Protein supplementation is often used by athletes and fitness enthusiasts to enhance appropriate anabolic and physiological responses to resistance exercise (RE). People have been encouraged to utilize soy protein as an alternative to meat, poultry, and other animal-based proteins as the popularity in health-conscious lifestyles along with vegetarian and vegan diets have increased. There is concern among many over the potentially pro-estrogenic or anti-anabolic effects of soy protein1. Despite this concern, there is limited research on soy protein supplementation and its acute effects on hormonal responses to heavy RE.

Hormones are crucial in the anabolic signaling that promotes adaptations to RE. These signals assist changes in tissue size and function and result in hypertrophy and gains in strength and power2. Testosterone is a major hormone that responds to acute intense RE and works both through direct and indirect pathways to produce anabolic signals that promote muscle protein synthesis (MPS)3. Soy protein and its possible effects on normal anabolic response to RE have raised concerns with much debate centered around isoflavones, which are naturally occurring compounds found in soy that possess estrogenic properties4.

Despite the benefits of soy (e.g. lower incidence of coronary heart disease and prostate, colon and breast cancers), evidence from many studies demonstrate the ability of soy to alter testosterone, sex hormone binding globulin (SHBG), estradiol and the testosterone:estradiol ratio5. This may occur by the isoflavone-induced increase in hepatic SHBG synthesis which decreases the amount of bioavailable testosterone to enter target cells and elicit a biological response. However, evidence involving soy isoflavone usage in humans has not been consistent.

The research . . .

Recently, an interesting piece of work was conducted to investigate the effects of short-term protein supplementation with whey protein, soy protein, or a carbohydrate control on the known hormonal response patterns to an acute bout of heavy RE6. This group or researchers hypothesized that whey protein would result in a more favorable acute exercise hormonal response in comparison to soy protein.

This study involved ten resistance-trained men and all men completed3 experimental treatment conditions supplementing with whey protein isolate (WPI), soy protein isolate (SPI), and maltodextrin (CHO) placebo control for 14 days. Participants ingested 20 grams of their assigned supplement each morning at approximately the same time each day. After consuming their respective protein supplement, these men performed an acute heavy RE test consisting of 6 sets of 10 repetitions in the squat exercise at 80% of their 1RM (repetition maximum).

The results . . .

This was a really good study design, and it showed some very interesting results! Here is what the researchers found:

  • Lower testosterone responses following supplementation with soy protein.
  • Positive blunted cortisol with the use of whey protein at some recovery time points.
  • No difference in SHBG between experimental treatments.
  • No difference between groups in changes in estradiol concentrations.
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The main findings of this research were that 14 days of supplementation with soy protein does appear to partially blunt serum testosterone. An important finding was that there were similar testosterone values in CHO and WPI which demonstrated that the SPI treatment caused changes in the testosterone response pattern. In addition, whey protein influences the response of cortisol following an acute bout of RE by blunting its increase during recovery. It was clear from this research that protein supplementation alters the physiological responses to a common resistance exercise workout with differences due to the type of protein utilized (whey vs. soy)!

Bottom line . . .

The research behind the positive benefits from whey protein isolate supplementation is numerous. AST’s VP2 Whey Isolate has been shown in clinical research to build lean muscle7. VP2 enhances absorption kinetics that shuttles large amounts of the amino acids essential for protein synthesis directly into muscle and other tissues. The result is that there is a more potent supply of critical amino acids within muscle which promotes a more constant, uninterrupted state of protein synthesis, cell volume, and muscle anabolism!


1. Setchell KD. Phytoestrogens: the biochemistry, physiology, and implications for human health of soy isoflavones. Am J Clin Nutr 1998;68:1333S-46S.

2. Hakkinen K, Pakarinen A. Acute hormonal responses to two different fatiguing heavy-resistance protocols in male athletes. J Appl Physiol (1985) 1993;74:882-7.

3. Spiering BA, Kraemer WJ, Anderson JM, et al. Effects of elevated circulating hormones on resistance exercise-induced Akt signaling. Med Sci Sports Exerc 2008;40:1039-48.

4. Barnes S. Soy isoflavonesphytoestrogens and what else? J Nutr 2004;134:1225S-8S.

5. Habito RC, Montalto J, Leslie E, Ball MJ. Effects of replacing meat with soyabean in the diet on sex hormone concentrations in healthy adult males. Br J Nutr 2000;84:557-63.

6. Kraemer WJ, Solomon-Hill G, Volk BM, et al. The effects of soy and whey protein supplementation on acute hormonal responses to resistance exercise in men. J Am Coll Nutr 2013;32:66-74.

7. Hayes A, Cribb PJ. Effect of whey protein isolate on strength, body composition, and muscle hypertrophy during resistance training. Curr Opin Clin Nutr Metab Care 2008;11:40-4.

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Whey vs Soy Protein Supplementation on Hormonal Responses to Resistance Exercise

by Paul C. Henning, Ph.D. CSCS time to read: 4 min
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