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Soy Protein – Does it Reduce Testosterone?

Introduction to Soy Protein

Soy protein is considered a complete protein by nature, meaning it contains an appropriate amount of all nine essential amino acids. This is important for the human body because it cannot produce essential amino acids on its own so it’s important to get these from diet. Soy protein seems to be a viable option to that of milk-derived proteins such as whey and casein. Or is it?

Soy’s Isoflavones (Phytoestrogens)

Soy protein contains components called isoflavones. Isoflavones are phytoestrogens – plant-derived compounds with estrogenic activity molecularly similar to that of estradiol (17ß-estradiol). Soy isflavones are present as glycosides (bound to a sugar) called genistin, daidzin, and glycitin. Once digested, the sugar molecule is released from the glycoside which leaves isoflavone aglycones called genistein, daidzein, and glycitein1.

Total Isoflavone Content of Selected Foods5

Food Serving Total Isoflavones (mg)
Soy protein concentrate, aqueous washed 3.5 oz 102
Soy protein concentrate, alcohol washed 3.5 oz 12
Miso ½ cup 59
Soybeans, boiled ½ cup 47
Tempeh 3 ounces 37
Soybeans, dry roasted 1 ounce 37
Soy milk 1 cup 30
Tofu yogurt ½ cup 21
Tofu 3 ounces 20
Soybeans, green, boiled (Edamame) ½ cup 12
Meatless (soy) hot dog 1 hot dog 11
Meatless (soy) sausage 3 links 3
Soy cheese, mozzarella 1 oz 2

Soy protein isolate’s isoflavone content can vary significantly depending on the method used to isolate it. Soy protein isolates prepared by an alcohol extraction process typically lose most of their isoflavone content, while those prepared by a water extraction process tend to retain them. According to an article published in the Plant Foods for Human Nutrition in 2007, soy protein isolate is said to contain 88 to 164 mg of isoflavones per 100 grams (or 0.88 mg to 1.64 mg per gram)2. However, most clinical studies have used Solae™ soy ingredients. Solae™ uses the water extraction method for their soy protein isolates which retains most of the isoflavone content3. NOW® Foods Soy Protein Isolate contains 2.10 mg of isoflavones per gram4.

Isoflavones and Testosterone

Since we know isoflavones have mild estrogenic effects, it’s easy to speculate that this could be a hormonal health concern for healthy men. A study published in the Cancer Epidemiology, Biomarkers & Prevention evaluates the effects of isoflavones on serum testosterone and luteinizing hormone (LH). Twelve healthy male volunteer subjects with an average age of 32.25 consumed 2-scoops (56 g) of Puritan’s Pride® Soy Protein Powder for 4-weeks. Serum testosterone and LH levels were checked on all subjects 7-days before soy supplementation began and then on days 0, 14, and 28. Levels were also checked on day 42, 2-weeks after soy supplementation was discontinued5.

According to this study there was a significant 19%(±22%) decrease in the percentage of serum testosterone from day 0 to day 28. The mean ± SD of testosterone was 411 ± 175.82 ng/dL (range 198 to 872 ng/dL) at the start of therapy, 393 ± 194.24 ng/dL (range 182 to 759 ng/dL) at day 28, and 438 ± 95.46 ng/dL (range 263 to 587 ng/dL) at day 42, 2 weeks after treatment was discontinued (Figure 1)5.

Figure 1: The lowest and highest ranges of testosterone levels at baseline (day 0), day 28 (at the completion of 4 wks of soy protein powder supplementation), and at day 42 (2 wks after the completion of the 4-wk therapy)5.

Serum LH was checked at the same time as serum testosterone to determine if reductions of LH directly effected changes in testosterone. While there was a decrease in serum LH during the 4-weeks of soy supplementation, it was statistically insignificant. The mean ± SD of LH was 5.2 ± 2.36 mIU/mL (range 2.5 to 11.1 mIU/mL) at the start of therapy, 4.5 ± 2.11 mIU/mL (range 1.3 to 8.5 mIU/mL) at day 28, and 5.7 ± 3.16 mIU/mL (range 1.8 to 10.5 mIU/mL) at day 42 (Figure 2)5.


