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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

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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)

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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?

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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

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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

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British J Nutr 108:S22-S30, 2012

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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.

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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?

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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

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Absorbed N

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Understanding Nutrition, 2008