Soy Protein

Legume Analysis Highlights Superior Quality of Soy

By August 11, 2017 No Comments

In plant-based diets beans play a critically important role as sources of protein.  In fact, it is difficult if not impossible to meet the requirement for the indispensable amino acid lysine without consuming legumes when total protein intake does not exceed the RDA.  Beans are also excellent sources of B vitamins and minerals such as potassium and resistant starch and fiber.1 For those individuals concerned about food costs they are an especially good choice.  While it is true that because of the presence of oligosaccharides, beans can cause flatulence, research indicates that this concern is much less of an issue than is commonly perceived and that over time flatulence appears to diminish.2 Besides, the oligosaccharides can function as prebiotics thereby stimulating the growth of health-promoting colonic bacteria.1

While there are exceptions (humus/falafel), for the most part other than soybeans, beans are consumed whole.  That is, other than boiling, they are typically consumed without additional processing.  In contrast, although soybeans can also be consumed after only minimal processing in the form of boiled beans, edamame and soynuts, they can also undergo additional processing to produce the traditional Asian soyfoods (tofu, miso, tempeh) or soy protein products (isolates, concentrates, flour).

Processing of beans typically increases protein digestibility which is why the digestibility of whole beans is not especially high.  Gilani et al.3 listed the digestibility of soybeans, soy flour and isolated soy protein as 78, 86 and 95%, respectively.  Although relatively little work has been published, that which has suggests the digestibility of protein from the traditional Asian soyfoods is quite high (>90%).

New work by Nosworthy et al.4 found that the digestibility of 9 different Canadian-grown legumes ranged from 70 to 90% with the digestibility of four of the beans being below 80%.  The protein digestibility corrected amino acid scores using the 1991 FAO/WHO scoring pattern ranged from 0.50 to 0.67.  By comparison, the PDCAAS of soy protein concentrate and isolated soy protein are approximately 1.0.5

As can be seen from the table below, in comparison to the 9 legumes studied by Nosworthy et al.4, the soybean has the highest concentration of sulfur amino acid (methionine and cysteine), which are the limiting amino acids in legumes.  So soyfoods have a superior amino acid profile and can be consumed in highly digestible forms.  Nevertheless, all beans are really nutritious foods that warrant a much bigger role in Western diets than they currently have.

Sulfur amino acid content of legumes

Beanmg /g protein
Red kidney beans117.5
Navy beans122.0
Whole green lentils117.9
Split red lentils114.9
Split yellow peas122.6
Split green peas114.9
Black beans119.2
Pinto beans121.2

1Nosworthy et al. 2USDA Nutrient Database



  1. Messina, V. Nutritional and health benefits of dried beans. Am J Clin Nutr. 2014, 100 Suppl 1, 437S-42S.
  2. Winham, D.M. and Hutchins, A.M. Perceptions of flatulence from bean consumption among adults in 3 feeding studies. Nutr J. 2011, 10, 128.
  3. Gilani, G.S., Cockell, K.A., and Sepehr, E. Effects of antinutritional factors on protein digestibility and amino acid availability in foods. J AOAC Int. 2005, 88, 967-87.
  4. Nosworthy, M.G., Neufeld, J., Frohlich, P., Young, G., Malcolmson, L., and House, J.D. Determination of the protein quality of cooked Canadian pulses. Food Sci Nutr. 2017, 5, 896-903.
  5. Hughes, G.J. The evolution of protein quality evaluation. INFORM. 2015, 26, 110-112.


Dr. Mark Messina

Author Dr. Mark Messina

PhD in Nutrition, Executive Director, Soy Nutrition Institute. Expert in soyfoods and isoflavones.

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