Is Soy Protein Safe?

by Robert Schinetsky July 01, 2021

Vegan (plant-based) proteins are gaining popularity as more individuals are looking to diversify their sources of dietary protein and/or move away from animal products entirely.

Concomitant with this surge in all things “plant strong,” the supplement industry has seen a steady influx of plant-based protein powders, which usually contain a mix of several plant proteins, including (but not limited to):

Pea protein
Brown rice protein
Hemp seed protein
Pumpkin seed protein
Sunflower protein

While there are more and more vegan protein powders entering the market, there has been one plant-based protein powder that has been a mainstay in the sports nutrition sphere for over 20 years -- soy.

Suffice it to say that soy protein is the OG vegan protein powder.

It’s been sold in bulk powder forms, and it’s included as an ingredient in many protein bars.

Moreover, soy is known to contain biologically active components that can help lower concentrations of total cholesterol, LDL cholesterol and triglycerides -- making it beneficial for cardiovascular health.

But, many concerns still abound as to whether it’s safe to consume, particularly for men due to presence of isoflavones -- phenolic compounds that are classified as both phytoestrogens and selective estrogen receptor modulators.

Are these concerns well-founded?

Should we be wary of consuming soy protein?

That’s what we’ll answer in this guide to all things soy protein.

Let’s first start by discussing what soy protein is, where it comes from, and how it stacks up compared to the grandmaster of all protein powders -- whey.

What is Soy Protein and Where Does It Come From?

Soy protein is derived from the humble soybean, a type of legume indigenous to Asia. Some accounts note that soy was used in China as far back as 2838 B.C.[11]

Soy was introduced to Europe in the early 1700’s and North America around 1765, where it was initially used for animal feed. Humans didn’t start consuming soy in North America until the early 1900s.

The United States is the world's biggest soybean producer, generating about 73 million tons per year. However, the average American only consumes only 40 grams of soy per year. This is less than the amount eaten in a single day by the average Japanese person.[10]

Today, soy-inclusive foods are some of the most rigorously investigated foods on the market, with some 2000 soy-related peer-reviewed articles published each year![2]

Before the sudden interest in the protein content of soy, it’s primary use was for the production of soybean oil, which is high in polyunsaturated fats (PUFAs) and low in saturated fats, leading many to consider it a “heart healthy” fat.

As research expanded into soy, the focus began to shift towards its protein content...

Nutritional Content of Soybeans

Soybeans are higher in protein and fat compared to most other legumes.

Between 35-38% of the calories in a soybean are protein, about 40% of calories are fat (most legumes -- except peanuts -- contain between 2-14% percent fat), and the remainder are carbohydrates.

Soybeans also contain essential vitamins, such as calcium and iron, as well as fiber.

Soy is consumed in a wide range of forms, such as:

Edamame
Tofu
Soy sauce
Miso
Tempeh
Natto
Soymilk
Textured vegetable protein (TVP)
Soy protein powder

What’s in Soy Protein?

Unlike most plant protein sources, the protein in soy is considered a “complete” protein on account of it including sufficient amounts of all nine essential amino acids needed to stimulate and fuel muscle protein synthesis (MPS).

For example, leucine (the anabolic “trigger” of protein synthesis), methionine, and lysine contents are typically lower in plant-based proteins than animal-based proteins (whey, casein, beef, etc.).

This is one of the main reasons why plant-based proteins are known to possess lesser anabolic properties -- lower essential amino acid content and/or deficiency of specific amino acids (leucine, lysine, and/or methionine).

Soy, however, contains adequate amounts of all the essential amino acids.

In fact, research notes that the protein digestibility corrected amino acid scores (PDCAAS) for soy protein range from 0.9 to 1.0 depending upon the specific soyfood in question, which is on par with whey (1.0), casein (1.0), egg (1.0), milk (1.0), and beef (0.92).[11]

There are other methods for ranking the quality of various protein sources.

For instance, the biological value (BV) provides a measurement of how efficient the body utilizes a given protein.

