Partially Complete Protein Example
Proteins of most cereal grains are low in lysine. If the lysine
content of a whole-wheat flour is 2.6% and the value for lysine in the scoring pattern based on the amino acid needs of the young child is 5.8%, the amino acid score for lysine in wheat proteins is 2.6/5.8 x 100 = 45.
The scores for all other amino acids are higher, so lysine is the
limiting amino acid. The amino acid score for wheat proteins is 45.
The score for whole egg proteins is 100. To meet the requirement for lysine, a young child would have to
consume more than twice as much protein from whole wheat as from whole egg. Scores Change With Age
Both protein and amino acids requirements are higher, in
milligrams per kilogram of body wei
ght, at birth than as we mature.
Amino acid requirements, in milligrams per kilogram of body
weight, decline relatively more rapidly with increasing age than protein requirements.
A protein that may not meet the indispensable amino acid
requirements of a child when consumed in an amount that meets the total nitrogen requirement may pr
ovide amounts of amino acids in
excess of the requirements of an adult.
For example, during the first two years of life, lysine
requirements decline from 5.8% of protein to 1.6%, more than one-thirds, while the protein requirement declines by half.
The partially complete whole-wheat flour, with a 2.6% value for
lysine, will have a 100 amino acid score for adults.
The Food and Agriculture Organization (FAO)/World Health
Organization (WHO) amino acid scores are most commonly used. Other groups’ values may be as much as 20% lower or higher.
Amino acid
Requirement
Composition
1
Milligrams per gram crude protein
Infant
2
2-5 yrs
10-12 yrs
Adult
Egg
Milk, cow
Beef
Histidine
26
19
19
16
22
27
34
Isoleucine
46
28
28
13
54
47
48
Leucine
93
66
44
19
86
95
81
Lysine
66
58
44
16
70
78
89
Methionine & cysteine
42
25
22
17
57
33
40
Phenylalanine & tyrosine
72
63
22
19
93
102
80
Threonine
43
34
28
9
47
44
46
Tryptophan
17
11
9
5
17
14
12
Valine
55
35
25
13
66
64
50
Total
460
339
241
127
512
504
479
Table 9. Amino acid requirements and composition of high-quality animal proteins, from FAO/WHO/UNU (1985). 1 Composition from Lunven
et al.,
1972.
2 Based on breast milk.
Adults Less Concerned About Quality
As can be seen in
Table 9
, a minimum of 460 milligrams out of
every 1,000 milligrams must contain essential amino acids for infants in order for protein to be of the highest quality.
The proportion of essential protein to the total protein
requirement falls from 46% for infants to about 13% for adults.
This is the basis for the conclusion of the FAO/WHO/UNO
committee that only digestibility, not protein quality, need be considered in estimating protein needs of adults.
This does not apply to young children, whose indispensable
amino acid requirements are several times those of adults.
Nutrition for Sports, Essentials of 60