PDCAAS

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Protein Digestibility Corrected Amino Acid Score (PDCAAS) is a method of evaluating the protein quality based on the amino acid requirements of humans. The PDCAAS rating is a fairly recent evaluation method (it was adopted by the US Food and Drug Administration (FDA) and the Food and Agricultural Organization of the United Nations/World Health Organization (FAO/WHO in 1993) as "the preferred 'best'" method to determine protein quality. These organizations have suggested that other methods for evaluating the quality of protein are inferior.[1]

PDCAAS value of protein

A PDCAAS value of 1 is the highest, and 0 the lowest as the table demonstrates the ratings of commons foods below.

whey (1.0)
egg white (1.0)
casein (1.0)
milk (1.0)
beef (0.92)
soy (0.91)
kidney beans (0.68)
rye (0.68)
whole wheat (0.54)
lentils (0.52)
peanuts (0.52)
seitan (0.25)

Methodology

The PDCAAS value is different in measuring the quality of protein from the protein efficiency ratio (PER) and the biological value (BV) methods.[2] The PER was based upon the amino acid requirements of growing rats, which noticeably differ to that of humans. The PDCAAS allows evaluation of food protein quality based on the needs of humans as it measures the quality of a protein based on the amino acid requirements (adjusted for digestibility) of a 2- to 5-year old child (considered the most nutritionally-demanding age group). The BV method uses nitrogen absorption as a basis. However, it does not take into account certain factors influencing the digestion of the protein and is of limited use for application to human protein requirements because what is measured is maximal potential of quality and not a true estimate of quality at requirement level. Nevertheless, BV can be used to assess requirements of protein derived from foods with known quality differences and measure the proportion of absorbed nitrogen which is retained and presumably utilized for protein synthesis as an accurate indicator for protein measurement.[3][4]

Using the PCDAAS method, the protein quality rankings are determined by comparing the amino acid profile of the specific food protein against a standard amino acid profile with the highest possible score being a 1.0. This score means that after digestion of the protein, it provides per unit of protein, 100% or more of the indispensable amino acids required.

The FDA gave two reasons for adopting the PDCAAS in 1993. 1) PDCAAS is based on human amino acid requirements, which makes it more appropriate for humans than a method based on the amino acid needs of animals. 2) The Food and Agricultural Organization/World Health Organization (FAO/WHO) had previously recommended PDCAAS for regulatory purposes.[5]

Limitations

Amino acids that move beyond the terminal ileum in the body are less likely to be absorbed for use in protein synthesis. They may pass out of the body, or may be absorbed by bacteria, and thus will not be present in the faeces, and will appear to have been digested. The PDCAAS takes no account of where the proteins have been digested.

Similarly, amino acids that are lost due to antinutritional factors present in many foods (such as tannins in soy) are assumed to be digested by the PDCAAS.

The PDCAAS method may also still be considered incomplete, since human diets, except in times of famine, almost never contain only one kind of protein—however, calculating the PDCAAS of a diet solely based on the PDCAAS of the individual constituents is impossible. This is because one food may provide an abundance of an amino acid that the other is missing, which means that in this case the PDCAAS of the diet is higher than that of any one of the constituents. To arrive at the final result, all individual amino acids would have to be taken into account, though, so the PDCAAS of each constituent is largely useless.

For example, grain protein has a PDCAAS of about 0.4 to 0.5, limited by lysine. On the other hand, it contains more than enough methionine. White bean protein (and that of many other pulses) has a PDCAAS of 0.6 to 0.7, limited by methionine, and contains more than enough lysine. When both are eaten in roughly equal quantities in a diet, the PDCAAS of the combined constituent is 1.0, because each constituent's protein is complemented by the other.

A more extreme example would be the combination of gelatine (which contains virtually no tryptophan and thus has a PDCAAS of 0) with isolated tryptophan (which, lacking all other essential amino acids, also has a PDCAAS of 0). Despite individual scores of 0, the combination of both in adequate amounts has a positive PDCAAS, with the limiting amino acids isoleucine, threonine and methionine. Further, according to a recent 2000 study by scientist Gerjan Schaafsma has concluded, "The questions about the validity of the amino acid scoring pattern and the application of the true fecal rather than the true ideal 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."[6] Also, the scientific community has raised critical questions about the validity of PDCAAS.Template:Specify[7][8]

In addition the fact that 4 proteins, all with different amino acid profiles, receive identical scores of 1.0 limits its usefulness as a comparative tool. Since they have different compositions it is natural to assume that they perform differently in the human body and should have different scores. In short, this method however gives no distinction of their performance relative to each other because after they pass a certain point they are all capped at 1.0 and receive an identical rating.[2][9][10] This is because in 1990 at a FAO/WHO meeting it was decided that proteins having values higher than 1.0 would be rounded or "leveled down" to 1.0 as scores above 1.0 are considered to indicate that the protein contains essential amino acids in excess of the human requirements.[11] This approach implies injustice to high-quality proteins which can compensate for low-quality ones by virtue of their high content of essential amino acids (egg has an actual PDCAA score of 1.19 compared to 0.91 for soy, however when leveled down, they appear much closer).[12]

References

  1. Boutrif, E., Food Quality and Consumer Protection Group, Food Policy and Nutrition Division, FAO, Rome: "Recent Developments in Protein Quality Evaluation" Food, Nutrition and Agriculture, Issue 2/3, 1991
  2. 2.0 2.1 The Journal of Sports Science and Medicine (2004) 3, 118-130.
  3. The Great Animal Versus Vegetable Protein Debate What Is The Best Protein For Muscle Growth?
  4. Srikantia, S. G., University of Mysore, Mysore: "The Use Of Biological Value Of A Protein In Evaluating Its Quality For Human Requirements", Joint FAO/WHO/UNU Expert Consultation on Energy and Protein Requirements EPR 81 29, Rome, Aug 1981.
  5. Cite error: Invalid <ref> tag; no text was provided for refs named Journal
  6. Gertjan Schaafsma; Center of Expertise Nutrition, DMV International-Campina Melkunie, 6700 AA, Wageningen, the Netherlands The Protein Digestibility-Corrected Amino Acid Score -- Journal of Nutrition. 2000;130(7):1865S-1867S. The Journal of Nutrition
  7. Darragh A. J., Schaafsma G., and Moughan P. J. Impact of amino acid availability on the protein digestibility corrected amino acid score. Proceedings of the Nutrition Week of the International Dairy Federation, Wellington, New Zealand, March 9–11, 1998 1998
  8. Dutch Dairy Foundation on Nutrition and Health Proceedings of the International Workshop on Nutritional Aspects of Milk Proteins in Comparison with Other Proteins, organized by the Dutch Foundation on Nutrition and Health, Utrecht, the Netherlands, March 13–14, 1995
  9. Sarwar G. Health Canada, Bureau of Nutritional Sciences, Banting Research Centre, Ottawa, Ontario. 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. J Nutr. 1997 May;127(5):758-64. Pub Med Reference
  10. Schaafsma G. TNO Nutrition and Food Research, PO Box 360, 3700 AJ Zeist, The Netherlands. The Protein Digestibility-Corrected Amino Acid Score (PDCAAS)--a concept for describing protein quality in foods and food ingredients: A Critical Review. J AOAC Int. 2005 May-Jun;88(3):988-94. The Validity Of PDCAAS Under Critical Review
  11. FAO/WHO [1990]. Expert consultation on protein quality evaluation. Food and Agriculture Organization of the United Nations, Rome.
  12. FAO/WHO/UNU [1985]. Expert consultation. Energy and protein requirements. Technical Report Series 724. World Health Organization, Geneva.

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