Introduction to Human Nutrition

70 Introduction to Human Nutrition

protein and amino acid needs. A listing of some calcu-

lated PDCAAS values for selected food protein sources

is given in Table 4.13 and a worked example for a

mixture of food proteins is presented in Table 4.14.

The development of an internationally derived

procedure for evaluating protein quality using the

amino acid scoring concept is a step that had long

been required. This PDCAAS procedure can be modi-

fi ed as new knowledge about specifi c amino acid

requirements emerges, as the determination of avail-

ability of dietary amino acids is improved, and as the

factors affecting digestibility and availability are better

understood. For the present, the PDCAAS procedure

would appear to be very useful for evaluating the

nutritional quality of human food protein sources.

Major sources of food proteins in the diet

The relative proportions in the diet of food proteins of

animal and plant origin differ according to geographi-

cal region and other socioeconomic and cultural

factors. Broadly, animal protein foods account for 60–

70% of the total protein intake in the developed regions

(Table 4.15). In contrast, plant proteins make up about

60–80% of the total protein intake in developing

regions, with cereals being the dominant source in this

case. Given the differences in amino acid content of

Table 4.13 Protein digestibility-corrected amino acid score (PDCAAS)
of wheat, rice, maize, sorghum, and millet

Protein source PDCAAS

Wheat 40 (L)
Rice 56 (L)
Maize 43 (L)
Sorghum 33 (L)
Millet 53 (L)
Beef >100 (S)

From Young VR, Scrimshaw NS, Pellett PL. Signifi cance of dietary
protein source in human nutrition: 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: 207.
L, lysine fi rst limiting amino acid; S, sulfur amino acids (methionine
and cystine).

to assess the quality of food protein in human diets.
An important benefi t is that the PDCAAS approach
uses human amino acid requirements as the basis of
evaluation, which ensures that appropriate levels of
indispensable amino acids will be provided in the diet.
In addition, use of the proposed amino acid scoring
procedure facilitates an evaluation of blending of
foods to optimize nitrogen utilization and meet

Table 4.14 Worked example of a protein digestibility-corrected amino acid score (PDCAAS) for a mixture of wheat, chickpea, and milk powder

Analytical data (mg/g protein)

Digestibility factor

Quantities in mixture (mg)

Weight (g) Protein (g/100 g) Lys SAA Thr Trp Protein (g) Lys TSAA Thr Trp

AB CDEFG

A × B = P

100 P × CP × DP × EP × F

Wheat 350 13 25 35 30 11 0.85 45.5 1138 1593 1365 501
Chickpea 150 22 70 25 42 13 0.80 33.0 2310 825 1386 429
Milk powder 50 34 80 30 37 12 0.95 17.0 1360 510 629 204
Totals 95.5 4808 2928 3380 1134

Amino acids mg/g protein
(total for each amino acid/total protein)

50 31 35 12

Reference scoring pattern used 58 25 34 11
Amino acids scoring for mixture amino acid/g
protein divided by reference pattern

0.86 1.24 1.03 1.09

Weighted average protein digestibility sum of [protein × factor (P × G)]
divided by protein total

0.85

Score adjusted for digestibility (PDCAAS) (0.85 × 0.86) 0.73 (or 73%) with lysine
limiting

From Food and Agriculture Organization of the United Nations. Protein Quality Evaluation. FAO Food and Nutrition Paper 51. FAO, Rome, 1991:
table 10.
Lys, lysine; SAA, sulfur amino acids; Thr, threonine; Trp, tryptophan; TSAA, total sulfur amino acids.