Journal Title (Medline/Pubmed accepted abbreviation):J. Am. Coll. Nutr.
Page numbers: 66-74
Background: Soy protein is very controversial in the United States because 1) it provides an excellent source of high quality protein, yet 2) it contains soy isoflavones which are similar in structure to estrogen. It is not clear whether soy disrupts normal, exercise-elicited responses of testosterone and other sex hormones.
Hypothesis: Post-workout supplementation with whey protein is superior to soy protein in regard to increasing muscle protein synthesis and increasing anabolic hormones.
Experimental Design: randomized, placebo-controlled, cross-over
Subjects: healthy males regularly involved in resistance training, age 21.7 ± 2.8 y, n = 10
Treatments: 20 g servings of whey protein, soy protein, or maltodextrin (carbohydrate placebo), each taken once per day in the morning with 475-600 mL of Crystal Light
Protocol: The participants were first familiarized with the resistance training protocol and evaluated for one-repetition maximum (1RM). They then supplemented with one of the treatments for 2 weeks. At the end of the two weeks, they reported to the laboratory for an acute heavy resistance exercise test. They came to the laboratory after an overnight fast, provided a fasting blood sample, and then consumed the appropriate treatment. After 20 min, they completed 6 sets of 10 repetitions of back squats at 80% 1RM with 2 min in between sets. Blood samples were taken at set times throughout the exercise protocol and up to 60 min after the last set. Blood was analyzed for total testosterone, estradiol, sex hormone binding globulin, and cortisol. They had a 2 week washout period, then repeated the cycle twice more so that each of the three treatments were consumed.
- Testosterone concentrations were significantly increased mid-way through the exercise routine up to 30 min after exercise in whey protein and carbohydrate trials.
- Testosterone levels were lower in the soy protein cycle at every time point during the study. The difference reached statistical significance at 5, 15, and 30 min after the last set of squats.
- With whey protein, cortisol concentrations peaked at the immediate-post-exercise time point and returned to baseline by 15 min. In contrast, cortisol concentrations increased with soy protein and maltodextrin steadily up until 30 min after exercise, then returned to baseline. Differences between whey protein and soy protein and whey protein and maltodextrin were statistically significant at 5, 15, and 30 min into recovery.
- There were no significant differences between whey protein, soy protein, and carbohydrate in regard to estradiol concentrations throughout the workout. With that said, estradiol concentrations were slightly lower in the carbohydrate cycle during recovery.
- Similarly, there were no differences between groups in sex hormone binding globulin throughout the study.
Key practice applications: Whey protein is better than soy protein at increasing testosterone and attenuating the exercise-induced cortisol response. However, soy protein did not increase estradiol or affect sex hormone binding globulin, as had been suggested by some in the body building community. It is thought that whey protein can create a more anabolic environment than soy because of its rapid absorption into the system and its high concentration of branched chain amino acids, especially leucine.
Limitations: These athletes performed the workout in the morning, and the only nutrition they consumed before the workout was the protein (or carbohydrate) supplement. It is more common for athletes to consume a more balanced meal that is high in carbohydrates before the workout, and then the protein supplement afterwards. It is unknown if the same hormonal response would be observed if the more typical protocol was followed.