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 in 1993) and emerged due to weaknesses in earlier evaluations of protein quality, such as the Protein Efficiency Ratio (PER) and the Biological Value (BV).

A PDCAAS value of 1 is the highest, and 0 the lowest. Some ratings of commons foods include soy (1.0), egg white (1.0), casein (1.0), milk (1.0), whey (1.0), beef (0.92), kidney beans (0.68), rye (0.68), whole wheat (0.54), lentils (0.52), peanuts (0.52), seitan (0.25).

Advantages

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The PDCAAS is superior to both the PER and the BV. The PER was based upon the amino acid requirements of growing rats, which are noticeably different to that of humans. The BV uses nitrogen absorption as a basis. However, it does not take into account certain factors influencing the digestion of the protein.

Limitations

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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.

External links

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• The Protein Digestibility–Corrected Amino Acid Score by Gertjan Schaafsma - nutrition.org

References

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