Protein Quality in Perspective: A Review of Protein Quality Metrics and Their Applications

Abstract

For design of healthy and sustainable diets and food systems, it is important to consider not only the quantity but also the quality of nutrients. This is particularly important for proteins, given the large variability in amino acid composition and digestibility between dietary proteins. This article reviews measurements and metrics in relation to protein quality, but also their application. Protein quality methods based on concentrations and digestibility of individual amino acids are preferred, because they do not only allow ranking of proteins, but also assessment of complementarity of protein sources, although this should be considered only at a meal level and not a diet level. Measurements based on ileal digestibility are preferred over those on faecal digestibility to overcome the risk of overestimation of protein quality. Integration of protein quality on a dietary level should also be done based on measurements on an individual amino acid basis. Effects of processing, which is applied to all foods, should be considered as it can also affect protein quality through effects on digestibility and amino acid modification. Overall, protein quality data are crucial for integration into healthy and sustainable diets, but care is needed in data selection, interpretation and integration.

Keywords: complementarity; digestibility; food processing; indispensable amino acids; protein quality.

Figure 1

Schematic overview of the key steps of protein digestion and absorption in humans.

Figure 2

Elements required to quantitively define protein quality.

Figure 3

Overview of site of measurement for different in vivo protein quality measurement methods.

Figure 4

Ranking preference of digesta used for determination of protein quality. Redrawn from [63].

Figure 5

Relation between faecal digestibility and ileal digestibility protein from milk and milk protein concentrates and isolates (●, ■), roasted nuts (●, ■), cooked cereals (●), raw cereals (●, ■), legume and cereal protein isolates and concentrates (●, ■) and cooked legumes (●) tested in pigs (squares) or rats (circles). Data from [57,72,73,74]; Black dotted line: trend line based on linear regression; grey dashed line: line of unity.

Figure 6

Relation between ileal digestibility of total amino acids (AA) and of the first limiting indispensable amino acid (IAA) histidine (●), sulphur-containing amino acids (●), lysine (●), valine (●) or leucine (●) of different food sources. Data from studies in pigs [57,76,77,78,79,80], and rats [72,81].

Figure 7

Concentration (expressed as a percentage of the recommendation for adults outlined in Table 2) and standardized ileal digestibility (SID) for histidine (●), isoleucine (●), leucine (●), lysine (●), sulphur containing amino acids (●), aromatic amino acids (●), threonine (●), tryptophan (●) and valine (●) in pork belly; - - -: adequacy marker for amino acid requirement; : cut-off for actual requirement as a combination of amino acid content and digestibility of the respective indispensable amino acid; (A; data from [76]), skim milk powder (B; data from [57]), soy flour (C; data from [57]), rice (D; data from [80]), cooked peas (E; data from [87]) and maize (F; data from [80]).

Figure 8

DIAAS value (A1,B1,C1,D1,E1,F1) and first limiting indispensable amino acid (A2,B2,C2, D2,E2,F2) for mixtures of protein from maize (A1,A2; data from [80]), rice (B1,B2; data from [80]), cooked peas (C1,C2; data from [87]), soy flour (D1,D2; data from [57]), pork belly (E1,E2; data from [76]) and skim milk powder (F1,F2; data from [57]) with any of the other 5 foods; Dashed line in (A1, B1, C1, D1, E1, F1) indicates a DIAAS value of 100%.