Soy: A Bodybuilder's Friend Or Foe?

In this article, I will assess the pro's and con's of soy-based foods and disseminate research studies and expert opinions to arrive to a strong and formidable conclusion about soy being a bodybuilder's friend or foe.

Common word of mouth and some studies have it that soy-based foods contain estrogenic properties that will halt anabolic and growth processes in bodybuilders. Unfortunately, the problem with information is that there are two kinds: information and misinformation.

In this article, I will assess the pro's and con's of soy-based foods and disseminate research studies and expert opinions to arrive to a strong and formidable conclusion about soy being a bodybuilder's friend or foe. Often times in the fitness industry, hot new trends get a spin.

For instance, one can find any one positive factor and put a spin on it to make it seem like a good thing. Obesity for example, we know it is detrimental to overall well-being, however, one could argue it is an effective means for preventing osteoporosis.

This same kind of spin has occurred with regards to soy. However, I am going to start this in-depth analysis as Bill O'Reilly starts his show The Factor with,

You are about the enter a no-spin zone.
The spin stops here!"

Adversaries propose the notion that plant estrogens called phytoestrogens (the chemical name is isoflavones) reduce testosterone levels and they also make the association that soy brings about feminine characteristics with excess estrogen levels.

It is highly probable that these same cynics are not natural bodybuilders, which is a conundrum in itself considering excess testosterone (anabolic steroids) actually induces real life feminine characteristics in men (Birkeland et al. 1994).

Furthermore, this is just folklore at best that soy-based foods or any foods that have estrogenic properties are associated with feminine characteristic or hormonal outcomes. Bottomline: It is all hogwash.

Now, the support for this magical food source that I am about to provide will be scientifically, logically and rationally-based and it will literally shock you as to how good soy foods really are.

What follows is an unbiased view from both sides on the issue. This is uncommon on most internet articles or even paper-copy articles of which most writers will only cite references to support their claim. However, in order to be fair in the critique and to avoid implications of bias, I will provide arguments from both sides.

Ultimately, a verdict will be reached regarding each claim based on the reliability, credibility, and logical inference and deduction from the supporting evidence.

Claim One

Claim: Soy Suppresses Testosterone

Supporting Evidence:

Some studies have shown that soy consumption is associated or inversely related to suppression of testosterone levels (Dillingham et al., 2005; Gardner-Thorpe et al., 2003; Habito et al., 2000; Nagata et al., 2000, Ziesel et al's study [as cited in Squires]).

Rebuttal: Soy Does Not Significantly Suppress Testosterone

Supporting Evidence:

Some studies have also shown that soy consumption is not associated or inversely related to suppression of testosterone levels (Kurzer, 2002; Maskarinec et al., 2006; Mitchell et al., 2001; Nagata et al., 2001).

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Does Soy Suppress Testosterone?

The Dillingham et al. (2005) study was meant to investigate soy protein's strong association with reducing prostate cancer risk by modulation of serum hormones. They made no such implication of the modest testosterone reductions being associated with feminine traits or negative health consequences. However, they did argue for soy having favorable effects on prostate health in men.

The Gardner-Thorpe et al. (2003) study also found soy to have advantageous effects against prostatic disease and heart disease. The idea of these testosterone reductions making these men less manly or smaller was not indicated.

Interestingly, in the Nagata et al. (2000) study, they may have concluded the inverse relationship of soy intake and testosterone levels, but they also stated that these correlations were of borderline significance.

In fact, in a study published 1-year later, Nagata et al. (2001) found no change in testosterone with soy consumption. In the Maskarinec et al. (2006) study, it was suggested that the reduction in prostate cancer risk could be regulated by a non-hormonal mechanism.

Thus, soy does not reduce testosterone concentrations, but actually lowers a man's risk of prostate cancer.

Furthermore, the studies that found testosterone levels to slightly decrease classified the relationship as correlation or association. This does not mean that the outcome was based on cause. In correlation, it is not a cause-effect outcome, rather, it is just a relationship, but the logic to that relationship is not fully understood.

In other words, who is to say that these lower testosterone levels in these subjects was not a result of eating a diet lower in saturated fat and higher in dietary fiber? It is commonly known that low-fat and/or high-fiber diets are correlated with lower testosterone levels (Berrino et al., 2001; Dorgan et al., 1996; Hamalainen et al., 1983; Hamalainen et al., 1984).

