The procedure that was adopted by a recent FAO/WHO Expert
Consultation ( 19) on protein quality evaluation is based on the
concept of an amino acid score. This concept was first introduced
in I 946 by Block and Mitchell (20), who observed a linear re
lationship between the biological value of proteins and the con
tent of their limiting amino acid. The amino acid score is defined
as the concentration of the limiting amino acid in the food protein
and is expressed as a proportion or percentage of the concentra
tion of the same limiting amino acid in a standard or reference
amino acid pattern (15). Hence, a particularly critical and im
portant issue becomes the choice of the amino acid reference
pattern to be used for assessing nutritional quality or for calcu
lating an amino acid score for a food protein or mixture of pro
teins of interest. When Block and Mitchell (20) proposed this
scoring procedure, the amino acid composition of egg proteins
was used as a standard. It was later determined that the relatively
high amounts of indispensable amino acids in egg proteins un
dervalued many proteins for human nutrition, which led to the
use of estimates of human amino acid requirements as a basis for
subsequent scoring systems (1 1, 21, 22).
A common feature of most of the amino acid scoring systems proposed before 1985, when the FAO/WHOIUNU report on en
ergy and protein requirements was published, was that a single reference amino acid pattern was used for studies on all ages,
despite published amino acid requirement data that generally
showed that infants needed w35% of their total amino acids in
the form of indispensable amino acids whereas adults apparently
needed only
.