J Nutr. 1997 May;127(5):758-64.
The protein digestibility-corrected amino acid score method overestimates quality of proteins containing antinutritional factors and of poorly digestible proteins supplemented with limiting amino acids in rats.
Sarwar G.
Health Canada, Bureau of Nutritional Sciences, Banting Research Centre, Ottawa, Ontario.
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The validity of the protein digestibility-corrected amino acid score (PDCAAS) method in predicting the quality of fourteen protein products was compared with the commonly used protein quality methods, protein efficiency ratio (RER) and net protein ratio (NPR). A rat growth and balance study was conducted to determine protein digestibility and quality of the animal and vegetable protein products by the PER and NPR methods. Amino acid compositions of the products were also determined, and PDCAAS were calculated using a rat and a human pattern of amino acid requirements. Compared to the biological methods, the scoring method overestimated protein quality of mustard flour [PDCAAS of 84-92% vs. relative PER (RPER) or relative NPR (RNPR) of 0], raw black beans (PDCAAS of 45-72% vs. RPER or RNPR of 0), alkaline-treated lactalbumin and soybean protein isolate (PDCAAS of 44-67% vs. RPER or RNPR of 0) and heated skim milk (PDCAAS of 29-31% vs. RPER and RNPR of 0-5%). The scoring method also overestimated the protein quality of zein (true protein digestibility of 63%) supplemented with Lys, Met, Thr and Trp (PDCAAS of 63-71% vs. RPER and RNPR of 3-44%). These data demonstrate that the PDCAAS method is inappropriate for predicting protein quality of those protein sources which may contain naturally occurring growth-depressing factors or antinutritional factors formed during alkaline and/or heat processing.

From the full text article:

The protein digestibility-corrected amino acid score (PDCAAS)2 method has been considered to be a simple and scientifically sound approach for routine assessment of dietary protein quality for humans (FAO/WHO 1991). FAO/WHO (1991) recommended use of the FAO/WHO/UNU (1985) amino acid requirement pattern for children of preschool age for the evaluation of dietary protein quality for all age groups except infants. The PDCAAS is now a federally approved alternative method to the protein efficiency ratio (PER) rat bioassay procedure still official in Canada and U.S.A. Major questions have, however, been raised about the validity of the PDCAAS relative to its inability to credit the extra nutritional values of proteins having scores higher than that of the reference protein, its failure to fully account for the possible adverse effects of antinutritional factors, and its assumption about complete biological efficiency of supplemental amino acids in improving quality of proteins, which may not be true in the case of poorly digestible, low quality proteins (Food Chemical News 1991). These concerns about the PDCAAS, however, require proper documentation (Young 1995).

Antinutritional factors may occur naturally or may be formed during heat processing. Some examples of naturally occurring antinutritional factors include glucosinolates in mustard and rapeseed protein products (Fenwick et al. 1982), trypsin inhibitors and hemagglutinins in legumes (Rackis and Gumbmann 1981), phytates in cereals and oilseeds (Sandberg 1991), and gossypol in cottonseed protein preparations (Martinez and Hopkins 1975), which could adversely affect nutrient utilization and may contribute to growth depression in animals.

During processing of foods, protein sources are treated with heat, oxidizing agents (such as hydrogen peroxide), organic solvents, alkalis, and acids for a variety of reasons such as to sterilize/pasteurize, to improve flavor, texture, and other functional properties, to deactivate antinutritional factors, and to prepare concentrated protein products (Cheftel 1979, Friedman et al. 1984, Schwass and Finley 1984). These processing treatments may cause the formation of Maillard compounds, oxidized forms of sulfur amino acids, D-amino acids, and crosslinked peptide chains (such as lysinoalanine and lanthionine), resulting in lower amino acid bioavailability and protein quality.

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Lysinoalanine is an unnatural amino acid derivative formed (during alkaline treatment of proteins) mainly by the addition of the -amino group of lysine residue to the double bond of a dehydroalanine residue, that has been generated by the -elimination reaction of cystine, phosphoserine or glycoserine residues (Friedman et al. 1984, Mega 1984). The formation of lysinoalanine adversely affects the nutritional value of proteins because bioavailable lysine, cystine and phosphoserine are lost. Moreover, sufficiently large doses of lysinoalanine, especially free lysinoalanine released from the protein-bound form during the digestion process, can induce a reversible form of renal toxicity known as nephrocytomegaly in rats. It has been proposed that lysinoalanine, which chelates certain metal ions, may exert its toxic effect by metal-binding in renal tubule cells (Hayashi 1982). Since extracts from human kidney were less effective in metabolizing lysinoalanine than corresponding extracts from several animal species, human kidney cells may be more susceptible to lysinoalanine toxicity than those from rats or other animals tested (Kawamura and Hayashi 1987). The formation of lanthionine during alkaline and heat processing of foods also results in significant loss of bioavailable cystine (Robbins et al. 1980).