Comparing the same protein sources above, the biological values are[11]:

Whey: 104
Egg: 100
Milk: 91
Beef: 80
Casein: 77
Soy: 74

For a closer comparison, here are the typical amino acid profiles of common protein sources in the diet:

Soy protein powder typically comes in one of three forms[11]:

Soy flour: 50% protein
Soy concentrate: 70% protein
Soy isolate: 90% protein

Soy concentrate is typically used in protein bars, cereals, and yogurts.

Isolates are typically used in protein powders, since they contain the highest concentration of protein. (Note: isolate is also the most refined soy protein product, not that that’s necessarily a bad thing.)

Soy Protein vs Whey Protein

Whey protein is the most popular form of protein powder on the market, routinely consumed by individuals looking to increase lean mass and support muscle recovery.

Given whey’s status as the leading protein powder on the market (plus the fact that it’s been shown in multiple studies to enhance resistance training adaptations -- gainz), and the fact that soy protein is a complete protein, researchers have compared the two proteins head-to-head to see how they impact performance, growth, and recovery.

Comparing the proteins side by side, soy protein contains a lower quantity of essential amino acids (EAAs) on a per gram basis as well as fewer branched-chain amino acids (BCAAs) compared to whey protein.[13]

Further comparing soy to whey, you’d need to consume ~40 grams of soy protein to the same amount of leucine (2.7g) and total amino acids (10.9g EAAs) as found in 25 grams of whey protein.[1]

Additional research has found evidence that consumption of soy protein does not stimulate acute post-exercise muscle protein synthesis to the same magnitude as whey or milk protein, either.[14,15]

Still, a 2018 meta-analysis including nine studies comparing soy protein supplements to animal-based protein supplements (with 5 of the 9 directly pitting soy against whey) found no significant differences in strength or lean mass gains between those who supplemented with whey vs those who supplemented with soy in conjunction with resistance training.[12]

Essentially, the authors of this meta-analysis concluded that soy was just as effective as whey protein in enhancing gains in strength and lean body mass when used in combination with resistance training.

A more recent 2020 study including 61, untrained men and women, again compared soy protein to whey protein in combination with a 12-week resistance training program. All participants performed supervised resistance training 3×/week and consumed either 19 grams of whey protein isolate or 26 grams of soy protein isolate, both containing 2 g (grams) of leucine.[16]

At the end of the 12-week study, researchers noted that both groups experienced similar increases in lean mass and strength when strength training and supplementing with soy or whey matched for leucine.[16]

Now, while soy may be a suitable option for supplementing your protein intake during the day, and supporting your quest to make gains in size and strength….the larger issue at hand is...

Is Soy Protein Safe for Men?

As we mentioned at the beginning of this article, isoflavones are phenolic compounds that are classified as both phytoestrogens and selective estrogen receptor modulators (SERMs). They are also noted to possess both estrogen-agonist and estrogen-antagonist properties, due to their structural similarity to 17-β-estradiol.[3]

Soy isoflavones can preferentially bind to and activate estrogen receptor-β (ER-β) — as opposed to ER-α — mimicking the effects of estrogen in certain tissues in the body and antagonizing (blocking) the effects of estrogen in other tissues.[4]

Furthering the concern over the intake of soy products are case studies noting an association between soy consumption and hypogonadism as well as erectile dysfunction.[5,6] Other research, though, disputes the potential feminizing effects of soy isoflavones.[7]

Additionally, a meta-analysis of 15 placebo-controlled studies concluded that soy protein nor isoflavones have no significant impact on[8]:

Testosterone
Free testosterone
Sex-hormone binding globulin (SHBG)
Free androgen index (FAI)

Still, concerns abound about soy protein, in large part due to consumer misconceptions stemming from fear-mongering articles that litter the internet.

So, let’s see what the latest research has to say regarding soy protein intake and men’s health.

Is it another quality source of protein that can be added to the diet, or...

Will consuming soy lead to rampant increases in estrogen levels, gyno, ED, and infertility?

Let’s find out!

Study #1

The first study to investigate the effects of soy protein consumption on body composition and sex hormone profiles was published in 2007.

Out of the 41 participants screened for the study, 20 subjects took part and completed the 12 week trial.

Subjects supplemented their diets with one of the following protein powders:

soy protein isolate
soy concentrate
whey blend (50% whey concentrate, 50% whey isolate)
50:50 mixture of soy isolate and whey blend

On training days, subjects were instructed to consume one serving of their respective protein supplement within 1 hour post-workout, and a second dose was consumed at some other point during the day.

