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Written by Michael J. Rudolph, Ph.D.
19 September 2016

16NN226-soy

Protein Wars: Soy vs. Whey

Which Creates Greater Muscle Growth?

 

The Anabolic Response

Weightlifting combined with protein supplementation enhances the anabolic response, generating greater levels of muscle growth and strength.1 Hormones play a substantial role in this anabolic response, promoting muscle hypertrophy2,3 with testosterone being one of the more crucial hormones driving muscle growth in response to intense resistance exercise.4,5 As for the many different types of protein consumed to trigger muscle growth, soy protein is one of the more popular ones, primarily because it is a rapidly absorbed protein6that may stimulate muscle growth. The popularity of soy protein also stems from its ability to reduce the incidence of diseases such as coronary artery disease7 and certain cancers such as breast cancer.8,9

Interestingly, soy protein’s disease-fighting qualities are partially due to its potent estrogenic-like properties, implying that the use of soy protein for muscle growth may not be optimal, especially for men. More specifically, concern over the reportedly negative influence soy protein has on muscle anabolism comes from a class of compounds found in soy known as isoflavones10that possess estrogenic-like properties11, which diminish the production and activity of anabolic hormones such as testosterone.

On the other hand, whey protein is another well-known protein source that does not possess any estrogenic-like qualities, ostensibly making it a more optimal choice for muscle growth. Whey is also superior to soy for muscle growth because it contains a higher amount of the branched-chain amino acid leucine, the most influential amino acid for promoting muscle growth.12

Whey’s superior stature as a protein source to potentiate muscle growth has been corroborated by several studies demonstrating that whey protein accelerates muscle protein synthesis13, especially in response to resistance exercise.14 Furthermore, whey protein activates a class of hormones secreted by the intestines known as incretins that minimize hunger, conceivably making it easier to cut calories for greater fat loss. Altogether, these qualities suggest that whey is a better protein source that can simultaneously bolster muscle size while driving fat loss.

 

Soy Protein Decreases Testosterone While Increasing Cortisol

In contrast to prominent misconceptions, soy protein supplementation does not appear to diminish anabolic hormone activity by directly increasing estrogen levels. However, the concerns regarding the estrogenic-like effects from soy consumption, that prevent muscle growth, have been validated by recent scientific work15 that incontrovertibly demonstrates soy protein reduces testosterone production following resistance exercise. The authors suggest that this decrease in testosterone is due to the isoflavone compounds in soy inhibiting a key enzyme involved in testosterone synthesis known as 17-beta hydroxysteroid dehydrogenase.16

This study also showed that consuming whey protein decreased the production of the catabolic hormone cortisol after exercise, while soy protein did not reduce cortisol production. Because elevated cortisol levels can lead to muscle protein degradation and muscle wasting17, the inability of soy to reduce circulating cortisol favors muscle loss while whey protein, which reduces cortisol production, hinders the loss of muscle mass. Importantly, there were no significant differences in estrogen concentrations between soy and whey-consuming groups— again refuting the fallacy that soy protein consumption leads to estrogen production.

Taken together, these results signify a diminished anabolic environment after consuming soy protein as opposed to whey protein, which improves muscle-building conditions after lifting weights.

 

Soy Consumption Inhibits Testosterone Activity and Could Increase Estrogen Function

The consumption of soy protein is thought to contribute to muscle loss as a result of the estrogenic-like properties of soy isoflavones— either through reducing the production of endogenous androgens like testosterone as previously mentioned, or by negatively influencing the activity of testosterone. The biological activity of testosterone is strongly influenced by the sex hormone-binding globulin (SHBG) protein, which binds to the testosterone molecule, rendering it inactive.18 Since testosterone circulates in the bloodstream in the inactive form when bound to SHBG, the ability of soy protein to increase SHBG levels should decrease testosterone activity. This was clearly shown in one study19, where researchers replaced meat protein with soy protein— which produced an overall decrease in testosterone activity that corresponded to an increase in SHBG. Even more importantly, since SHBG has a greater affinity for testosterone than estrogen20, this increase in SHBG preferentially inhibited testosterone over estrogen, leading to greater levels of estrogen activity18, further minimizing the potential to augment muscle mass.

While reduced levels of active testosterone accurately reflect the adverse influence that soy protein has on muscle growth, evaluating androgen receptor expression patterns provides additional evidence concerning the role of soy as an anti-anabolic agent21 since the androgen receptor mediates testosterone activity. One study in particular found that consumption of isoflavone-rich soy protein for six months lowered androgen receptor expression levels considerably.22 In addition, the researchers showed that consumption of isoflavone-free soy protein did not significantly lower androgen receptor expression, yet still caused some reduction in receptor— suggesting that the isoflavones alone may not be responsible for the reduction in androgen receptor expression.

 

Whey Protein Suppresses Hunger More Efficiently Than Soy

Dietary protein can play an important role in bodyweight regulation by suppressing appetite. However, the satiating effect can vary based on the different influence each protein has on certain hormones following consumption. In particular, glucagon-like peptide GLP-1, a gastrointestinal hormone that has been previously shown to play an important role in reducing hunger23, was recently shown to be abundantly produced in response to whey protein consumption— whereas soy protein had no influence on GLP-1 levels.24

In line with increasing GLP-1, whey consumption also reduced hunger more effectively than soy. The increase in GLP-1 concentration after whey consumption can be explained, to some extent, by the finding that whey inhibits the enzyme dipeptidyl peptidase-IV, which degrades GLP-1, thus increasing GLP-1 levels and prolonging the hunger-reducing action of this hormone.25

In summary, whey protein is seemingly a better protein source for muscle growth compared to soy protein, based on the remarkably different influence each protein has on the hormonal response after consumption. While whey protein increases the anabolic to catabolic hormone ratio while simultaneously boosting testosterone activity, soy protein has precisely the opposite impact— which presumably reduces the muscle-building influence typically associated with post-workout protein consumption.

 

For most of Michael Rudolph’s career he has been engrossed in the exercise world as either an athlete (he played college football at Hofstra University), personal trainer or as a Research Scientist (he earned a B.Sc. in Exercise Science at Hofstra University and a Ph.D. in Biochemistry and Molecular Biology from Stony Brook University). After earning his Ph.D., Michael investigated the molecular biology of exercise as a fellow at Harvard Medical School and Columbia University for over eight years. That research contributed seminally to understanding the function of the incredibly important cellular energy sensor AMPK— leading to numerous publications in peer-reviewed journals including the journal Nature. Michael is currently a scientist working at the New York Structural Biology Center doing contract work for the Department of Defense on a project involving national security.

 

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