Truncation of the amino acid score

In the 1991 FAO/WHO report on protein quality assessment⁽¹⁾, PDCAAS values exceeding 100 % were truncated to 100 %. It has been put forward⁽Reference Schaafsma²⁾ that truncation does not give credit for the extra dietary indispensable amino acids in high-quality supplementary proteins that are used to balance the amino acid composition of a mixed diet, which is a nutritionally relevant intrinsic value of high-quality proteins. This was recognized in the 2007 FAO/WHO/UNU report⁽³⁾. In that report it was clarified that Biological Value, e.g. utilization of absorbed dietary essential and non essential amino acids, cannot exceed the value 1 and that therefore amino acid scores and not the final PDCAAS-values of dietary proteins (as proposed in the 1991 report) should be truncated to 1. This means that in cases where amino acid scores exceed the value 1, PDCAAS-values are equal to True Digestibility.

The limitation of PDCAAS-values, resulting from truncated amino acid scores, and thus not giving credit for the extra indispensable amino acids, is admitted in the 2007 WHO/FAO/UNU report⁽³⁾. This report suggests that this limitation can be overcome by the development of an additional protein quality index, in which the contents of the essential amino acids that are most often limited in single proteins (e.g. lysine (cereals), sulphur amino acids (legume proteins), threonine (some cereals) and tryptophan (maize)), are represented. Such an index (which could be called supplementation power: SP) for a particular protein can be obtained easily by multiplying the score of each of these amino acids with true digestibility values. An example is given in Table 1.

Table 1 Supplementation power (SP) of different proteins to balance diets deficient in lysine, sulphur amino acids (SAA), threonine or tryptophan.

SP values (non-truncated amino acid score x true digestibility), obtained from data on amino acid content, true digestibility values and 1–2 year old children amino acid scoring pattern, as reported by WHO/FAO/UNU⁽³⁾.

From the data shown in Table 1, it can be computed that 1·28 g of milk protein is needed to balance 1 g of the lysine deficient wheat protein. The formula for this is: amount of milk protein (g) = (1-LSPwheat)/(LSPmilk-1), where LSP wheat = lysine supplementation power of wheat and LSP milk = lysine supplementation power of milk. To balance this wheat protein with chick pea, one would need 8·5 g of this protein.

Validity of the scoring patterns

Amino acid scoring patterns (mg/g protein) are obtained by dividing dietary essential amino acid requirements (mg/kg body weight) by the minimum amount of protein needed for adequate growth and maintenance (g/kg body weight). They depend on age, physiological status and health status. The 1989 FAO/WHO Expert Consultation⁽¹⁾ on protein quality evaluation realized that age specific amino acid requirement patterns, that existed at that time, were based on limited evidence and therefore it was proposed for PDCAAS purposes to use, as an interim procedure, the 1985 WHO/FAO/UNU⁽⁴⁾ amino acid requirement pattern of the preschool child. This pattern however did not represent the pattern of normal healthy children, since it was obtained from a limited number of children that were recovering from malnutrition. It was therefore assumed (but not validated) that this requirement pattern reflected upper requirement levels of normal healthy children. Instead of dividing the pattern by the minimum protein requirement, it was divided by the safe level of (high-quality) protein intake. Since that time knowledge about age-specific amino acid requirements has increased. Therefore, in the 2007 WHO/FAO/UNU report⁽³⁾, revised amino acid scoring patterns were published for different age groups. These patterns were obtained by dividing essential amino acid requirements by minimum requirements of high-quality proteins, which is the correct procedure. Table 2 shows the patterns for three different age groups (0·5 years, 1–2 years and adults). For age groups between 1–2 years and adults, intermediate values were published. The slight decreases in content of essential amino acids with advancing age are attributable to differences in requirement values for maintenance and growth and the relative decrease of requirements for growth with aging.

Table 2 Age-specific amino acid scoring patterns (mg/g protein).

From WHO/FAO//UNU⁽³⁾.

Scoring patterns are thus based on minimum requirements of normal healthy subjects. They do not necessarily represent optimal protein nutrition. The PDCAAS approach argues that increased amino acid oxidation reflects inefficient use of amino acids, but this ignores roles of amino acids beyond requirements for maintenance and growth, such as any transient signalling influence before oxidation⁽Reference Millward, Layman and Tomé⁵⁾. An example of this is the signalling function of leucine in stimulating muscle protein synthesis⁽Reference Layman⁶⁾. Other examples are effects on satiation and satiety⁽Reference Tome⁷⁾ and the power of sulphur amino acids to increase the synthesis of glutathione and taurine⁽Reference Schaafsma⁸⁾. The scoring pattern is also unlikely to represent all situations where an additional need for specific amino acids may exist, such as in pregnancy, lactation, old age, the negative energy balance during calorie restriction, clinical conditions and in sports people. Conditionally indispensable amino acids are not included in PDCAAS, so it is uncertain that PDCAAS-values are applicable in situations where a special need for these amino acids may exist, like in pathological conditions or extreme physiological situations.