However, with a bodybuilder who includes variety in the diet such as eating lean red meat, dairy products and poultry in moderation in conjunction to proper intense training, soy's modest testosterone reducing effects would be counterbalanced, but the prostate and other health benefits would be reaped.

Another factor to investigate in the testosterone suppression claims is the methodology of their supporting studies. Some of these studies had their participants either exclusively consume soy (Habito et al., 2000; Zeisel et al's study [as cited in Squires]) OR they mega dosed on soy to isolate the effects of soy (Habito et al., 2000; Ziesel et al's study [as cited in Squires]).

In Habito et al's (2000) study, the only difference was that in one group subjects had 150 grams of lean meat protein, in the other group; they had 290 grams of tofu. Nearly double the amount of soy protein in comparison to the lean meat protein.

Again, in the real world, most people would not just eat soy-based foods in exclusion of others, nor would they mega dose. Hence, these findings have limited ecological validity (real-world application).

Squires (2004) reported the following:

Most concerns about soy have centered on the fact that it is a rich source of isoflavones, substances that mimic the effects of the female hormone estrogen.

To determine what these plant-based chemicals might do, Steven Zeisel and his colleagues at the University of North Carolina at Chapel Hill fed mega doses of soy to men as part of a recent National Cancer Institute study.

Nipple discharge, breast enlargement and slight decreases in testosterone occurred with the mega doses. But, "We still couldn't find anything that was serious, and we went up to doses that are probably 30 times what you could get from normal foods," Zeisel said. "I don't think that there are a lot of estrogenic worries. Your testicles will not shrink and you won't have massive breast enlargement" from eating soy.(p. HE01).

Conversely, ones testicles would shrink from long-term use of anabolic steroids.

Verdict Of The Claim: Guilty On All Counts

  1. Distortion and bias of research findings to sell or promote meat and dairy proteins as the be all.
  2. Not viewing both sides of the issue, but instead only citing studies that support the claim.
  3. Extrapolating insignificant findings to support an irrelevant claim.
  4. Putting a spin on the findings by making an extrapolation or blind inference into hormonal alterations of which were actually found to promote better health.

Claim Two

Claim: Soy Protein Does Not Aid In Muscle Growth

Supporting Evidence:

One study using pigs as subjects, suggested that soy protein intake induced protein degradation (Lohrke et al., 2001).

Rebuttal: Soy Protein Aids In Muscle Growth

Supporting Evidence:

One study using rats as subjects, suggested that soy protein prevented protein degradation (Nikawa et al., 2002). Three studies actually looked at the effects of supplementing with animal protein vs. soy protein during a resistance training program in humans (Brown et al., 2004; Candow et al., 2006; Haub et al., 2002).

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Does Soy Consumption Aid In Muscle Growth?

In Haub et al's (2002) study, it was shown that there were no significant differences in strength, hypertrophy, muscle cross-sectional area, or muscle creatine levels when comparing meat protein vs. soy protein consumption during resistance training (Haub et al., 2002).

Unbelievably, I came across an anti-soy article online that distorted the findings indicating that there was a 30% increase in muscle cross sectional area compared to soy!

Personally, I do not know where this interpretation came from because I actually looked at the full-text article and the authors made it statistically and graphically clear that there were no significant differences in muscle cross-sectional area and concluded that the key factor in regulating muscle strength and hypertrophy was adequate protein intake, independent of the protein source (Haub et al., 2002).

In Candow et al's (2006) study, it was concluded that protein supplementation increase muscle strength and size, independent of the protein source.

In another study looking at the effects of a whey and soy supplemented diet in mice found that both protein sources had positive effects on antioxidant status and resulted in enhanced body composition (Elia et al., 2006).

Another study showed that skim milk led to greater muscle growth compared to soy protein (Wilkinson et al., 2007). However, it was found that milk and soy each led to a positive net protein balance (Wilkinson et al., 2007).

Therefore, as I said earlier, skim milk certainly is a great source of protein and it has its place in my diet, but, so does soy. In the Brown et al (2004) study, a comparison was made between whey and soy protein bars and their influence on lean body mass in college males in a weight training class.

It was found that both protein sources led to greater gains in lean body mass, however, only the soy protein preserved two aspects of antioxidant function, whereas the whey protein did not counteract the oxidative stress of exercise as effectively (Brown et al., 2004).

