The maintenance of skeletal muscle mass can be defined as the net result of protein synthesis and degradation. In most healthy persons the consumption of regular meals without training results in a relatively stable balance of muscle tissue over time. Generally it is acknowledged that a combination of training and adequate nutrition promotes the accretion of lean body tissue; the presence of a training stimulus and a positive protein balance fosters skeletal muscle fiber hypertrophy 12.

An aspect of adequate nutritional intake is related to the bioavailability of ingested dietary protein and its source (i.e. essential amino acids) which can directly influence the magnitude of adaptation of a given training stimulus. Previous work has suggested that soy protein is preferentially directed towards the splanchnic region and milk proteins, (i.e. whey and casein), to peripheral regions such as muscle tissues 34. When compared to soy proteins, milk proteins provide greater amounts of the branched chain amino acids (BCAAs) leucine, isoleucine and valine, as well as methionine and lysine 5678. Recent work has identified the importance of the BCAA leucine in the activation of myogenic translation initiation factors such as MTOR, P70 S6 kinase and eIF4E – eIF4G [for review see: 91011], which are considered important for muscle hypertrophy. As such the relatively low BCAA content found in soy protein may decrease the effectiveness of downstream leucine signaling 5.

Phytoestrogens (isoflavones) are a group of natural estrogen receptor modulators that are highly concentrated in soy foods, including soy protein isolates 12. Soy isoflavones have comparable molecular weights and are structurally similar to 17-beta estradiol, which may enable them to exert estrogenic and antiestrogenic activities through their associated receptor-binding site 13. In-vitro studies lend support to this relationship by demonstrating the ability of soy to inhibit a variety of androgenic and estrogenic hormones including: testosterone 14, sex hormone binding globulin (SHBG) 15, esterone 16 and testosterone/estradiol ratio 17. However, in-vivo evidence shows that the source and concentration of soy isoflavones does not impact levels of circulating sex hormones 181920. It has been demonstrated circulating sex hormone levels are closely linked to the adaptive response to resistance exercise 21. This provides one such premise regarding the perception that soy protein sources are inferior to milk proteins such as whey for supporting lean mass accretion in males engaged in resistance exercise 22.

Although there is some evidence regarding the benefits of whey over soy as an efficient adaptogenic protein source for muscle tissue, no human studies have compared the two proteins directly in response to resistance training. Based on this background, the present study assessed the effect of 12 weeks of resistance training and dietary supplementation with soy, whey or a combination, on body composition and plasma sex hormone concentrations.

This study is the first to investigate the effects of whey and soy protein supplements on androgenic hormones during 12 weeks of resistance training. The main finding was that 12 weeks of resistance training induced similar gains in lean body mass in subjects consuming a diet supplemented with whey or soy protein (Figure 1). This finding is contrary to the hypothesis that soy is an inferior source of protein in comparison to whey.

The SI supplemented group neared significance regarding lean mass increases over the 12 week training schedule (Figure 1). Previously, studies over a similar time frame have shown no significant differences between soy and whey supplementation with equal protein content 25. This suggests that over the short term the bioactive composition of these proteins has little additive effect on the magnitude of change in lean tissue mass. However, the subjects enrolled in the study were relatively novice exercisers with little previous exposure to training program. This may result in only early stage hypertrophic adaptation 26. The rationale is that early adaptive response to resistance exercise is neuromuscular in etiology and subsequent skeletal muscle fiber hypertrophy plays only a minor part in the total adaptive response 26.

Soy is well known for its estrogenic effects, which may occur through the hypothalamic-pituitary-gonadal axis 27. Isoflavones inhibit key steroidogenic linked enzymes including aromatase enzyme, 17β-hydroxysteroid oxidoreductase and cytochrome p-450 with the latter responsible for estrogen hydroxylation 28274. These postulated mechanisms provide a probable rationale for the decline in serum estradiol across all soy groups within the current study (Figure 6). Reduction in serum estrogens has previously been shown following soy-milk supplementation for 8 weeks 29. A declining trend in estradiol was also evident following supplementation in the WB group, although not statistically significant (Figure 6). Although it is not clear why whey protein may down-regulate estrogen synthesis, there may be a link to the activity of CYP 3A4 30, or similar estrogen influencing biochemical pathways following whey consumption. The potential interaction of whey protein and estrogen synthesis is an area worthy of further study.

