Soy Protein: Hormone Disruptor or the Ultimate Protein?

You’ve likely heard the rumors—ahem, myths—about soy, and concern over whether it affects hormone levels. Those myths could be convenient for the meat industry, given the science-based truth: Soy is one of the best proteins you could consume. In fact, soy protein can pack 17 to 35 grams of protein per cup (depending on the type) with bioavailability that’s better than beef and on par with whey protein at 1.0 PDCAAS. What’s more, it’s packed with fiber and antioxidants, to boost your overall health.  

Read on to learn more about how soy protein actually affects you.

The Hormone Myth: What Science Shows

The most persistent myth surrounding soy centers on its supposed ability to disrupt male hormones. That’s because soy contains isoflavones, which are plant compounds that have a weak estrogen-like structure. However, just because isoflavones have a similar molecular structure does not mean they affect the human body the way estrogen does.  

A report published in Reproductive Toxicology contained data from multiple studies and revealed that soy protein did not affect total testosterone, free testosterone, or estradiol levels in the male study participants. The research findings were also not impacted by dosage or study duration—showing that soy consumption did not interfere with male reproductive hormones.

Other research published in the journal Nutrition showed that soy protein intake by active young men raised plasma antioxidant capacity without altering plasma testosterone levels. Another study examining college-aged men found that soy protein supplementation was "not androgenic or estrogenic" when combined with resistance exercise training.

Soy Protein: A Complete Nutritional Powerhouse

Beyond dispelling hormonal concerns, it's important to understand why soy protein deserves recognition as a high-quality protein source. Back to bioavailability: Soy protein boasts a protein digestibility-corrected amino acid score (PDCAAS) of 1.00, putting it on par with animal proteins like chicken, meat, and dairy. This score indicates that soy provides all essential amino acids in proportions your body needs.

Soy has a complete amino acid profile. While some plant proteins lack one or more essential amino acids, soy contains all nine essential amino acids the body cannot produce on its own. This completeness helps you maximize muscle protein synthesis.

Soy Protein for Muscle Growth

If you’re focused on building muscle or boosting your athletic performance, soy protein could be your best bud. A randomized controlled trial published in the International Journal of Sport Nutrition and Exercise Metabolism found no significant differences in muscle growth and strength development between men and women consuming soy versus whey protein supplements when matched for leucine content over 12 weeks of resistance training.

This finding is particularly significant because leucine is considered the key amino acid for triggering muscle protein synthesis. When soy protein supplements are formulated to match the leucine content of whey, they perform equally well for muscle building purposes.

Soy protein also delivers additional antioxidant benefits that may aid in recovery. Unlike many other protein sources, soy contains natural compounds that help combat exercise-induced oxidative stress. Translation? With soy, you can get muscle protein synthesis while helping to reduce inflammation and speed recovery between workouts.

Soy Can Boost Your Overall Health

Soy protein’s benefits go way beyond muscle building. For example, research has shown soy consumption is associated with reduced risk of prostate cancer. Those same isoflavones people mistakenly blamed for hormonal effects actually appear to protect against certain types of cancer.

Cardiovascular health is another area where soy protein shines. Studies show replacing animal proteins with soy protein can help lower cholesterol levels and improve overall heart health.

Soy Delivers Variety

One of the best parts about soy is that it’s easy to work into your diet and day. Soy protein powder can be mixed into shakes, and soy foods like tofu, tempeh, and edamame make great recipe staples and snacks.

Soy’s versatility as a plant protein makes it a clear winner—with no effect on your T levels.

References

Andrés, S., Abraham, K., Appel, K. E., & Lampen, A. (2011). Risks and benefits of dietary isoflavones for cancer. Critical Reviews in Toxicology, 41(6), 463-506.

Applegate, C. C., Rowles, J. L., Ranard, K. M., Jeon, S., & Erdman, J. W. (2018). Soy consumption and the risk of prostate cancer: An updated systematic review and meta-analysis. Nutrients, 10(1), 40.

Banaszek, A., Townsend, J. R., Bender, D., Vantrease, W. C., Marshall, A. C., & Johnson, K. D. (2019). The effects of whey vs. pea protein on physical adaptations following 8-weeks of high-intensity functional training (HIFT): A pilot study. Sports, 7(1), 12.

Cianci, D. J., Alaniz, R. C., Bolling, B. W., Cho, K., Colby, S. E., Corwin, R. L., ... & Olfert, M. D. (2018). The role of soy foods in the treatment of chronic disease. Nutrients, 10(11), 1623.

Dahlquist, D. T., Dieter, B. P., & Koehle, M. S. (2019). Plausible ergogenic effects of vitamin D on athletic performance and recovery. Journal of the International Society of Sports Nutrition, 16(1), 15.

Gilani, G. S., Xiao, C. W., & Cockell, K. A. (2012). Impact of antinutritional factors in food proteins on the digestibility of protein and the bioavailability of amino acids and on protein quality. British Journal of Nutrition, 108(S2), S315-S332.

Haun, C. T., Vann, C. G., Roberts, B. M., Vigotsky, A. D., Schoenfeld, B. J., & Roberts, M. D. (2018). A systematic review of dietary protein during caloric restriction in resistance trained lean athletes: A case for higher intakes. International Journal of Sport Nutrition and Exercise Metabolism, 28(4), 359-371.

Kalman, D., Feldman, S., Martinez, M., Krieger, D. R., & Tallon, M. J. (2007). Effect of protein source and resistance training on body composition and sex hormones in men. Journal of the International Society of Sports Nutrition, 4(1), 4.

Messina, M. (2010). Soybean isoflavone exposure does not have feminizing effects on men: A critical examination of the clinical evidence. Fertility and Sterility, 93(7), 2095-2104.

Messina, M. (2016). Soy and health update: Evaluation of the clinical and epidemiologic literature. Nutrients, 8(12), 754.

Reed, K. E., Camargo, J., Hamilton-Reeves, J., Kurzer, M., & Messina, M. (2021). Neither soy nor isoflavone intake affects male reproductive hormones: An expanded and updated meta-analysis of clinical studies. Reproductive Toxicology, 100, 60-67.

Schoenfeld, B. J., Aragon, A., Wilborn, C., Urbina, S. L., Hayward, S. E., & Krieger, J. (2017). Pre-versus post-exercise protein intake has similar effects on muscular adaptations. PeerJ, 5, e2825.

Tokede, O. A., Onabanjo, T. A., Yansane, A., Gaziano, J. M., & Djoussé, L. (2015). Soya products and serum lipids: A meta-analysis of randomised controlled trials. British Journal of Nutrition, 114(6), 831-843.

Yan, Z., Zhang, X., Li, C., Jiao, S., & Dong, W. (2017). Association between consumption of soy and risk of cardiovascular disease: A meta-analysis of observational studies. European Journal of Preventive Cardiology, 24(7), 735-747.