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The Soy Nutrition Institute provides information, research, and discussion about the benefits of soy to your health.

SOY PROTEIN

Complete, high-quality plant protein

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Soy is a high-quality, complete source of plant protein. It is the only plant protein that carries FDA’s heart health claim confirming it may be able to reduce the risk of coronary heart disease. It’s also a source of folate, potassium, and fiber, and the quality of soy protein is similar to animal protein and higher than the quality of nearly all other plant proteins.

PROTEIN QUALITY

The U.S. adult protein recommended dietary allowance (RDA) and the protein requirement established by the Food and Agriculture Organization (FAO) of the United Nations and other health agencies is 0.8 g/kg body weight.1 The U.S. RDA is based on the intake of good-quality protein, although no definition of “good” was provided by the National Academy of Medicine (formerly the Institute of Medicine) when it established the RDA other than to say the protein should have a well-balanced amino acid profile in relation to human needs.2

Protein quality is determined by two factors— digestibility and amino acid profile. Regarding the former, proteins must be broken down into their constituent amino acids in order for them to be absorbed. There are factors in foods that affect digestibility, such as phytate and fiber. Consequently, protein from plants tends to be less well digested than protein from animal sources. Regarding amino acid profile, of the 20 common amino acids in foods, 9 are considered essential or indispensable amino acids (IAA). IAA cannot be made by the body in sufficient amounts to meet requirements. The 9 essential amino acids are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Of the two factors that determine protein quality, IAA profile or content plays a larger role than digestibility, because the variability in digestibility among proteins is narrower than the variation in IAA content. Over the years, a variety of methods have been used to determine protein quality. Early on, a common method was the protein efficiency ratio (PER), which measures the ability of a protein to support the growth of young rats and is reported as the gain in weight per gram of protein consumed. However, because rats have a higher need for the sulfur amino acids (SAA) (methionine and cysteine) than humans, the PER underestimates the quality of some proteins, especially protein from legumes.3-4 For this reason, the PER has fallen out of favor.

The classic approach to the study of amino acid and protein requirements is the nitrogen balance study as pioneered by Rose and colleagues.5 Since nitrogen (~16% of total protein weight) is a fundamental component of amino acids and amino acids account for most nitrogen consumed, measuring nitrogen inputs and losses can be used to study protein metabolism. In the 1980s, a series of nitrogen balance studies by Young and colleagues6-11 demonstrated the high quality of soy protein. However, the NAM noted the shortcomings of this method and concluded that it should no longer be regarded as the “gold standard” for the assessment of the adequacy of protein intake and that alternative means should be sought.”12

In the early 1990s, the protein digestibility corrected amino acid score (PDCAAS) was adopted by the FAO and the U.S. Food and Drug Administration as the method of choice to determine protein quality. The PDCAAS is determined by comparing the IAAs in a food protein with the biological requirement for the IAAs and then correcting for true fecal digestibility in a rat assay. The reference amino acid pattern was based initially on the IAA requirements of the preschool-age child (2-5-year-old).  In 2007, the pattern for 1- to 2-year-olds was established by the FAO as the basis for calculating the PDCAAS. When determining protein quality using this method, scores for proteins exceeding 1.0 are not considered to contribute additional benefit in humans and are therefore truncated.

In 2011, Hughes et al.13 established the high quality of soy protein for the soy protein ingredients, soy protein isolate (SPI) and soy protein concentrate (SPC), which are comprised of ≥90% and 65-90% protein, respectively.14 Utilizing two different laboratories, these authors determined that the untruncated PDCAAS of 3 different soy protein isolates ranged from 0.95 to 1.02, and the scores for the single soy protein concentrate examined were 1.02 and 1.05. These values are similar to those determined by Rutherfurd et al.15 for soy protein isolate and by Mathai et al. 16 for soy protein isolate and soy flour. According to the USDA, in order for a food to qualify as a “high-quality protein,” it must have a score of at least 0.8. 

The FAO recently convened a series of meetings with experts in protein quality methodology. The reports of these meetings recommend gradually shifting from the PDCAAS to one of five potential methods for assessing protein quality. The most well-known is the digestible indispensable amino acid score (DIAAS)17 which has been widely used in the animal feed industry and has received the most support.  Given that some methodological issues remain to be resolved and limited data exist on the quality of proteins using this method,18 it will likely be several years before the DIAAS is accepted by regulatory bodies. However, protein quality scores using the DIAAS are increasingly presented in the scientific literature.

Preliminary data using the DIAAS support the high quality of soy protein although in general, the quality of plant protein is rated slightly lower using this method versus the PDCAAS.15  Fanelli et al.19 recently determined that the DIAAS for the Impossible Burger (primary protein source is soy) was similar to the DIAAS for 80% ground beef when calculated using the scoring pattern for the older child, adolescent and adult. And, when using this pattern, Reynaud et al.20 reported scores of 97% and 117% for tofu and soymilk, respectively.

CHOLESTEROL REDUCTION

Soy protein directly lowers blood cholesterol levels. Numerous meta-analyses published over the past 15 years indicate the reduction in LDL-cholesterol is approximately 4%.21-25  Over a period of years, each 1% reduction in cholesterol is thought to reduce risk of coronary heart disease by approximately 1-2%.26,27

In 1999, the U.S. Food and Drug Administration approved a health claim for soy foods and coronary heart disease based on the cholesterol-lowering effects of soy protein.28 At least 12 countries have approved similar claims over the past 20 years;29 the most recent country to do so was Canada, which approved a claim in 2015.24

LEAN TISSUE ACCRETION

Soy protein supplementation has been shown to lead to similar gains in strength and muscle mass among individuals engaged in resistance exercise training as supplementation with whey protein, the latter of which is typically viewed as the gold standard protein for building muscle.30  Recommendations for those engaged in resistance exercise training wanting to increase muscle mass and strength are to consume at least 1.6 g protein per kilogram body weight, which is double to recommended dietary allowance for the general adult population.31,32

ALLERGIES

Soy protein is one of the Big 8 (soon to be Big 9 with the addition of sesame), the eight foods responsible for approximately 90% of the food allergic reactions in the United States.  However, the prevalence of allergy to each of these foods varies markedly.