Remember that when it comes to making gains in the gym, it is not necessarily the workouts that promote growth; workouts stimulate growth, while nutrition determines how effective the response is to the stimulus. Thus, with the inclusion of soy, one may see greater and better recovery time, which would mean one could see better gains in the gym.

Squires (2004) reported the following:

As a protein source, soy ranks at the top of the list. But no need to take massive doses of soy protein supplements or powder to build more muscle. Eating a soy burger, a handful of soy nuts or a soy smoothie after a weight training session is plenty to help repair muscle, according to Jeff Potteiger, an exercise physiologist at Miami University in Ohio (p. HE01).

Now, I know many may disagree with this report above with soy being at the top of the list, I do as well. I personally believe whey, casein, and branched chain amino-acids (leucine in particular) to be at the top of the list.

In fact, it is well-known that whey and casein protein proteins are the best absorbed and provide the best results (Hansen, 2005). Furthermore, Hansen (2005) had no negative things to say about soy protein. He just mentioned that fact that since soy is derived from plant sources, it is an incomplete protein and may not be digested or absorbed as well as milk proteins (Hansen, 2005).

However, if soy protein is taken in combination with whey and/or casein, the protein quality or lack of essential amino acids are no longer issues (Hansen, 2005; Margen et al., 1991). Thus, the point being made here is that including soy as a protein source can add variety to one's diet and protein-source arsenal.

Verdict Of The Claim

This 30% difference claimed by this anti-soy writer is similar to saying 3/10 people saw better gains with the meat protein, which is not even significant looking at from a common-sense point of view. However, as a well-informed and critical reader of information, one should certainly respect the statistical level of significance of 0.05 when interpreting findings.

So, in this study (Haub et al., 2002), there were no significant differences, thus one can slice the results any which way, but at the end of the day, no significant differences. Therefore, one needs to question the motives (i.e. personal bias, selling something, protection of product sales) for certain individuals taking results from research studies out of context.

Paul and Elder (2001) suggested the following excerpt for identifying questionable professional thinking:

This professional seems to be viewing the situation in a questionable manner because... (here you are trying to determine whether the professional seems to have been influenced by some vested interest to exclude relevant information or whether the professional was simply engaging in poor-quality thinking because of naiveté or similar reason (pg. 303).

Claim Three

Claim: Soy Protein Is Associated With Testicular Cell Death And Infertility

Supporting Evidence:

Some studies have suggested that one of the compounds in soy (genistein) induces testicular death and infertility in laboratory animals and human cells in test tubes (Anderson et al. 1997; Casanova et al., 1999; Hopert et al., 1998; Kuma-Diaka et al., 1999; Setchell et al., 1987; Strauss et al., 1998).

Rebuttal: Soy Protein Suppresses Cancer Growth

Supporting Evidence:

Many studies have shown that soy consumption actually deters proliferation of cancer cells (Geller et al., 1998; Hillman et al., 2001; Santibanez et al., 1997; Squadrito et al., 2003).

A preponderance of studies have also shown soy to be beneficial in prevention of heart disease, prostate cancer, diabetes, and osteoporosis (Colacurci et al., 2005; Jacobsen et al., 1998; Omoni & Aluko, 2005; Taku et al., 2007; Tikkanen et al., 1998; Vitolins et al., 2001; Wiseman et al. 2000; Zhan & Ho, 2005).

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Does Soy Protein Suppress Cancer Growth?

Verdict Of The Claim: Guilty As Charged

Sometimes in research, findings have statistical significance, but this does not mean they have practical significance.

Gay & Airasian (2003) stated the following:

The fact that results are statistically significant does not automatically mean that they are of any educational value, that is, that they have practical significance...

Statistical significance only means that your results would likely occur by chance a certain percentage of the time, say 5%. This only means that the observed statistical relationship or difference is probably a real one, but not necessarily an important one (pg. 495).

Interpreting findings from animal studies is in of itself an irrelevant inference.

Brody (1998) reported the following:

Though nearly all that is known about the cancer risk of chemicals in foods comes from studies of laboratory animals exposed to very high doses of each suspect chemical, the committee urged caution in drawing conclusions from animal studies about cancer risks in humans.

