The soyabean is very high in protein (38%). So whole soyabeans and traditional soya foods, such as tofu, tempeh and soya milk are good protein sources. But it is not only the total amount of protein that is important, you also have to take the quality of the protein into account. Currently, a PDCAAS (protein digestibility corrected amino acid score) is used to evaluate protein quality. This evaluation takes into account the amino acid composition and the digestibility of the protein.

The protein digestibility of most soya products is generally excellent. In fact, the digestibility of soya protein is comparable to milk and meat proteins and can reach 100%.

The chemical index or amino acid score is the ratio of the most limiting essential amino acid, calculated by comparing content of the protein with that of amino acid requirements as defined by the FAO/WHO/UNU (Food and Agriculture Administration/ World Health Organisation/ United Nations University). Different needs for different age groups have been defined. In the illustration, the amino acid pattern of soya protein is compared with the amino acid needs for the most demanding category, the 2 to 5 year olds, (see picture).

The digestibility and chemical index together determine the PDCAAS value of the protein. The theoretical maximum value is 1. The protein in most soy products has a protein digestibility corrected amino acid score that approaches a score of 1.0, the highest possible rating, indicating that both amino acid pattern and digestibility of soy protein are excellent.5, 6. In addition to helping to meet the biological requirement for protein consuming soyfoods is an excellent way to increase the protein content of the diet as means of maintaining ideal weight. Research suggests that higher protein diets are more satiating and useful for weight management.16 Although high protein diets may increase risk of developing kidney disease in susceptible individuals,17 several studies indicate that soy protein favorably affects renal function in comparison to animal proteins.18-21

References :

1. Codex Alimentarius Commission. Document Alinorm 89/30. Report of the working group on protein    quality measurement. FAO/WHO, Rome, 1989.
2. FAO/WHO/UNU. Protein quality evaluation. Report of a joint FAO/WHO/UNU Expert consultation. Rome: FAO food and nutrition paper Nr. 51, 1991.
3. Joint FAO/WHO/UNU Expert consultation. Energy and protein requirements. Technical Report Series, Nr. 724, WHO, Geneva, 1985.
4. Dillon JC. Les méthodes d'évaluation de la valeur nutritive des protéines en alimentation humaine. Cah Nutr Diet 1992;27:54-58.
5. Young VR et al. Plant proteins in relation to human protein and amino acid nutrition. Am J Clin Nutr 1994;59(5 Suppl):1203S-1212S.
6. Young VR. Soya protein in relation to human protein and amino acid nutrition. J Am Diet Assoc 1991 Jul;91(7):828-835.
7. Lecerf JM et al. L'Intéret nutritionnel du soja. Nutr Clin Métabol 1995;9:137-144.