Surveys conducted over the past 10 years indicate that the prevalence of soy allergy is lowest among the Big 8.  Estimates are that between 1 and 6 adults per 1,000 are allergic to soy protein with most surveys pointing toward the lower range.33-35

Estimates are that 70% of children will outgrow their soy allergy by age 10.36, 37

SUSTAINABLY GROWN

U.S. soybean farmers are doing more with less and continue to strive to improve sustainability on their farms to preserve our natural resources. The soybean plays an important role in U.S. agriculture providing both protein and oil that is used throughout the world.

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REFERENCES

  1. EFSA Panel on Dietetic Products Nutrition and Allergies (NDA). Scientific Opinion on Dietary Reference Values for Protein. EFSA J 2012;10:2557.
  2. Institute of Medicine 2005. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: The National Academies Press. https://doi.org/10.17226/10490.
  3. 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 Assoc Official Anal Chem 1985;68:689-93.
  4. Babji AS, Fatimah S, Ghassem M, Abolhassani Y. Protein quality of selected edible animal and plant protein sources using rat bio-assay. Int Food Res J 2010;17:303-8.
  5. Rose WC, Wixom RL. The amino acid requirements of man. XVI. The role of the nitrogen intake. J Biol Chem 1955;217:997-1004.
  6. Istfan N, Murray E, Janghorbani M, Evans WJ, Young VR. The nutritional value of a soy protein concentrate (STAPRO-3200) for long-term protein nutritional maintenance in young men. J Nutr 1983;113:2524-34.
  7. Istfan N, Murray E, Janghorbani M, Young VR. An evaluation of the nutritional value of a soy protein concentrate in young adult men using the short-term N-balance method. J Nutr 1983;113:2516-23.
  8. Scrimshaw NS, Wayler AH, Murray E, Steinke FH, Rand WM, Young VR. Nitrogen balance response in young men given one of two isolated soy proteins or milk proteins. J Nutr 1983;113:2492-7.
  9. Wayler A, Queiroz E, Scrimshaw NS, Steinke FH, Rand WM, Young VR. Nitrogen balance studies in young men to assess the protein quality of an isolated soy protein in relation to meat proteins. J Nutr 1983;113:2485-91.
  10. Young VR, Wayler A, Garza C, Steinke FH, Murray E, Rand WM, Scrimshaw NS. A long-term metabolic balance study in young men to assess the nutritional quality of an isolated soy protein and beef proteins. Am J Clin Nutr 1984;39:8-15.
  11. Beer WH, Murray E, Oh SH, Pedersen HE, Wolfe RR, Young VR. A long-term metabolic study to assess the nutritional value of and immunological tolerance to two soy-protein concentrates in adult humans. Am J Clin Nutr 1989;50:997-1007.
  12. Institute of Medicine of the National Academies. 2005. P. 686 Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids. National Academy Press, Washington, DC.
  13. Hughes GJ, Ryan DJ, Mukherjea R, Schasteen CS. Protein digestibility-corrected amino acid scores (PDCAAS) for soy protein isolates and concentrate: Criteria for evaluation. J Agric Food Chemistry 2011;59:12707-12.
  14. Codex General Standard for Soy Protein Products, Codex Standard 175-1989. 1989.
  15. Rutherfurd SM, Fanning AC, Miller BJ, Moughan PJ. Protein digestibility-corrected amino acid scores and digestible indispensable amino acid scores differentially describe protein quality in growing male rats. J Nutr 2015;145:372-9.
  16. Mathai JK, Liu Y, Stein HH. Values for digestible indispensable amino acid scores (DIAAS) for some dairy and plant proteins may better describe protein quality than values calculated using the concept for protein digestibility-corrected amino acid scores (PDCAAS). Br J Nutr 2017;117:490-9.
  17. World Health Organization. Dietary protein quality evaluation in human nutrition.  Report of an FAO Expert Consultation. FAO Food and Nutrition Paper 92. Rome; 2013.
  18. World Health Organization. Research approaches and methods for evaluating the protein quality of human foods.  Report of a FAO Expert Working Group 2 – 5 March 2014, Bangalore, India.
  19. Fanelli NS, Bailey HM, Thompson TW, Delmore R, Nair MN, Stein HH. Digestible indispensable amino acid score (DIAAS) is greater in animal-based burgers than in plant-based burgers if determined in pigs. Eur J Nutr 2022;61:461-75.
  20. Reynaud Y, Buffiere C, Cohade B, Vauris M, Liebermann K, Hafnaoui N, Lopez M, Souchon I, Dupont D, Remond D. True ileal amino acid digestibility and digestible indispensable amino acid scores (DIAASs) of plant-based protein foods. Food Chem 2020;338:128020.
  21. Jenkins DJ, Mirrahimi A, Srichaikul K, et al. Soy protein reduces serum cholesterol by both intrinsic and food displacement mechanisms. J Nutr. 2010;140(12):2302S-11S.
  22. Blanco Mejia S, Messina M, Li SS, et al. A meta-analysis of 46 studies identified by the FDA demonstrates that soy protein decreases circulating LDL and total cholesterol concentrations in adults. J Nutr. 2019;149(6):968-81.
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