While research has indicated some potential benefits of soy isoflavones, there is increasing concern about their hormonal effects. According to an evidence-based monograph prepared for MedlinePlus by the Natural Standard Research Collaboration, there’s no conclusion whether isoflavones act as an estrogen receptor agonist (stimulator) or antagonist (blocker)6. This uncertainty is very alarming and consumers should be aware of the potential side-effects of using soy-based products.

Healthy males who consume various protein-based products may want to check their supplement labels for soy ingredients. While this is not an anti-soy campaign by any means, this blog is designed to increase your awareness about soy isoflavones and their potential to significantly disrupt your normal hormone balance. For competitive athletes using soy-based products / powders, this may be the difference between finishing first or not.

Interestingly enough, some self-proclaimed “hardcore” supplement manufacturers use soy protein in many of their products. This is typically done to reduce product costs in comparison to using higher-grade proteins such as whey and casein. If you are purchasing a protein for performance purposes, check your product’s ingredient label for soy protein. If you find soy on the label you might consider switching brands. According to a 2007 study published in the American Journal of Clinical Nutrition, the consumption of fat-free milk (80% Casein, 20% Whey) post-training resulted in greater muscle hypertrophy (growth) when compared to soy protein7.

If you’re a vegetarian or vegan you might consider pea protein. Pea Protein is a highly bioavailable protein and provides better muscle support than soy protein.

Puritan’s Pride® is a registered trademark of NBTY, Inc.


1 Jane Higdon, Ph.D., Victoria J. Drake, Ph.D. Soy Isoflavones. http://lpi.oregonstate.edu/infocenter/phytochemicals/soyiso/. 2009 Accessed on 10/30/2009.

2 Genovese MI, Barbosa AC, Pinto Mda S, Lajolo FM. Commercial soy protein ingredients as isoflavone sources for functional foods. Plant Foods Hum Nutr. 2007 Jun;62(2):53-8.

3 Solae™, LLC. Company Overview. http://www.solae.com/. Accessed on 10/30/2009.

4 NOW® Foods. Soy Protein Isolate – Non GE. http://www.nowfoods.com/. Accessed on 10/30/2009.

5 Goodin S, Shen F, Shih WJ, Dave N, Kane MP, Medina P, Lambert GH, Aisner J, Gallo M, DiPaola RS. Clinical and biological activity of soy protein powder supplementation in healthy male volunteers. Cancer Epidemiol Biomarkers Prev. 2007 Apr;16(4):829-33.

6 National Standard. MedlinePlus. http://www.nlm.nih.gov/medlineplus/druginfo/natural/patient-soy.html. August 2009. Accessed on 10/31/2009.

7 Hartman JW, Tang JE, Wilkinson SB, Tarnopolsky MA, Lawrence RL, Fullerton AV, Phillips SM. Consumption of fat-free fluid milk after resistance exercise promotes greater lean mass accretion than does consumption of soy or carbohydrate in young, novice, male weightlifters. Am J Clin Nutr. 2007 Aug;86(2):373-81.

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Written on October 13, 2009 by Brian Putchio
Last Updated: June 16, 2010

This information is not intended to take the place of medical advice.Please check with your health care providers prior to starting any new dietary or exercise program. CasePerformance is not responsible for the outcome of any decision made based off the information presented in this article.

About the Author: Brian Putchio is owner/operator of NUTRI-BODIES, LLC in Dubuque, Iowa. Through his extensive knowledge and experience in the nutraceuticals industry since 1999, Brian offers a unique perspective to his blog readers. Brian's refusal to simply flow with the marketing strategies of the industry conveys a strong sense of credibility that helps consumers successfully navigate the nutraceutical minefield.