Soy Protein, a High-Quality Protein

The Protein Digestibility Corrected Amino Acid Score (PDCAAS) is superior to other methods for evaluating the protein quality of food proteins for humans because it measures the quality of a protein based on the amino acid requirements of a 2- to 5-year old child (the most demanding age group), adjusted for digestibility.

It has replaced the Protein Evaluation Ratio (PER).

Adoption of PDCAAS allows evaluation of food protein quality based on the needs of humans. Isolated soy protein such as Solae™, with a PDCAAS of 1.0, is a complete protein and has the same score as milk protein and egg white.

PDCAAS is based on a food protein's amino acid content, its true digestibility, and its ability to supply indispensable amino acids in amounts adequate to meet the amino acid requirements of a 2- to 5-year old child, the age group used as the standard.

The highest PDCAAS value that any protein can achieve is 1.0. This score means that after digestion of the food protein, it provides per unit of protein, 100% or more of the indispensable amino acids required by the two- to five-year old child. A score above 1.0 is rounded down to 1.0. Any amino acids in excess of those required to build and repair tissue would not be used for protein synthesis, but would be catabolized and eliminated from the body or stored as fat.

The following steps are necessary to calculate the PDCAAS of a food protein.

1. The food protein must be analyzed for its proximate composition, its nitrogen content.
2. Protein content is calculated by multiplying the nitrogen content by 6.25.
3. The food protein is analyzed to determine its indispensable amino acid content.
4. The uncorrected amino acid score is calculated by dividing the milligrams of a particular indispensable amino acid in one gram of the test protein by the milligrams of the indispensable amino acids in one gram of the reference protein which is the amino acid requirement pattern for the 2- to 5-year old child.
5. The digestibility of food protein needs to be determined. The classical procedure for determining digestibility is a rat balance method which has been recommended as the most suitable method for humans by the FAO/WHO Expert Consultation.
6. The PDCAAS is calculated by multiplying the lowest uncorrected amino acid score by the food protein's digestibility.
   

For example, Isolated soy protein has 26 milligrams of histidine per gram of protein. The digestibility factor of 97% means that out of 26 milligrams of histidine reaching the intestinal tract, 25.2 milligrams are absorbed. A 2- to 5-year old child requires 19 milligrams of histidine per gram of protein, giving isolated soy protein a PDCAAS of 1.3 for this amino acid. This means that isolated soy protein provides 130% of the histidine required by the reference pattern, which reflects the requirements of a 2- to 5-year old child.

The PDCAAS for a food protein is equal to the lowest score for a single indispensable amino acid (or amino acid pair). In this case, methionine and cystine have a PDCAAS of 1.0, which becomes the score for the entire protein. Had all of the individual amino acid scores exceeded 1.0, the PDCAAS would still have been rounded down to 1.0, the highest PDCAAS possible.

The PDCAAS is currently used for labeling protein on food products for adults and for children over one year of age.

The FDA gave two reasons for adopting the PDCAAS over PER.

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. FAO/WHO is a recognized international organization, which is experienced in establishing these types of standards.

Isolated soy protein has a PDCAAS equal to the protein in milk and in egg white.

As an example, an adult who needs 50 grams of protein per day could satisfy all of his or her protein needs by consuming 50 grams of a certain brand of isolated soy protein such as Solae™. Whole wheat has a PDCAAS of 0.40. This means that it would take about 125 grams of protein from whole wheat to supply the amounts of all the indispensable amino acids provided by 50 grams of this isolated soy protein.