Introduction to Human Nutrition

Nutrition and Metabolism of Proteins 69

This can affect the effi ciency with which the amino
acids derived from the protein ingested are used to
support overall body nitrogen and amino acid homeo-
stasis and balance.

Protein nutritional quality

Not all proteins have the same capacity to meet the
physiological requirements for total nitrogen and the
indispensable amino acids. The concentration and
availability of the individual indispensable amino
acids are major factors responsible for the differences
in the nutritive values of food proteins. Thus, the
content and balance of indispensable amino acids
differ among plant and animal protein foods. For the
present purpose a summary is given in Table 4.12,
listing the four indispensable amino acids that are
most likely to be limiting, or in shortest supply and
especially in food proteins of plant origin. As can be
seen, lysine is present in a much lower concentration
in all the major plant food groups than in animal
protein foods and is most frequently the most limit-
ing amino acid.

The nutritional signifi cance of these differences

can be assessed in a number of ways. One useful

approach is an amino acid scoring procedure that

compares the content of amino acids in a protein with

a reference human amino acid requirement pattern.

In 1991 a UN Expert Consultation reviewed the

appropriate methods for measuring quality of food

proteins for the nutrition of human populations. This

consultation concluded that the most appropriate

method available was the protein digestibility-

corrected amino acid score (PDCAAS) method, and

it was recommended for international use. This amino

acid scoring procedure, including a correction for

digestibility, uses the amino acid requirement pattern

for a 2–5 year old child (as shown in Table 4.9). This

is the reference amino acid requirement pattern for

this purpose, expressing the amino acid requirement

in relation to the total protein requirement.

The PDCAAS is estimated from the following

equation:

PDCAAS =

Concentration of most limiting,

digestibility-correccted amino

acid in a test protein

Concentration of that amiino acid

in the 1991 FAO WHO amino

acid scoring reference ppattern

(preschool child: see Table 4.9)

In addition to establishing the amino acid reference

pattern for use in the PDCAAS method, the UN

Consultation considered the procedures for measur-

ing and estimating amino acids and digestibility. This

approach offers considerable benefi ts over that of

animal bioassays, which traditionally have been used

Table 4.11 The uptake of dietary amino acids by the visceral tissues

Percentage of intake

Amino acid

Utilization by the liver

and gut (human)

Utilization by the

gut (piglet)

Leucine 26 37
Lysine 32 45
Phenylalanine 39 53
Threonine No data 65
Glutamine 53 50
Glutamate 88 95

Table 4.12 The amino acid content of different food protein sources

Food source

mg/g protein (mean ± SD)

Lysine Sulfur amino acids Threonine Tryptophan

Legumes 64 ± 10 25 ± 3 38 ± 3 12 ± 4

Cereals 31 ± 10 37 ± 4 32 ± 4 12 ± 2

Nuts, seeds 45 ± 14 46 ± 17 36 ± 3 17 ± 3

Fruits 45 ± 12 27 ± 6 29 ± 7 11 ± 2

Animals foods 85 ± 938 44 12

From Young VR, Scrimshaw NS, Pellett PL. Signifi cance of dietary protein source in human nutri-

tion: Animal and/or plant proteins? In: Waterlow JC, Armstrong DG, Fowder L, Riley, eds. Feeding

a World Population of More Than Eight Billion People. Oxford University Press in association with

the Rank Prize Funds, Oxford, 1998: 206.