Soy Tanks Testosterone in Strength-Training Men

cortisol soy strength training testosterone Apr 29, 2014

For years, top body builders such as the late Vince Gironda touted diets rich in eggs, raw milk, red meat and other animal products while warning about the dangers of soy protein.1    Gironda went so far as to call soy protein that s*&%#**    The soy industry, of course, touts its soy protein products as the ideal way for athletes and fitness buffs to build strength and lean muscle.   The  research doesn’t exactly bear that out though the soy industry is gifted at putting a positive spin on anything and everything, including a recent study out of the University of Connecticut,2 that they should have tried to spin it deep into the ground.

In a randomized cross over, placebo-designed study, researchers compared the estrogen production of men drinking soy protein to those drinking whey.  They looked at changes in estradiol concentrations after only fourteen days, and reported some good news — the soy drinkers did not end up with more of the female hormone.   The researchers  did, however, find lower testosterone levels and higher cortisol levels.  In other words, less of the manly hormone and more of the stress hormone.  Sounds like stressed out soy boys to me!

In today’s soy-pushing climate, the U Conn researchers describe these legitimate concerns of Gironda and others about feminization and testosterone depletion as “fears” that “largely stem from body building mythologies.”  However, soy protein has been shown less effective than whey protein because its amino acids are more likely to go into splanchnic circulation (stomach, small intestines, colon, liver, pancreas and spleen) than into peripheral regions such as muscle tissues.4,5   This makes sense because whey protein provides greater amounts of the branches chain amino acids (BCAAs) leucine, isoleucine and valine as well as more methionine and lysine,  all of which are critically needed for muscle building.6-9   Researchers have also found the low BCAA content of soy protein adversely affects muscle building by disrupting both leucine signaling10 and the activation of myogenic translation initiation factors.11-14

Prior to the U Conn study, only one previous study compared the effect of soy and whey protein supplementation on the hormones of men doing resistance training.15   That study reported “no significant differences” between the soy and whey groups for total testosterone, free testosterone and sex-hormone-binding globulin (SHBG) after twelve weeks of supplementation.   But as the U Conn team suggests, soy may well put testosterone at risk.   In fact, over the past few decades, many researchers have found phytoestrogens have adverse effects on both the production and utilization of hormones in males.16-18  Indeed, scientists have even induced “testosterone deprivation” in animals simply by feeding them diets rich in soy isoflavones.19

For the most part, the soy industry has tried to promote any testosterone-tanking effects as beneficial.   Just as they promote hormonal changes that can lead to anovulatory cycles and infertility in women as valuable tools in the war against breast cancer,  they tout testosterone-lowering in men as protective against prostate cancer and atherosclerosis.20,21    Although the possibility that soy foods or supplements could prevent these deadly conditions tends to make headlines, few men hear that the downside is demasculinization.    Testosterone might appear to be just a macho thing, but it’s a vital hormone for growth, repair, red blood cell formation, healthy sleep cycles, sex drive and immune function.22     Low levels of testosterone have also been linked to low thyroid function, another unwanted and common side effect of soy consumption.  And low thyroid function leads to loss of libido in both men and women.23

Perhaps the most startling study came out of  the University of North Carolina at Chapel Hill a few years back.24   Completed for the National Cancer Institute, it found soy-eating men experienced “nipple discharge, breast enlargement, slight decreases in testosterone and hot flashes.”   In an attempt to downplay these rather sensational findings, lead researcher Steven H. Zeisel MD, reported nothing “serious” was found, even though they administered doses up to thirty times what might get from “normal foods.”  To reassure men, he stated, “I don’t think there are a lot of estrogenic worries.  Your testicles will not shrink and you won’t have massive breast enlargement.” 25

To their credit the U Conn researchers did not step up to reassure our men!



