Soya protein contains all the essential amino acids in sufficient quantities and of high digestibility (> 90%) comparable to animal protein.
Current assessments of protein quality of individual foods are determined by two criteria:
· Essential amino acid content. The ability of the food to meet all essential and conditionally essential amino acid thresholds set for human requirements.
· Digestibility defined as the percentage absorbed after oral intake of a given quantity protein of an individual food.
The Protein Digestibility Corrected Amino Acid Score or PDCAAS is a method used to measure protein quality, taking into account both the amino acid composition and digestibility of a set quantity of protein of a specific food. Food proteins are categorised according to these criteria, with a maximum score of 1 corresponding to the highest quality protein. The PDCAAS for soya protein range from 0.9 to 1.0. Out of all the plant proteins, soya protein has the highest biological value, comparable in quality to milk protein.
Figure from Hughes GJ et al. Protein Digestibility-Corrected Amino Acid Scores (PDCAAS) for Soya Protein Isolates and Concentrate: Criteria for Evaluation. J Agric Food Chem 2011.
In 2013 the FAO proposed to abandon the PDCAAS index, in favour of another method, the Digestible Indispensable Amino Acid Score (DIAAS). The DIAAS is considered to be more accurate for measuring the digestibility of individual amino acids in the small intestine. While the DIAAS value for soya protein may be slightly lower than its PDCAAS score, the DIAAS has a value of around 0.9.
Important to note: single food scores of protein quality such as the PDCAAS and DIAAS do not reflect the essential amino acid provision in the total diet. Indeed, all plant foods contain all essential amino acids, be it some in low quantities and below the threshold set to meet all of human requirements. It has been demonstrated that it is the overall intake of amino acids and meeting energy requirements over the course of a day that determines the ability of a diet to meet essential amino acid needs. Therefore, diets based solely on plants, which meet the daily energy requirements of an individual, will also meet all essential amino acid needs.
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