It is well accepted that the nutritional value of proteins may differ substantially depending on their (essential) amino acid composition and digestibility. For many years, bioassays, mainly with rats, were the methods of choice to assess the nutritional value of proteins. This value was expressed in parameters such as protein efficiency ratio, net protein utilization and biological value. In 1989, a joint FAO/WHO Expert Consultation on Protein Quality Evaluation (FAO/WHO 1990) concluded that protein quality could be assessed adequately by expressing the content of the first limiting essential amino acid of the test protein as a percentage of the content of the same amino acid in a reference pattern of essential amino acids. This reference pattern was based on the essential amino acid requirements of the preschool-age child as published in 1985 (FAO/WHO/UNU 1985) (Table 1). Subsequently, this percentage is corrected for the true fecal digestibility of the test protein, as measured in a rat assay. This scoring method, known as the protein digestibility–corrected amino acid score (PDCAAS),2 was adopted as the preferred method for measurement of the protein value in human nutrition. Proteins with PDCAAS values exceeding 100% were not considered to contribute additional benefit in humans and were truncated to 100%. The PDCAAS formula is shown later.
TABLE 1
FAO/WHO/UNU amino acid requirement pattern based on amino acid requirements of preschool-age child1
| Amino acid | Requirement |
|---|---|
| mg/g crude protein | |
| Isoleucine | 28 |
| Leucine | 66 |
| Lysine | 58 |
| Total sulfur amino acids | 25 |
| Total aromatic amino acids | 63 |
| Threonine | 34 |
| Tryptophan | 11 |
| Valine | 35 |
| Total | 320 |
Table 2 shows values for protein efficiency ratio, true fecal digestibility, amino acid score and nontruncated PDCAAS for some selected proteins.
TABLE 2
True fecal digestibility, amino acid score and PDCAAS for selected proteins1
| Protein | PER | Digestibility | AAS | PDCAAS |
|---|---|---|---|---|
| % | ||||
| Egg | 3.8 | 98 | 121 | 118 |
| Cow’s milk | 3.1 | 95 | 127 | 121 |
| Beef | 2.9 | 98 | 94 | 92 |
| Soy | 2.1 | 95 | 96 | 91 |
| Wheat | 1.5 | 91 | 47 | 42 |
Now, after ∼10 y of experience with the PDCAAS method, it can be concluded that the method has been adopted widely. On the other hand, critical questions have been raised in the scientific community (Dutch Dairy Foundation on Nutrition and Health 1995, Darragh et al. 1998) about the following three PDCAAS issues: 1) the validity of the preschool-age child amino acid scoring pattern, 2) the validity of the true fecal digestibility correction and 3) the truncation of PDCAAS values to 100%.
These issues are discussed later; it is concluded that it is timely to evaluate the PDCAAS method in its current form.
Validity of preschool-age child amino acid scoring pattern
This reference scoring pattern (Table 1) is based on amino acid balance studies performed ∼20 y ago by Torun et al. (1981) and Pineda et al., 1981) in a limited number of 2-y-old children. These children were recovering from malnutrition and thus not representing normal healthy preschool-age children. The results of these studies, which so far have not yet been published in peer-reviewed international journals, were expressed in mg/kg of body weight/d and were assumed to include a safety margin of the same magnitude as that of the FAO/WHO safe level of high quality protein (meat, fish, egg, milk) intake for this particular group of children. The reference pattern was obtained by computing the ratios between the essential amino acid requirement values (mg/kg body weight/d) and this safe level of high quality protein intake (g/kg body weight/d), thus resulting in values of mg/g of protein for each essential amino acid.
Although there is no evidence to reject the assumption that both numerator and denominator of these ratios include similar margins of safety, this has not been validated. A difference in safety margins of nominator and denominator would result in an incorrect reference pattern with underestimated or overestimated values.
Another issue is that the current reference pattern is restricted to the indispensable amino acids and does not include amino acids that become indispensable under specific physiological or pathological conditions, such as cystine, tyrosine, taurine, glycine, arginine, glutamine and proline. This implies that these latter amino acids should also contribute to the nutritional value of a protein (van Hooydonk 1994).
These considerations plead for a critical contemplation of the current scoring pattern.
Validity of true fecal digestibility correction
As recognized by the FAO/WHO Expert Consultation on protein quality evaluation (1990), the intestinal flow of amino acids beyond the terminal ileum is an important route for bacterial metabolic consumption of amino acids. Amino acids that appear in the colon are most probably lost for body protein synthesis. Therefore, ileal rather than fecal digestibility is the critical biologically relevant parameter for amino acid or protein digestibility. The Expert Consultation recognized the shortcomings of the true fecal digestibility correction and recommended methodological studies to resolve uncertainties about the contribution and variation of endogenous amino acid losses at the terminal ileum before the determination of ileal digestibility could be recommended to replace fecal digestibility. Since then, several studies in this field were published (e.g.,Caine et al. 1997a, and 1997b, Huisman et al. 1993, Rowan et al. 1994, Van Leeuwen et al. 1996) indicating that antinutritional factors associated with dietary proteins may enhance substantially endogenous losses of amino acids and therefore decrease the nutritional value of the protein. Only true ileal digestibility of amino acids will take these losses into account (Darragh et al. 1998), and it is therefore timely to consider the use of ileal instead of fecal digestibility values.
Truncation of PDCAAS values to 100%
According to the current PDCAAS method, values that are higher than 100% are truncated to 100%, arguing that digestible essential amino acid concentrations in a protein in excess of those in the preschool-age child reference pattern do not provide additional nutritional benefit. This statement is correct when the protein in question is the sole source of protein in the human diet, as occurs in infant feeding practices and under special conditions, like enteral feeding. However, under all other conditions, humans consume mixed diets with proteins from a variety of sources. Under such conditions, the power of high quality proteins to balance the amino acid pattern of the mixed diet is extremely relevant. A classic and widely accepted example in this regard is the combination of milk and wheat, in which the relatively high lysine concentration of milk proteins compensates for the low concentration of this essential amino acid in wheat. So it can easily be computed that 1.2 g of casein can balance 1 g of wheat protein, whereas 6.2 g of soy protein would be needed to do so (Table 3). The truncation of PDCAAS values thus largely eliminates the differences in the power of high quality proteins to balance the amino acid composition of inferior proteins. This is highly relevant, not only for the low lysine content of cereals but also for the low content of S-containing amino acids and threonine of many plant protein sources. Thus, truncated PDCAAS values do not provide information about the potency of a protein to balance inferior proteins, and a solution for this problem should be found.
TABLE 3
Amount of protein needed to upgrade 1 g of wheat protein to obtain the preschool-age child’s lysine requirement level of 58 mg/g mixed crude protein
| Protein | Protein supplement needed |
|---|---|
| g | |
| Beef | 1.0 |
| Cow’s milk | 1.6 |
| Egg | 2.6 |
| Soy | 6.2 |
Amount of protein needed to upgrade 1 g of wheat protein to obtain the preschool-age child’s lysine requirement level of 58 mg/g mixed crude protein
The questions about the validity of the amino acid scoring pattern and the application of the true fecal rather than the true ileal digestibility correction as well as the truncation of PDCAAS values warrant a critical evaluation of PDCAAS in its current form as a measure of protein quality in human diets.
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