Protein Content and Protein Quality

One of the concerns of vegetarian diets is the achievement of protein requirements. The soybean is considered not only for its total protein content but also for the quality of soy protein which is higher than that of other plant proteins and similar to animal protein.

The protein content of soybean is 36%. Among these, storage proteins are predominant. The main types of storage protein in soybeans are glycinin (11S globulin) and conglycinin(7S globulin), which make up approximately 80% of the total protein content. These proteins may trigger allergic reactions in some people. Moreover, there are other less abundant types of storage proteins such as 2S, 9S, and 15S globulins. In addition to globulins, complete soy protein content is the combination of enzymes, protease inhibitors (Kunitz and Bowman‐Birk), lectin, and others. 

There are two different methods involved to describe soy proteins.

1. An isolation of proteins from the commercial soy products and
2. The use of analytical techniques for protein determination.

Soy components such as minerals, phytic acid, ascorbic acid, and fiber may interact with soy proteins. These interactions depend on the processing and treatment of soy, and can decrease the bioavailability of minerals and proteins.

The main functional properties of soybean are: (7)

1. Swelling,
2. Solubility,
3. Viscosity, and
4. Capacity to form a gel, an emulsion, or a foam 

Protein quality is an estimation of a single food, through protein extraction, to know the information about protein synthesis after the intake. Different methods to define protein quality of foods such as Net Protein Utilization (NPU), Nitrogen Balance, Protein Efficiency Ratio (PER), Biological Value (BV). The most preferred method adopted by FAO/WHO  is the Protein Digestibility Corrected for Amino Acid Score (PDCAAS) for the measurement of the protein value in human nutrition. 

PDCAAS was adopted by (WHO), (FAO) and (USDA), due to the combination of the chemical value of amino acid composition of the test food, with the biological value of true absorbability in its integrity and not only from protein fraction after extraction. For example, from a data on animal test, BV method implied the use of protein extract, and used egg or milk as reference values. Whereas from a data on human studies, PDCAAS uses infant amino acid requirements as reference pattern and digestibility of whole foods. The maximum value that can be reached is 100% or 1.00 of PDCAAS. When children aged upto 6 months old (previously at 2 years old but with unsatisfactory information available) and above and adults are provided with an appropriate amount of protein intake, PDCAAS indicates an adequate amount of proteins by a specific food that ensures all the essential amino acids. Even if the PDCAAS method was used for a long time, there are few studies on soy.  The protein digestibility corrected amino acid scores (PDCAAS) for soy protein range from 0.9 to 1.0 depending upon the specific soy food.(3)

Soy protein quality through different methods.(3)

Small differences in PDCAASs for soy protein from different soy products exist because of minor differences in amino acid profile and digestibility that result from processing and because of the presence or absence of components such as fiber and phytate. 

The 2011 FAO Expert Consultation recognized that the Digestible Indispensable Amino Acid Score (DIAAS) was superior to the PDCAAS for determining protein quality. The DIAAS value for soy protein will be lower than the PDCAAS value, this is because the digestibility of each amino acid will be used as the measure of digestibility rather than fecal digestibility of the overall protein. But soy protein will still have a DIAAS of approximately 0.9. However, there was insufficient human data on amino acid digestibility before adopting the DIAAS. More human data was needed before DIAAS could be implemented.(1)

A meta-analysis of nitrogen balance studies for estimating protein requirements in healthy adults, recommends that there is no significant difference between the intake of vegetal or animal sources to meet nitrogen equilibrium, recommending that processed soy proteins are equivalent to animal proteins.(8) Therefore, soy-based vegetable analogues can help to meet protein requirements.

Carbs

The carbohydrate content of soybeans is low which means that many traditional Asian soy foods are also low in carbs which may make them beneficial for people with diabetes.(1)

Being low in carbs, they have low glycemic index (GI), which is a measure of how fast the food is converted into sugars after the meal.(9)

Also, much of the soybean carbohydrate is composed of oligosaccharides. Soybean oligosaccharides are potentially classified as prebiotics that may be used to improve immune function. Prebiotics are poorly digestible food ingredients by intestinal enzymes that are considered to be advantageous to the host, by selectively stimulating the growth and/or activity of one or a limited number of bacteria such as bifidobacteria, in the colon.

Soybean oligosaccharides (SBOSs), which are isolated from soybean seeds, are well-established prebiotics approved by the United States Food and Drug Administration as a generally recognized as safe (GRAS) ingredient. SBOSs mainly consist of raffinose, stachyose, and sucrose, common components formed by various linkages of mono- and oligosaccharides.

In the intestine, SBOSs can be fermented by certain bacteria, such as lactobacilli and bifidobacteria that also support the growth and survival of these bacteria in the gastrointestinal tract of animals. In addition, soybean meal oligosaccharides (SMOs) have been reported to potentially promote competitive exclusion of pathogens and hence their consumption may be useful for maintaining populations of beneficial microbes in the gut.(10)

Fat

Soybeans are very high in fat as compared to other legumes and soyfoods. Soy foods are naturally cholesterol-free and low in saturated fat. Animal protein foods high in saturated fat and cholesterol increase your risk of developing cardiovascular disease. Substituting them with soy a few times each week can help cut saturated fats and reduce your overall risk for disease. 

Soy foods are a good source of polyunsaturated fat. Polyunsaturated fats have a number of heart health benefits, such as lowering cholesterol. Choosing minimally processed soy foods will help you benefit from these heart-friendly fats.(11)

The fat content of soybeans is comprised of approximately 10%–15% saturated, 19%–41% monounsaturated and 46% to 62% polyunsaturated fatty acids (PUFA) (per 100 g soybeans that equates to approximately 2.9, 4.4 and 11.3 g saturated, monounsaturated and PUFA, respectively).

The PUFA comprises both essential fatty acids, i.e the omega-6 fatty acid linolenic acid and omega-3 fatty acid alpha-linolenic acid (ALA). The soybean and full fat soyfoods are some of the few foods that are good sources of both essential fatty acids.(1)

Fatty acid composition of soybean oil by seed coat color (grams per 100 g of oil) (1)

Summary

Among available plant proteins, soy is the only one that has been shown to have high quality proteins, based on human nitrogen balance studies. Soy has been analyzed for protein quality and has been found to be very similar to cow milk and egg proteins, traditionally used as standard references.(3)

Low GI makes soybeans suitable for people with diabetes. Also, much of the soybean carbohydrate is composed of oligosaccharides (predominantly stachyose).

Soybeans are very high in fat as compared to other legumes and soyfoods. Its high fiber content is beneficial for gut health. Including fiber-rich soy foods like edamame (green soybeans), black soybeans, soy nuts, soy flour and tempeh in your diet can help you boost your daily dietary fiber.