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Soy Protein Intake May Adversely Affect Testosterone Levels in Men
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In evaluating the best protein for promoting anabolism and recovery, most research has focused on measures of muscle protein synthesis.  That research clearly distinguishes whey over other protein sources such as soy protein.  Another important aspect of the recovery process is related to promoting an optimal hormonal environment, especially as it pertains to the anabolic hormone testosterone and the catabolic hormone cortisol.  Studies evaluating the effects of protein quantity and quality have produced mixed results, but there is some evidence that soy protein may adversely affect hormonal profiles in men.  One reason is due to the presence of isoflavones in soy, which could increase estradiol.  Soy protein may also affect other aspects of testosterone metabolism such as inhibition of enzymes involved in testosterone synthesis.  Until recently, no studies directly examined the acute hormonal response to resistance exercise after consumption of soy and whey protein.  Researchers at the University of Connecticut examined how consuming equal amounts of whey and soy protein affected hormonal responses to an intense squat workout in resistance-trained men.  For 2 weeks subjects consumed one serving of whey, soy or carbohydrate.  Then they performed 6 sets for 10 repetitions of squat exercise 20 min after consuming a serving of the respective supplement.  Soy protein was found to have no effect on blood levels of estradiol.  However compared to whey, soy resulted in significantly lower testosterone and higher cortisol responses to exercise.  These results suggest that the superior anabolic effects of whey may be mediated in part by a more favorably anabolic hormonal response to resistance exercise.

Kraemer WJ, Solomon-Hill G, Volk BM, Kupchak BR, Looney DP, Dunn-Lewis C, Comstock BA, Szivak TK, Hooper DR, Flanagan SD, Maresh CM, Volek JS. 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.