As previously discussed, isoflavones may have a significant effect on circulating levels of testosterone. To date 11 previous studies have evaluated the effects of isoflavones and soy on sex hormone levels 3112141718192232331620, of which only two have shown a significant decline in testosterone 1214. Although the effects on total testosterone were minor, the relative response correlated to increasing isoflavone content. Gardner-Thorpe et al. gave 120 mg of isoflavones per day, a level comparable to the 138 mg/day supplementation for the SC group in the current study (Table 2). That the SC group was the only soy containing group showing a numerical reduction (not statistically significant) towards declining testosterone concentrations further suggests that a future study may want to evaluated if SC in a dose dependent manner affects serum testosterone levels. The biologically active and less commonly assessed free testosterone was not significantly reduced across all groups. Based on mean data, a trend towards a decline in the SC group was again evident (Figure 3). However, the lack of significance on circulating free testosterone levels corroborates previous studies on high protein soy isolates and soy-milk 1516 but not for milk proteins 3435 which may be as a direct result of differences in training methodology between studies.

The significant increase in the testosterone/estradiol ratio further contradicts any suggested relationship with declining androgenic activity following soy consumption (Figure 4). Both the biological activity and systemic clearance are influenced by the binding capacity of SHBG 363738. Since SHBG may alter the balance of both bound and unbound estrogens and androgens and has a greater binding to testosterone, the lack of effect on serum testosterone (free and total) should be reflected in shifts in SHBG concentrations. In agreement with previous studies on protein sources of phytoestrogens (i.e. soy), the current study has shown no significant change (Figure 5) 141916 in testosterone or SHBG. The lack of significant changes in SHBG may be due to confounding dietary variables, physical activity, lifestyle factors 394041, and bodyweight 42. Factors such as diet not controlled in the present study cannot be discounted. Although dietary intake (fat and calories) was significantly different between groups, 24 h dietary recall has significant limitations with regard to precision; nutrient intake estimates can ranging from 4 to 400% when compared with observed intakes 43. However, work comparing 24 hr versus 3 day dietary record shows comparable results as general indicators of dietary intake 44. Even with the use of biomarkers dietary analysis can prove inaccurate 44. As this was a free-living study without controlled diets 24 hr recall was deemed an appropriate indicator of estimated dietary intake patterns, however precise nutrient intake records maybe considered a limitation of the current study. Although a free living based nutritional study design is efficacious to external validity, the integration of 30-day or multiple 24 hour recall assessment could be employed to better detect dietary influences on training and hormonal adaptation.

The resistance training protocol utilized in the present study effectively increased the accretion of lean tissue irrespective of intervention. Increases in lean mass are consistent with the results of previous investigations 4546. The application of 12 weeks of resistance training was based on previous research describing the occurrence of significant muscle hypertrophy within this time frame 4748 and that significant diet-related differences in muscle hypertrophy occur within this period 4748. The results of the present study confirm these earlier studies regarding adaptive responses to diet and training.

No consistent effects were evident in the fatigue and vigor scores, although the POMS-Vigor score increased significantly in the highest isoflavone containing group (SC). Although, previous work has shown improvements in mood with exercise 49 and mood in females following isoflavone consumption 50, this is the first study to show any effect on males.

The current randomized and blinded intervention study in healthy men demonstrated that protein, irrespective of source (SI, SC, SW or WB), coupled with resistance training results in a significant enhancement of lean body mass. There was no significant decrease in serum androgenic hormones following supplementation with any protein intervention. The biological significance of the sex hormone changes within the current study resulting from lower estradiol, and increased testosterone/estradiol ratio in response to soy and whey supplementation is unknown at this time. The isoflavone content may affect the magnitude of hormonal change as may the duration of dietary exposure to supplemental isoflavones.

This is the first study to report low-level isoflavone content in commercial whey proteins, and future studies should take this into consideration when looking to designing an isoflavone free control study. Previous work has shown occurrence of isoflavones in the range of 1–30 ng/ml in bovine milk 51 suggesting that whey may not be an isoflavone free protein source. Although the sex hormone measurements were not statistically different in comparison to soy, factors such as sample size and duration of supplementation may have influenced outcome. Other factors such as equol responders and non-responders were not addressed in the current study. Because equol is know to influence the biochemical responses to isoflavone containing proteins further study is needed to asses its impact on lean mass changes 52. Because both protein source and bioactive content may bring about specific biochemical non-hypertrophic responses to exercise; such as increased antioxidant capacity 25, future work should look to measure performance markers to detect additional benefits from dietary protein supplementation. In conclusion, it appears that both soy and whey supplementation in free living resistance training men results in lean body mass accretion without negatively affecting serum androgen levels.