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.

The digestibility and chemical index together determine the PDCAAS value of the protein. The theoretical maximum value is 1.

The protein in most soya products has a PDCAAS that approaches a score of 1.0, indicating that both amino acid pattern and digestibility of soya protein are excellent (1,2). 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 (3). Although high protein diets may increase risk of developing kidney disease in susceptible individuals (4), several studies indicate that soya protein favorably affects renal function in comparison to animal proteins (5-8).

The American Food and Drug Administration and the British Joint Health Claim Initiative approved the health claim that “Intake of 25 grams of soya protein a day, as part of a diet low in saturated fat, may reduce the risk of heart disease”.

1. Sarwar G, Peace RW, Botting HG. Corrected relative net protein ratio (CRNPR) method based on differences in rat and human requirements for sulfur amino acids. J Am Oil Chem Soc 1985;68:68:689-93.
2. Sarwar G. The protein digestibility-corrected amino acid score method overestimates quality of proteins containing antinutritional factors and of poorly digestible proteins supplemented with limiting amino acids in rats. J Nutr 1997;127(5):758-64.
3. Astrup A. The satiating power of protein--a key to obesity prevention? Am J Clin Nutr 2005;82(1):1-2.
4. Knight EL, Stampfer MJ, Hankinson SE, Spiegelman D, Curhan GC. The impact of protein intake on renal function decline in women with normal renal function or mild renal insufficiency. Ann Intern Med 2003;138(6):460-7.
5. Kontessis P, Jones S, Dodds R, et al. Renal, metabolic and hormonal responses to ingestion of animal and vegetable proteins. Kidney Int 1990;38(1):136-44.
6. Anderson JW, Blake JE, Turner J, Smith BM. Effects of soy protein on renal function and proteinuria in patients with type 2 diabetes. Am J Clin Nutr 1998;68(6 Suppl):1347S-53S.
7. Stephenson TJ, Setchell KD, Kendall CW, Jenkins DJ, Anderson JW, Fanti P. Effect of soy protein-rich diet on renal function in young adults with insulin-dependent diabetes mellitus. Clin Nephrol 2005;64(1):1-11.
8. Teixeira SR, Tappenden KA, Carson L, et al. Isolated soy protein consumption reduces urinary albumin excretion and improves the serum lipid profile in men with type 2 diabetes mellitus and nephropathy. J Nutr 2004;134(8):1874-80.