It’s long been rumored that soy products are similar enough to natural estrogens that males should beware heavy consumption. Bodybuilders especially were concerned that soy protein supplements might be doing them more harm than good. The mechanism for this effect is unclear, but it does seem to exist.
A study (“The Effects of Soy and Whey Protein Supplementation on Acute Hormonal Responses to Resistance Exercise in Men”) from researchers at the Human Performance Laboratory at the University of Connecticut seems to confirm this; from the abstract:
Objective: For many resistance-trained men concerns exist regarding the production of estrogen with the consumption of soy protein when training for muscle strength and size. Thus, the purpose of this investigation was to examine the effects of soy and whey protein supplementation on sex hormones following an acute bout of heavy resistance exercise in resistance trained men.
Methods: Ten resistance-trained men (age 21.7 ± 2.8 [SD] years; height 175.0 ± 5.4 cm; weight 84.2 ± 9.1 kg) volunteered to participate in an investigation. Utilizing a within subject randomized crossover balanced placebo design, all subjects completed 3 experimental treatment conditions supplementing with whey protein isolate (WPI), soy protein isolate (SPI), and maltodextrin placebo control for 14 days with participants ingesting 20 g of their assigned supplement each morning at approximately the same time each day. Following supplementation, subjects performed an acute heavy resistance exercise test consisting of 6 sets of 10 repetitions in the squat exercise at 80% of the subject’s one repetition maximum.
Results: This investigation observed lower testosterone responses following supplementation with soy protein in addition to a positive blunted cortisol response with the use of whey protein at some recovery time points. Although sex hormone binding globulin (SHBG) was proposed as a possible mechanism for understanding changes in androgen content, SHBG did not differ between experimental treatments. Importantly, there were no significant differences between groups in changes in estradiol concentrations.
Conclusion: Our main findings demonstrate that 14 days of supplementation with soy protein does appear to partially blunt serum testosterone. In addition, whey influences the response of cortisol following an acute bout of resistance exercise by blunting its increase during recovery. Protein supplementation alters the physiological responses to a commonly used exercise modality with some differences due to the type of protein utilized.
Blunted testosterone response would be expected to reduce muscle gains from intense exercise. So it appears the widely-held view that soy products and soy protein are counterproductive when trying to build muscle mass is likely correct. The alternative, whey protein, is presumably preferable. Muscle and Fitness Magazine disagrees.
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