The Maillard reaction between proteins and reducing sugars is primarily responsible for the loss in nutritional value of food proteins during heat processing (Hurrell 1984), such as overheating of skim milk powder in this study. The overheating of skim milk and the alkaline treatment of lactalbumin and SPI may also have resulted in racemization of amino acid residues (Friedman et al. 1981). Protein-bound D-amino acids formed during processing may have adverse effects on biological value and safety of processed foods (Friedman et al. 1984). Since the routine amino acid methodology used in the determination of PDCAAS does not distinguish between D- and L-forms of amino acids, the scoring method does not take into account the presence and nutritional implications of D-amino acids in processed foods. Special analyses for D-amino acid compositions of processed foods are needed to assess the importance of this problem to human nutrition.

The PDCAAS method also does not take into account bioavailability of individual amino acids, which may be up to 44% lower than the overall digestibility of protein in the same food product (Eggum et al. 1989, Sarwar and Peace 1986). Therefore, the PDCAAS method may give misleading results about the quality of proteins co-limiting in more than one essential amino acid (Sarwar and Peace 1994). In the present study, it was difficult to identify the true first-limiting amino acid in SPI, alkaline-treated SPI, raw SBM, heated SBM, raw black beans, heated black beans and AA-supplemented zein using the PDCAAS method based on human requirements (Table 5). Depending upon the bioavailability of individual amino acids, these protein sources could be first-limiting in methionine + cystine, lysine, threonine or tryptophan which would affect the true score. Moreover, biological experiments would be required to document beneficial effects of supplementation with limiting amino acids. Emmert and Baker (1995) illustrated the benefit of using amino acid analysis and bioavailability data in correctly predicting the protein quality of several processed soybean protein products. In chick assays, SBM was found to be limiting in methionine + cystine, while soybean protein concentrate and SPI were limiting in methionine + cystine as well as threonine (Emmert and Baker 1995). Between the two samples of SPI (functional and edible), the functional SPI was superior to the edible SPI in terms of protein quality.

The amino acid scoring method also overestimated the protein quality of AA-supplemented zein (PDCAAS of 71% vs. RPER and RNPR values of 3-44%) (Table 6). The PDCAAS assumes complete biological efficiency of supplemental amino acids in improving protein quality. This was, however, not true in the case of AA supplementation of zein, a protein of low digestibility and poor quality. A marked difference between the PDCAAS and RPER or RNPR of amino acid-supplemented zein would suggest incomplete biological efficiency of the supplemental amino acids. The poor biological response to amino acid supplemented zein may have been due to the poor bioavailability of essential amino acid(s) other than those supplemented.

Since scores > 100 are considered to be 100, the PDCAAS does not credit the extra nutritional value of a protein having a score higher than that of the reference protein such as egg, fish, milk and most meat protein products. There is a need to revise the calculation of PDCAAS to permit values > 100 for high quality proteins, especially if they are intended to be used as supplements to other low quality proteins. However, this revision may not be required in calculating the PDCAAS for combined proteins, where the supplemental effect of excess amino acids is included in the calculation.

In the amino acid scoring method, methionine and cyst(e)ine concentrations are combined. Due to lack of knowledge of the proportion of the total sulfur amino acid requirement which can be met by cystine, expression of protein values based on the sum total of methionine and cyst(e)ine has limitations (FAO-WHO 1991). Moreover, dietary methionine may be less effective in meeting the cellular requirements for cysteine as is a preformed dietary supply of cyst(e)ine (Baker and Han 1993, Young 1995). This may be especially applicable to protein sources with low cysteine:methionine ratios (0.2) such as casein, where some methionine would be needed for cysteine synthesis which is only about 80% efficient on a weight basis (Baker and Han 1993). Therefore, there may be a need for the inclusion of a desirable cysteine:methionine ratio in the scoring pattern. There is a large variation in cysteine:methionine ratios of dietary protein sources. For example, the cysteine:methionine ratios (molar basis) in beef, egg white, soy protein isolate, rapeseed protein concentrate, wheat flour and pea flour have been reported to be 0.5, 0.9, 1.2, 1.5, 1.6 and 1.8, respectively (Sarwar et al. 1983). Another protein source with one of the highest cysteine:methionine ratios (1.9) is mature human milk (Räihä 1985). The cysteine:methionine ratios (2.32 to 2.38) were especially high in preterm and term transitional human milks (Sarwar et al. 1996).

Since the introduction of the PDCAAS method by FAO/WHO (1991), several questions about the validity of the method have been raised (Food Chemical News 1991, Sarwar and Peace 1994). Some of these concerns were also documented by this investigation. Therefore, there is a need to address these issues, and to suggest proper revisions to the scoring method. Meanwhile, the PDCAAS remains the preferred method for routine prediction of protein quality of properly processed (containing minimal amounts of residual antinutritional factors) and highly digestible (where the overall digestibility of protein is a good approximation of bioavailability of individual amino acids) food products for human consumption.

Categories: 1997, Legumes, Soy, Dairy, Protein, Nutrition and diet, Trypsin inhibitors, Antinutrients