The FAO has held recent discussions about replacing the protein digestibility–corrected amino acid score (PDCAAS) with the digestible indispensable amino acid score (DIAAS) although it is likely that the PDCAAS will remain the assay used by regulatory bodies for evaluating protein quality for several more years. There are two primary differences between the PDCAAS and the DIAAS. One, as the name implies, is that the digestibility of individual amino acids rather than the entire protein is used in the calculation of protein quality. The other is that digestibility is to be determined at the end of the small intestine (ileal digestion) rather than the large intestine (fecal digestion).
Soy protein is a very high-quality protein with PDCAAS values ranging from about 0.9 to 1.0 for different soy products. This research by Rutherford et al. shows that soy protein is a high-quality protein when evaluated using the DIAAS. The digestibility of the indispensable amino acids ranged from approximately 90 to 98% and the content of the sulfur amino acids (the limiting amino acids in soy protein) for the two soy proteins evaluated averaged about 26.5 mg/g. This value compares favorably to the FAO recommended scoring pattern of 27 mg/g protein for children 6 months to 3 years of age and 23 mg/g protein for children older than 3 years, adolescents and adults. Consequently, DIAAS value for soy protein was approximately 0.9, which was much higher than the other plant proteins evaluated. Proteins with scores above 0.8 are considered high-quality proteins.
This research by Rutherford et al. is also notable because they found that 98% of the lysine was available. The FAO has recommended that available or reactive lysine be determined in proteins because of the potential for processing to convert lysine to lysinoalanine, a form of lysine unusable for protein synthesis. Although exposure to high alkaline conditions can cause this conversion to occur, the work by Rutherford et al. shows little conversion occurs in commercial soy proteins.
Rutherfurd SM, Fanning AC, Miller BJ, et al. Protein digestibility-corrected amino acid scores and digestible indispensable amino acid scores differentially describe protein quality in growing male rats. J Nut. (2015).