discrepancy?
5
6
7
Methods for determining Amino Acid Requirement
(primarily focusing on the EAAs)
1. Balance study
2. Plasma amino acid level
3. Amino acid oxidation
Balance Study
Diet given contains all essential amino acids (EAA)
except one, then this missing amino acid is added back in small
increments until Nitrogen Balance is achieved
i.e. zero balance for healthy adults
N input = N output
8
Plasma Amino Acid Level
Concept is similar to that of the balance study, but
plasma amino acid instead of nitrogen balance is monitored.
However, this method works for some but not all essential amino acids
Point of inflection*
* is a point on a curve at which the curvature changes signs.
Indicator Amino Acid Oxidation
This approach is based on the principle that when requirement
is met, essential amino acids in excess will be oxidized for
energy and generating CO2
9
British J Nutr 108:S22-S30, 2012
10
Essential (Indispensable) Amino Acids
Branched Chain Amino Acids (BCAA) ILE, LEU, VAL
Aromatic Amino Acids PHE, TRP
LYS
THR
MET (sulfur containing/ methy group)
HIS+
*Neutral amino acids
General Summary
There is a continual catabolism of EAA. This leads to a
continuing need for dietary protein.
Metabolic need < Dietary Requirement < RDA
54 71 91 mg N/day
Average true digestibility of dietary protein is ~90%
Basal N loss is a function of body weight.
When normalized (weight 0.75), this varies little with age.
Nitrogen / protein requirement (per kg body weight) decreases
with age (up to adulthood).
Proportion of EAA needed also decreases with age.
11
British J Nutr 108:S22-S30, 2012
Note:
1)Compare to 1985 FAO, requirement of some amino
acids are increased in 2007FAO/WHO/UNU.
2)Will requirement be increased further with the IAAO
data?
12
Protein requirement is determined based on the use of a diet
that contains high quality protein.
Translate into daily living, we need MORE protein because
the quality of protein in foods is not always of high quality.
Evaluating Protein Quality
AA score = Amino acid Score
PER = Protein Energy Ratio
NPU = Net Protein Utilization
BV = Biological Value
PER of casein = 2.5
13
Absorbed N
14
Understanding Nutrition, 2008