written by: Douglas Kalman email, Samantha Feldman email, Michele Martinez email, Diane R Krieger email and Mark J Tallon email
source: Journal of the International Society of Sports Nutrition 2007, 4:4doi:10.1186/1550-2783-4-4

Abstract

Background

Evidence suggests an inverse relationship between soy protein intake and serum concentrations of male sex hormones. Anecdotal evidence indicates that these alterations in serum sex hormones may attenuate changes in lean body mass following resistance training. However, little empirical data exists regarding the effects of soy and milk-based proteins on circulating androgens and exercise induced body composition changes.

Methods

For 12 weeks 20 subjects were supplemented with 50 g per day of one of four different protein sources (Soy concentrate; Soy isolate; Soy isolate and whey blend, and Whey blend only) in combination with a resistance-training program. Body composition, testosterone, estradiol and sex hormone binding globulin (SHBG) were measured at baseline and week 12.

Results

Protein supplementation resulted in a significant increase in lean body mass independent of protein source (0.5 ± 1.1 and 0.9 ± 1.4 kg, p = 0.006, p = 0.007). No significant differences were observed between groups for total and free testosterone, SHBG, percentage body fat, BMI or body weight. The Testosterone/Estradiol ratio increased across all groups (+13.4, p = 0.005) and estradiol decreased (p = 0.002). Within group analysis showed significant increases in the Testosterone/Estradiol ratio in soy isolate + whey blend group (+16.3, p = 0.030). Estradiol was significantly lower in the whey blend group (-9.1 ± 8.7 pg/ml, p = 0.033).

Conclusion

This investigation shows that 12 week supplementation with soy protein does not decrease serum testosterone or inhibit lean body mass changes in subjects engaged in a resistance exercise program.

 

Background

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 [1,2].

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 [3,4]. 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 [5-8]. 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: [9-11]], 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 [18-20]. 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.

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Miami Research Associates, Nutrition/Endocrinology Division, Miami, Florida, USA

The electronic version of this article is the complete one and can be found online at: http://www.jissn.com/content/4/1/4
Received:     14 June 2007
Accepted:     23 July 2007
Published:     23 July 2007

 

© 2007 Kalman et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.