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[Josette Gaan]

Protein Digestibility Corrected Amino Acid Score or PDCAAS, what is the score of soy protein?

Does soy protein provide the body with bioavailable amino acids?

[Mark Messina]

To answer the questions, I offer the following:

Soy protein is a complete protein but it is not correct to say that soy is the only plant protein to contain all of the essential amino acids.  In fact, the whole concept of complete and incomplete proteins is a bit misleading as is the notion that some proteins are missing essential amino acids.  This is because all proteins contain all of the essential amino acids.  Some proteins are however low in one or more essential amino acids relative to the biological requirement for these amino acids.  These proteins are often referred to as incomplete proteins whereas complete proteins provide all of the essential amino acids in sufficient amounts to meet biological requirements.  All animal proteins are complete whereas most plant proteins are incomplete.  Of course, vegans, who do not use animal products, can meet protein requirements by eating grains and legumes.  This is because the amino acids that are low in grains tend to be high in beans and visa versa. 

There are several different methods for evaluating protein quality.  One of the most common and the one used by most health agencies, is the protein digestibility corrected amino acid score (PDCAAS).  The PDCAAS is not a perfect rating system, however.  The highest PDCAAS is 1.0.  Egg protein has a rating of 1.0 and other animal proteins generally about 0.9.  Egg protein is far and away the highest quality protein.  Soy protein also receives ratings of about 0.9 although this can vary according to the specific soy product providing the protein.  This is because the digestibility (the other component that determines protein quality) of soy protein varies among different soyfoods.  Other types of assays for assessing protein quality suggest soy protein might be a little bit lower than indicated by the PDCAAS, and somewhat lower than some animal proteins, but still a very high quality protein. 

In contrast to soy and animal proteins, wheat protein has a very low PDCAAS, only about half that of soy protein whereas other bean proteins lay somewhere between wheat and soy protein.   Finally, generally speaking, once a protein is digested into its constituent amino acids the source of protein doesn