Does soy decrease testosterone levels?
Soy and Testosterone
Fitness enthusiasts use protein supplements to increase performance gains. With interest in healthy lifestyles on the rise, many are searching for alternatives to animal products such as whey or casein proteins. Soy protein is an easy alternative as it’s plant based and contains all of the essential amino acids. Resistance to soy protein is fierce due to its supposed association with pro-estrogenic and anti-anabolic effects.
Isoflavones, compounds found in soy, are thought to exert estrogen-like properties as they have the ability to bind to estrogen receptors in the body. There is however, evidence linking these compounds with benefits including lowering the risk of heart disease, cancer, and osteoporosis, lowering cholesterol levels, and improving weight loss outcomes1. The evidence on whether isoflavones alter testosterone levels is mixed.
What does the research say about soy and testosterone?
Soy Negatively Affects Testosterone
The Effects of Soy and Whey Protein Supplementation on Acute Hormonal Responses to Resistance Exercise in Men1
In the first study, researchers tested 3 treatments: soy protein, whey protein, and a placebo. Each treatment lasted 14 days and all 10 subjects went through all 3 treatments. Researchers compared the effects of each treatment on hormone levels. Subjects consumer 20 grams of protein once per day.
The results of this study did find different hormone responses in whey compared to soy. Testosterone levels were elevated in all 3 treatments, however, the whey and placebo treatments saw a greater testosterone increase than soy.
Cortisol is a catabolic hormone (its signals promote muscle breakdown) and levels begin to increase after a workout2. The results of this study showed whey was much more effective in lowering cortisol levels post workout. The whey treatment decreased cortisol at five minutes post workout. The soy treatment did not see a decrease until 60 minutes following the workout.
The results did not show any differences in estrogen levels between the 3 treatments. Researchers made a few conclusions. Their primary finding was that soy did not influence estrogen levels. While they did find lower testosterone increases with soy protein, there was still an increase. Soy’s effect on cortisol was similar to the placebo’s, meaning whey protein likely has the ability to decrease the cortisol response during recovery.
Soy Protein Isolates of Varying Isoflavone Content Exert Minor Effects on Serum Reproductive Hormones in Healthy Young Men3
Authors of this study looked at the effects of high and low isoflavone soy protein on hormones in men. They wanted to test the hypothesis that isoflavones are to blame for negatively affecting hormone levels. Thirty five subjects participated in this study and completed three separate treatments over 32 weeks. Each treatment lasted 57 days and was separated by a 28 day washout period to ensure treatments did not affect each other. The three treatments were: milk protein isolate, low-isoflavone soy protein, and high-isoflavone soy protein. Each protein supplement contained roughly 32 grams of protein per serving.
The results showed dihydrotestosterone (DHT) levels were lower with both low and high isoflavone soy protein compared to milk protein. Testosterone decreased with low isoflavone soy but showed no change with milk protein or high isoflavone soy. Estrogen levels decreased with milk protein and increased in the low isoflavone soy protein; it remained unchanged with the high isoflavone soy.
Researchers concluded that soy protein had significant effects on hormones regardless of isoflavone content. They noted that these results indicate that soy protein may prevent prostate cancer.
Soy Does Not Negatively Affect Testosterone
Soy protein intake by active young adult men raises plasma antioxidant capacity without altering plasma testosterone4
This study compared the effects of soy and whey protein on testosterone levels. Thirty subjects were recruited and assigned to one of three groups: soy protein, whey protein, or a placebo. The serving size was determined by body weight and set at 0.135 grams per pound of body weight per day (20 grams for a 150 lb individual). Blood samples were taken before and after the 4 week trial period.
The results did not show any difference in testosterone levels before or after the 4 week period. The type of soy protein used in this study was high in isoflavones. The authors of this study speculated that decreases in testosterone in other studies may have more to do with amino acid profiles of the soy products used rather than isoflavones.
Effect of protein source and resistance training on body composition and sex hormones5
This study compared the effects of soy and whey proteins on hormone levels. Twenty subjects were included and split into four groups: soy isolate, soy concentrate, whey blend (50% whey concentrate, 50% whey isolate), or soy isolate plus whey blend. Subjects took two servings of their respective supplement per day. Each serving contained 25 grams of protein. Subjects engaged in weight training 3 days per week for 12 weeks.
The results showed supplementation across all four groups did not significantly change testosterone levels from before to after the 12 week trial. The soy concentrate and whey blend groups saw small decreases in testosterone while the soy isolate and soy/whey mix groups saw small increases. This study also looked at changes in lean body mass (muscle) across the groups. It found lean body mass increased in all four groups, however, those assigned soy isolate saw the biggest increases.
The researchers of this study concluded that protein, regardless of source, in addition to resistance training resulted in increased lean body mass without causing a decrease in testosterone.
Meta-Analysis: Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis6
The final article is a meta-analysis. This is perhaps the most important piece of evidence we have as the authors reviewed over 30 studies to come to their conclusion. The studies they included in their analysis met the following criteria: 1) the study included men that consumed either soy foods, soy protein, or isoflavone extracts, and, 2) the study tracked testosterone and sex hormone binding globulin.
The analysis found that soy protein did not significantly affect testosterone. It also found isoflavone content did not affect testosterone levels. The authors concluded, “…neither soy protein nor isoflavone intake significantly alters any of these measures [bioavailable testosterone, sex hormone binding globulin, or free androgen index].”
The Bottom Line – What does this research mean, is soy protein safe?
While there is some evidence showing soy negatively affects testosterone levels, the vast amount of evidence, specifically in the meta-analysis, shows that soy protein does not decrease testosterone levels. There is some evidence presented above that soy even increases testosterone and results in more lean muscle gains than whey protein. While this is unlikely to quell anecdotes and rumors in the gym world, research has shown that soy protein is a safe and effective way to increase protein consumption, especially in populations that wish to decrease animal protein intakes.
Soy Protein Series
- Kraemer, W. J., Solomon-Hill, G., Volk, B. M., Kupchak, B. R., Looney, D. P., Dunn-Lewis, C., . . . Volek, J. S. (2013). The Effects of Soy and Whey Protein Supplementation on Acute Hormonal Responses to Resistance Exercise in Men. Journal of the American College of Nutrition, 32(1), 66-74. doi:10.1080/07315724.2013.770648
- Burley, S. D., Whittingham-Dowd, J., Allen, J., Grosset, J., & Onambele-Pearson, G. L. (2016). The Differential Hormonal Milieu of Morning versus Evening May Have an Impact on Muscle Hypertrophic Potential. Plos One, 11(9). doi:10.1371/journal.pone.0161500
- Dillingham, B. L., Mcveigh, B. L., Lampe, J. W., & Duncan, A. M. (2005). Soy Protein Isolates of Varying Isoflavone Content Exert Minor Effects on Serum Reproductive Hormones in Healthy Young Men. The Journal of Nutrition, 135(3), 584-591. doi:10.1093/jn/135.3.584
- Disilvestro, R. A., Mattern, C., Wood, N., & Devor, S. T. (2006). Soy protein intake by active young adult men raises plasma antioxidant capacity without altering plasma testosterone. Nutrition Research, 26(2), 92-95. doi:10.1016/j.nutres.2005.12.002
- 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. Journal of the International Society of Sports Nutrition, 4(1). doi:10.1186/1550-2783-4-4
- Hamilton-Reeves, J. M., Vazquez, G., Duval, S. J., & Phipps, W. R. (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. doi:10.1016/j.fertnstert.2009.04.038