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Updated: July 26, 2011      

In this on-going series, Dr. Layne Norton will focus on clearing up some of most egregious B.S. that surrounds protein intake. For the better part of a decade, Layne has been researching protein metabolism as it pertains to building muscle. The emphasis of his PhD thesis was on dietary protein intake, quality and its influence on muscle metabolism and body composition. He has also put his methods into practice, getting himself and hundreds of clients into razor-sharp shape. He knows the difference between something that looks good on a PubMed abstract but that won’t work in the real world. Layne is going to use his integrated research and practical knowledge to destroy the myriad of myths that surround this misunderstood muscle-building macronutrient. Get ready.

PROTEIN MYTH: 

“If you consume a lot of soy protein you will soon have the testosterone levels of a Miss Universe contestant and the pecs of Sofia Vergara.”

PROTEIN TRUTH:
“Soy isn’t nearly as feminizing as people believe, but you probably shouldn’t have too much of it.”

Soy Story:

In recent years soy protein has garnered a lot of public support. In fact, mainstream media stories touting the health benefits of soy have spawned a generation of soy-based products that claim to help extend lifespan and cure everything from cancer to heart disease. However, significant concerns were raised in the bodybuilding world with soy, namely its potential negative effects on anabolic hormones and the muscle building process. In this article we break down the good, the bad, and the ugly of soy protein.

The Good: 

Soy protein has been demonstrated to activate AMP Protein Kinase (AMPK), a complex that enhances insulin sensitivity and glucose tolerance, mostly by increasing GLUT-4 (a glucose receptor responsible for insulin stimulated glucose disposal in muscle) content in the muscle cell (1). There is quite a bit of evidence to suggest that supplementation with soy protein can increase insulin sensitivity and may even help inhibit fat gain in animal models of diet-induced obesity (2,3). Furthermore, there is evidence that soy protein can help improve blood lipid profiles and reduce the risk of heart disease (4).

The Bad: 

Probably the biggest knock on soy is that it lowers testosterone. Certainly there is some evidence that soy isoflavones can lower testosterone significantly in some animal models (5). In humans, there is some evidence that soy supplementation can modestly reduce testosterone in healthy men (6-8) and even reduce androgen receptor expression (9). These studies however, only found small reductions in testosterone. Furthermore, several studies have shown no impact on soy protein supplementation on testosterone levels (9,10). Perhaps the most relevant study to this discussion was published in the Journal of the International Society of Sports Nutrition, which detailed subjects who followed a weight training program for 12 weeks while supplementing with 50g a day of soy isolate, soy concentrate, whey protein, or a blend of whey and soy (11).  At the end of the study all groups experienced increases in lean body mass but there were no significant differences in testosterone, body composition, or increases in lean body mass between the groups fed whey or soy. This seems to suggest that in moderation, soy protein does not appear to cause significant reductions in testosterone.

The Ugly:

Although the researchers in the above-mentioned study found no differences between soy and whey for changes in lean body mass, there is data indicating that soy does have an inhibitory effect on an anabolic signaling pathway in muscle cells called the mammalian target of rapamycin (mTOR). Think of mTOR as the “switch” that turns on protein synthesis in muscle. A study published in the Journal of Nutrition demonstrated this as mice fed soy had lower levels of mTOR activation than those fed whey protein (12). This was later confirmed by data in humans where whey protein was significantly more anabolic than soy (13). This is not real surprising however given that whey has about 40% greater leucine content than soy, and leucine is the amino acid responsible for stimulating protein synthesis in muscle. Further support for soy’s ‘anti-anabolic’ effects were found in a study done at my old lab at the University of Illinois, demonstrating that subjects consuming soy protein had far less lean body mass and muscle at the end of 11 weeks compared to subjects who consumed equal amounts of egg and whey protein (14). Various other studies have determined that soy is inferior to milk proteins in terms of anabolism (15, 16)

The Wrap-Up:

While soy protein may not cause the spiral crash in testosterone levels as previously thought, it appears that it can impair anabolism if over-consumed. I’m not saying you should never ever consume soy. In moderate amounts it is most likely fine. But if you’re looking to supplement with a protein powder, whey is most likely preferable to your goals given that many of the health benefits from soy can also be obtained by using whey protein, which is significantly more anabolic as well (14,17-19).

References

1. Jessen N, Pold R, Buhl ES, Jensen LS, Schmitz O, Lund S.  Effects of AICAR and exercise on insulin-stimulated glucose uptake, signaling, and GLUT-4 content in rat muscles.  J Appl Physiol. 2003 Apr;94(4):1373-9.

 
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