Ileal vs. Faecal Digestibility

True faecal N digestibility does not take into account essential amino acids that are not absorbed in the ileum and are lost into the colon. This is an important route for metabolic consumption of amino acids by the intestinal flora. Based on studies in humans, Gaudichon et al. ⁽Reference Gaudichon, Bos and Morens⁹⁾ suggested that ileal losses of amino acids are an important component of amino acid requirements. True faecal N digestibility values in the rat also potentially overestimate digestibility because of coprophagy.

A low ileal protein digestibility, which thus increases the flow of nitrogen into the colon, increases the bacterial formation of toxic compounds, such as ammonia, phenols and indoles. For these reasons the use of ileal digestibility in fistulated pigs, instead of faecal digestibility in rats, has been proposed as a better approach for the assessment of PDCAAS values⁽Reference Schaafsma^2, Reference Darragh and Hodgkinson¹⁰⁾. The advantage of using pigs is also that the gastro-intestinal physiology of this meal-eating species resembles more closely that of humans. It should be stressed, however, that the possibility of recycling of colonic and bacterial amino acids into the body has not been studied⁽Reference Fuller and Tomé¹¹⁾. Recently Deglaire et al. ⁽Reference Deglaire, Bos and Tomé¹²⁾, compared the ileal digestibility of dietary protein in the ileal fistulated growing pig and ileal sampled adult human. Their results supported the use of pigs as a model for predicting differences among dietary ileal protein digestibilities.

Amino acid availability

Current PDCAAS assessment does not take into account potential differences in bioavailabilty between amino acids from a food protein source, so processing or storage of foods at higher temperatures may selectively decrease the bioavailability of lysine by Maillard reactions⁽Reference Moughan¹³⁾. Routine amino acid analysis does not identify the biologically inactive D-amino acids that can be formed by racemization from L-amino acids through heat treatment under alkaline conditions. Oxidized sulphur amino acids are less bioavailable, and their presence in a food protein source may lead to overestimation of its PDCAAS. Another issue here is the supplementation of a food protein source with its limiting free amino acid to increase its PDCAAS. Such a supplementation may however not improve PDCAAS to the desired level. This has been demonstrated for leucine. Nolles⁽Reference Nolles¹⁴⁾ showed that, because of different absorption kinetics, supplemented (extrinsic) leucine is preferentially oxidized compared to its intrinsic form.

Anti-nutritional factors

Anti-nutritional factors, such as trypsin inhibitors, tannins and lectins, are often present in plant food protein sources. They may inhibit protein digestion and/or increase endogenous nitrogen losses into the faeces, causing a decrease in the apparent protein digestibility and an increase of protein requirements⁽Reference Van Leeuwen, Veldman and Boisen^15, Reference Huisman, Verstegen and van Leeuwen¹⁶⁾. Such effects are not included in the PDCAAS and this score thus overestimates the protein quality of products containing these naturally occurring anti-nutritional factors. The endogenous nitrogen flow into the ileum of pigs was 1·3, 3·1, 3·3, 4·0 and 10·8 g/100 g protein for skim milk, wheat, soya protein isolate, barley and phaseolus beans respectively⁽Reference Souffrant¹⁷⁾. Processing may decrease but not completely eliminate the activity of anti-nutritional factors⁽Reference Liu^18, Reference Yuan, Chang and Liu¹⁹⁾. In feedstuffs (less processing) the problem is more important than in foods.

Requirement for an animal experiment

Although PDCAAS is a rapid and relatively cheap routine method for the assessment of protein quality, it still needs an animal experiment for the measurement of true N digestibility. Considering the negative public opinion on the use of animals for research, it would be an advantage when protein quality measurement could be performed in-vitro. As shown before⁽Reference Schaafsma²⁾, true ileal digestibility of dietary essential amino acids can be measured in a computer-controlled dynamic digestion system. The advantage of such a system is that it can mimic physiological conditions of different animal species, so that it becomes possible to assess species-specific ileal amino acid digestibility coefficients.