  1. Roach, Randy.  Muscle, Smoke and Mirrors, Volumes I and II.  (Author House, 2008 and 2011).
  2. Kraemer WJ, Solomon-Hill G, et al.  The effects of soy and whey protein supplementation on acute hormonal responses to resistance exercise in men.   J Am Coll Nutr, 2013;32(1):66-74.
  3. Kraemer  66.
  4. Gaudichon C, Mahe S, et al.   Net postprandial utilization of [15N]-labeled milk protein nitrogen is influenced by diet composition in humans. J Nutr. 1999;129:890–895.
  5. Makela S, Poutanen M, et al.   Inhibition of 17beta-hydroxysteroid oxidoreductase by flavonoids in breast and prostate cancer cells. Proc Soc Exp Biol Med. 1998;217:310–316.
  6. Baum JI, O’Connor JC, Seyler JE, Anthony TG, Freund GG, Layman DK. Leucine reduces the duration of insulin-induced PI 3-kinase activity in rat skeletal muscle. Am J Physiol Endocrinol Metab. 2005;288:E86–91.
  7. Bos C, Metges CC, et al.  Postprandial kinetics of dietary amino acids are the main determinant of their metabolism after soy or milk protein ingestion in humans. J Nutr. 2003;133:1308–1315.
  8. Layman DK. Role of leucine in protein metabolism during exercise and recovery.  Can J Appl Physiol, 27, 646-663.  2002. Physiol Endocrinol Metab. 2005;288:E86–91.
  9. Boza JJ, Martinez-Augustin O et al. Protein v. enzymic protein hydrolysates. Nitrogen utilization in starved rats. Br J Nutr. 1995;73:65–71.
  10. Bos, Metges.
  11. Baum, O’Connor.
  12. Blomstrand E, Eliasson J,et al. Branched-chain amino acids activate key enzymes in protein synthesis after physical exercise. J Nutr. 2006.  Jan; 136 (1 Suppl): 269S–273S.
  13. Kimball SR, Jefferson LS. Signaling pathways and molecular mechanisms through which branched-chain amino acids mediate translational control of protein synthesis. J Nutr. 2006. pp. 227–231
  14. Norton LE, Layman DK. Leucine regulates translation initiation of protein synthesis in skeletal muscle after exercise. J Nutr. 2006. pp. 533–537.
  15. Kalman D, Feldman S et al.   Effect of protein source and resistance training on body composition and sex hormones. J Int Soc Sports Nutr 4:4, 2007
  16. Gardner-Thorpe D, O’Hagen C, Young I, Lewis SJ: Dietary supplements of soya flour lower serum testosterone concentrations and improve markers of oxidative stress in men. Eur J Clin Nutr, 2003, 57:100–106.
  17. Habito RC, MontaltoJ, et al.  Effects of replacing meat with soyabean in the diet on sex hormone concentrations in healthy adult males. Br J Nutr 2000, 84:557–563.
  18. Dillingham BL, McVeigh BL et al.  Soy protein isolates of varying isoflavone content exert minor effects on serum reproductive hormones in healthy young men. J Nutr 2005, 135:584–591.
  19. Weber KS, Setchell KDR et al.   Dietary soy phytoestrogens decrease testosterone levels and prostate weight without altering LH, prostate 5a reductase  or testicular steroidogenic acute regulatory peptide levels in adult male Sprague-Dawley rats   J Endocrinol, 2001, 170, 591-599.
  20. Pollard M, Wolter W, Sun L.  Diet and the duration of testosterone dependent prostate cancer in Lobund-Wistar rats Cancer Lett, 2001, 173, 2 127-131.
  21. Gardner-Thorpe D, O’Hagen C et al.  Dietary supplements of soya flour lower serum testosterone concentrations and improve markers of oxidative stress in man   Eur J Clin Nutr, 2003, 57, 1, 100-106.
  22. Van de Graff, Ken M. Fox,  Stuart Ira.    Concepts of Human Anatomy and Physiology (Boston, MA, Wm C. Brown Publishers, 1995).
  23. Shames Richard and Karilee Halo Shames. Thyroid Power (New York, NY HarperResource, 2001).
  24. Fischer L, Mahoney et al.  Clinical characteristics and pharmacokinetics of purified soy isoflavones: multiple-dose administration to men with prostate neoplasia. Nutr Cancer  2004;48(2):160-70.
  25. Steven Zeisel as quoted by Sally Squires in “Nutrition not for women only: boys and men can benefit from soy too.”   Washington Post, June 8, 2004.

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