Abstract
Indispensable amino acid (IAA) composition and standardized ileal digestibility (SID) of five animal- and 12 plant-based proteins were used to calculate their respective Digestible Indispensable Amino Score (DIAAS) according to the three age categories defined by the Food and Agriculture Organization (FAO). Mean IAA content and mean SID obtained from each protein dataset were subsequently used to simulate optimal nutritional quality of protein mixtures. Datasets revealed considerable variation in DIAAS within the same protein source and among different protein sources. Among the selected protein sources, and based on the 0.5- to 3-year-old reference pattern, pork meat, casein, egg, and potato proteins are classified as excellent quality proteins with an average DIAAS above 100. Whey and soy proteins are classified as high-quality protein with an average DIAAS ≥75. Gelatin, rapeseed, lupin, canola, corn, hemp, fava bean, oat, pea, and rice proteins are classified in the no quality claim category (DIAAS <75). Potato, soy, and pea proteins can complement a broad range of plant proteins, leading to higher DIAAS when supplied in the form of protein mixtures and at specific ratios. Such complementarity highlights the potential to achieve an optimal nutritional efficiency with plant proteins alone.
Keywords: DIAAS; digestibility; essential amino acids; sustainability; vegetable protein.
INTRODUCTION
New protein sources have emerged in recent years to support the transition toward more sustainable food production dedicated to human nutrition. Animal-sourced proteins, for example, from milk and meat, have shown to have a substantial impact on greenhouse gases and are known to contribute to depletion of natural sources (Aiking, 2014; Godfray et al., 2018). These proteins are included to a large extent in Western diets, and their consumption is gradually increasing in developing countries (Godfray et al., 2018). While it has been recommended to shift global protein consumption toward plant-based proteins (Boland et al., 2013; Pyett, Vet, Trindade, Zanten, & Fresco, 2019), attention should be paid to the nutritional quality of new and alternative protein sources.
Dietary protein quality is primarily characterized by their indispensable amino acid (IAA) content. IAAs cannot be synthetized by the human body and must be obtained from the diet. The current standard to evaluate the nutritional quality of proteins is the Protein Digestibility Corrected Amino Acid Score (PDCAAS). Due to several drawbacks of this method (Sarwar, 1997), the Food and Agriculture Organization (FAO) recommends another procedure called Digestible Indispensable Amino Acid Score (DIAAS) (FAO, 2013). DIAAS addresses the limitations of the PDCAAS method by considering the ileal digestibility of individual amino acids (AAs), with the growing pig as preferred model over the rat, and by avoiding truncation of the score obtained. Furthermore, reactive digestible lysine rather than total digestible lysine should be considered to determine the DIAAS of processed and cooked foods. The FAO DIAAS report also recommends classification of proteins using quality categories based on the DIAAS value: <75
(no quality claim); 75–99 (high-quality protein); and ≥100 (excellent quality protein). The FAO has updated the age-related AA reference scoring patterns and recommends that the general population be categorized according to three distinct age-related reference patterns: 0–6 months (infant), 0.5–3 years (children), and > 3 years (rest of the population).
Recent studies compared various protein sources based on their AA composition (Gorissen et al., 2018; Sá, Moreno, & Carciofi, 2020; van Vliet, Burd, & van Loon, 2015). However, these data do not inform to which extent a specific protein can be digested to meet the human body requirements. The most extensive studies to date provide
DIAAS values of eight protein sources (Cervantes-Pahm, Liu, & Stein, 2014; Mathai, Liu, & Stein, 2017). To broaden the comparison of DIAAS to more protein sources, IAA profiles and IAA standardized ileal digestibility (SID) data were used to calculate the DIAAS values of 17 protein sources according to the three reference scoring patterns.
Standardized ileal digestibility is preferred over apparent ileal digestibility (AID) since the latter does not correct for the AA endogenous loss inherent to the body function (Stein, Sève, Fuller, Moughan, & de Lange, 2007). Endogenous AAs are secreted into the small intestine, and an estimated 20%–35% is not reabsorbed by the body before reaching the distal ileum (de Lange, Sauer, Mosenthin, & Souffrant, 1989; Souffrant et al., 1993). Applying AID as digestibility coefficient therefore leads to underestimation of the actual AA digested. We also combine protein sources to optimize their nutrition profile, for which protein digestibility coefficients must be additive. SID therefore provides a more accurate and appropriate measurement of the AA ileal digestibility of single protein sources or protein mixtures (Stein, Pedersen, Wirt, & Bohlke, 2005). To improve comparison between results, studies involving rodents, gilts, sows, piglets, weanling pigs, and finishing pigs were excluded and instead only data obtained with growing pigs were used. In addition, only protein material containing at least 10% crude protein, and falling in categories such as seeds, meals, flours, concentrates, and isolates, was included in the investigation. Material in the forms of hulls, brans, or peels was excluded, being less suitable sources of protein for human consumption. The obtained datasets were subsequently used to simulate protein mixtures and highlight the complementarity of proteins at specific ratios to obtain higher values than that of the individual proteins.
CONCLUSIONS
In conclusion, this investigation shows that protein quality based on DIAAS differs not only between animal and plant proteins but also between plant protein sources. Based on the 0.5- to 3-year-old scoring pattern, potato and most animal-derived proteins tend to reach the excellent protein quality category (DIAAS ≥100). Soy and whey
proteins fall into the high-quality protein range (DIAAS = 75–99). Gelatin, corn, wheat, hemp, fava bean, oat, pea, canola, rapeseed, lupin, and rice proteins lie in the no quality claim category (DIAAS <75). Such scores differ greatly for the infant population, for which only casein is of good quality in the current selection. The differences observed in the protein quality lead to opportunities to enhance their nutritional efficiency in the form of protein mixtures. Any protein material can reach a higher DIAAS value when combined with an adequate complementary source. In the current selection, potato, soy, and pea proteins seem the most promising plant-based complementary sources to reach high or excellent quality mixtures to support a plant-based lifestyle. Clear dissimilarities were observed within each protein dataset suggesting that: (a) harmonized methods to evaluate protein ileal digestibility; (b) proper characterization of (cooked) protein material; and (b) digestible reactive lysine data are needed to obtain a coherent DIAAS database.
ACKNOWLEDGMENTS
The authors thank Robin Spelbrink for his support and constructive comments in completion of this manuscript. Our acknowledgments are also addressed to Michael Polhuis for proofreading this manuscript.