On non-training days, subjects consumed two doses of protein at different times throughout the day.

Resistance training during the study consisted of 3 full body workouts per week for 12 weeks, with individual instruction once per week by a qualified personal trainer to make sure subjects were using proper form, pushing themselves hard enough, etc.

At the end of the 12 weeks, researched documented similar gains in lean body mass in subjects consuming whey or soy protein, which at the time of this publication (2007) was quite surprising since it was believed that whey was “far superior” for building muscle than soy.

The researchers do note, though, that “the subjects enrolled in the study were relatively novice exercisers with little previous exposure to training program.”

Whether soy is effective for muscle building isn’t really the point here, what we’re really interested in is if consuming soy shrinks your balls and increases your cup size.

So, what did the study find concerning soy intake and male hormone profiles?

“There was no significant decrease in serum androgenic hormones following supplementation with any protein intervention. The biological significance of the sex hormone changes within the current study resulting from lower estradiol, and increased testosterone/estradiol ratio in response to soy and whey supplementation is unknown at this time.”[9]

Basically, consuming two soy protein shakes per day for 12 weeks (3 months) didn’t cause the male subjects to grow breasts or tank their testosterone.

But, one study isn’t enough.

Fortunately, there are several more studies investigating this same line of inquiry…

Study #2

A 2018 study by Cody Haun and colleagues investigated the effects of different protein supplements on serum sex hormones, androgen signaling markers in muscle tissue, and estrogen signaling markers in subcutaneous adipose tissue.

47 untrained men took part in the twelve week study and were distributed into one of three groups:

Soy protein concentrate
Whey protein concentrate
Placebo

Subjects performed three total body resistance training sessions per week for 12 weeks and consumed two servings of either soy protein, whey protein, or placebo every day during the 12-week study period.

At the end of the study, researchers documented no significant interactions of supplement and time on adipose tissue estrogen receptor α/β protein levels, muscle tissue androgen receptor protein levels, or mRNAs in either tissue indicative of altered estrogenic or androgenic activity.

Based on these findings, the researchers concluded:

“These data suggest that, while isoflavones were detected in SPC, chronic WPC or SPC supplementation did not appreciably affect biomarkers related to muscle androgenic signaling or SQ estrogenic signaling.”

Essentially, while soy protein contains isoflavones, their ingestion did not affect androgenic or estrogenic signaling in college-aged men, even when consumed twice a day, everyday for 12 weeks.

Further Evidence

The European Food Safety Authority (EFSA) has also stated that isoflavones do not adversely affect the breast, thyroid or uterus of postmenopausal women.[2]

And, on a final note, a 2010 review of the literature by Messina (who has spent 20 years researching soy protein) concluded[18]:

“Finally, other than allergic reactions, there is almost no credible evidence to suggest traditional soyfoods exert clinically relevant adverse effects in healthy individuals when consumed in amounts consistent with Asian intake.”

Another meta-analysis from 2010 also found that:

“soy foods nor isoflavone supplements alter measures of bioavailable T concentrations in men.”[19]

The Bottom Line on Soy Protein Supplements

At the end of the day, the current body of evidence indicates that soy protein supplements may not only be effective for enhancing gains in strength and lean body mass, but they are also safe for men to consume in reasonable doses.

Evidence to date shows that the consumption of soy protein does NOT raise estrogen levels in men, or adversely affect testosterone production.

So, if you happen to eat some protein bars that include soy protein, or have the occasional plant-based protein shake that includes soy protein, you can rest easy knowing that neither your gains (or manhood) is threatened.

References

Gorissen SHM, Crombag JJR, Senden JMG, et al. Protein content and amino acid composition of commercially available plant-based protein isolates. Amino Acids. 2018;50(12):1685-1695. doi:10.1007/s00726-018-2640-5
Messina M. Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature. Nutrients. 2016;8(12):754. Published 2016 Nov 24. doi:10.3390/nu8120754
Wood CE, Register TC, Franke AA, Anthony MS, Cline JM. Dietary soy isoflavones inhibit estrogen effects in the postmenopausal breast. Cancer Res. 2006 Jan 15;66(2):1241-9. doi: 10.1158/0008-5472.CAN-05-2067. PMID: 16424064.
Wang LQ. Mammalian phytoestrogens: enterodiol and enterolactone. J Chromatogr B Analyt Technol Biomed Life Sci. 2002 Sep 25;777(1-2):289-309. doi: 10.1016/s1570-0232(02)00281-7. PMID: 12270221.
Siepmann T, Roofeh J, Kiefer FW, Edelson DG. Hypogonadism and erectile dysfunction associated with soy product consumption. Nutrition. 2011 Jul-Aug;27(7-8):859-62. doi: 10.1016/j.nut.2010.10.018. Epub 2011 Feb 25. PMID: 21353476.
Martinez J, Lewi JE. An unusual case of gynecomastia associated with soy product consumption. Endocr Pract 2008;14:415–8
Messina M. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertil Steril 2010;93:2095–104.
Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010 Aug;94(3):997-1007. doi: 10.1016/j.fertnstert.2009.04.038. Epub 2009 Jun 12. PMID: 19524224.
Kalman, D., Feldman, S., Martinez, M. et al. Effect of protein source and resistance training on body composition and sex hormones. J Int Soc Sports Nutr 4, 4 (2007). https://doi.org/10.1186/1550-2783-4-4
UN Food and Agriculture Organization
Hoffman JR, Falvo MJ. Protein - Which is Best?. J Sports Sci Med. 2004;3(3):118-130. Published 2004 Sep 1.
Messina M, Lynch H, Dickinson JM, Reed KE. No Difference Between the Effects of Supplementing With Soy Protein Versus Animal Protein on Gains in Muscle Mass and Strength in Response to Resistance Exercise. Int J Sport Nutr Exerc Metab. 2018 Nov 1;28(6):674-685. doi: 10.1123/ijsnem.2018-0071. Epub 2018 Oct 26. PMID: 29722584.
Van Vliet S., Burd N.A., Van Loon L.J.C. The Skeletal Muscle Anabolic Response to Plant- versus Animal-Based Protein Consumption. J. Nutr. 2015;145:1981–1991. doi: 10.3945/jn.114.204305.
Yang Y., Churchward-Venne T.A., Burd N.A., Breen L., Tarnopolsky M.A., Phillips S.M. Myofibrillar protein synthesis following ingestion of soy protein isolate at rest and after resistance exercise in elderly men. Nutr. Metab. 2012;9:57. doi: 10.1186/1743-7075-9-57
Wilkinson S.B., Tarnopolsky M.A., Macdonald M.J., Macdonald J.R., Armstrong D., Phillips S.M. Consumption of fluid skim milk promotes greater muscle protein accretion after resistance exercise than does consumption of an isonitrogenous and isoenergetic soy-protein beverage. Am. J. Clin. Nutr. 2007;85:1031–1040. doi: 10.1093/ajcn/85.4.1031.
Lynch HM, Buman MP, Dickinson JM, Ransdell LB, Johnston CS, Wharton CM. No Significant Differences in Muscle Growth and Strength Development When Consuming Soy and Whey Protein Supplements Matched for Leucine Following a 12 Week Resistance Training Program in Men and Women: A Randomized Trial. Int J Environ Res Public Health. 2020;17(11):3871. Published 2020 May 29. doi:10.3390/ijerph17113871
Mobley CB, Haun CT, Roberson PA, et al. Effects of Whey, Soy or Leucine Supplementation with 12 Weeks of Resistance Training on Strength, Body Composition, and Skeletal Muscle and Adipose Tissue Histological Attributes in College-Aged Males. Nutrients. 2017;9(9):972. Published 2017 Sep 4. doi:10.3390/nu9090972
Mark Messina, Insights Gained from 20 Years of Soy Research, The Journal of Nutrition, Volume 140, Issue 12, December 2010, Pages 2289S–2295S, https://doi.org/10.3945/jn.110.124107
Hamilton-Reeves, J. M., Vazquez, G., Duval, S. J., Phipps, W. R., Kurzer, M. S., & Messina, M. J. (2010). Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: Results of a meta-analysis. Fertility and Sterility, 94(3), 997-1007. https://doi.org/10.1016/j.fertnstert.2009.04.038