Animal studies have limitations, the report pointed out, because the bodies of animals and people might handle a chemical differently, and the effects of ingesting a single substance in isolation may differ from the effects of consuming it as part of a varied diet. Also, an animal study uses very large doses of a chemical that people are exposed to in only minute amounts (pg. 184).

The committee referred to in the above excerpt was the Research Council Committee, and the report was "Carcinogens and Anticarinogens in the Human Diet," which was based on an exhaustive review of scientific reports and other relevant information (Brody, 1998).

The Benefits Of Soy

For the intensive purposes of this discussion, I will use soy milk as the common example reference food. Furthermore, I will use milk as the opposing example reference food. This is an important comparison as it is relevant to the concept of this article.

In past years, milk has always been referred as a man's kind of food source with the media mentioning that milk does a body good, milk is good for bones, and that milk will make one grow big and strong. However, I am in no way making the claim that whey and casein are not good protein sources, nor that milk in moderation is not good. I am simply using both food sources as an analogy to symbolize soy as those supporting soy and milk for those against soy

First of all, it was Arnold Schwarzenegger himself who became a believer that a vegetarian could build big muscle and become a champion bodybuilder (Schwarzenegger, 1998). This vegetarian goes by the name of Bill Pearl. Accordingly, Soy milk has several benefits worth investigating.

A Good Protein Source For Bodybuilders

Soy milk has a good amount of protein per serving (~7 grams per cup). When one thinks of soy-based foods, do me this favor. Check the protein content in the ingredients please. In most cases, you will find a wonderful macronutrient profile which is mostly protein, complex carbs, and healthy fat with fiber to boot!

Additionally, there are different brands of soy milk that contain different concentrations of carbohydrates, fat, total calories, and fiber. Research has shown that the lower carbohydrate and higher fiber brands have lower glycemic and insulinemic indices (Torres et al., 2006).

Low Glycemic Index and Insulinemic Index and Soy's Fat-Burning Effects

A lot of people know skim milk has a low glycemic index, but many do not know that skim milk has a high insulinemic index (Ostman et al., 2001).

In other words, although blood glucose levels stay normal, milk spikes insulin. And if we recall, insulin is a potent inhibitor of fat oxidation (Manore & Thompson, 2000; McArdle et al., 2001). However, soy milk has been shown to have both a low glycemic and insulinemic index (Blair et al., 2006; Torres et al., 2006). Thus, soy milk allows one to stay in fat-burning mode longer.

In addition, there have been studies that have actually found soy protein intake to be associated with improved fat oxidation markers (Morifuji et al., 2006). In fact, it has been found that soy protein leads to greater reductions in body fat content and blood glucose levels compared to casein (Nagasawa et al., 2002) or whey protein (Aoyama et al., 2000).

Environmental Enhancement

As if the aforementioned reasons are not good enough, soy milk is even involved in saving our planet! Some products are proud to claim their products to be made by 100% wind energy (Helping the Environment, 2007). Thus, soy indirectly helps in preserving the atmosphere and preventing further global warming.

Antioxidant Capacity And Healthy Fats

For this one, I'd like to borrow a line from Ocean's Eleven. Remember when Brad Pitt was calmly and confidently making demands to the Casino owner on his cell phone in the casino? The line that comes to mind is, "I got two words for you, mini-bar." Well, I got two words for you, "Iso-flavones."

Isoflavones have favorable effects on reducing prostate cancer, heart disease, and cholesterol (Taku et al., 2007; Vitolins et al., 2001; Zhan & Ho, 2005). Incredibly, soy milk also has alpha-lipoic acid (ALA).

Some soy milk products contain upwards of 300mg of ALA omega-3's; a serving of 2% milk contains only 20mg of ALA omega-3's. Research has shown ALA to aid in the fight against free radicals and oxidative stress (Packer et al., 1995; Packer et al., 1997).

Tastes Great

Sorry, but this is true. Personally for me anyway, I find soy milk to taste so much better than milk. However, to provide better proof than personal opinion, soy was the proud recipient of the 2007 American Culinary Chefs Best Award for Best Taste.

Forum Threads:

  • Soy Milk or Skim Milk? - Started By DCpapi07
    "Which is better for mucle growth?"
  • To all Soy-haters: let's have a look at milk. - Started By Dr.P
    "The interesting thing is that cow milk may very well have some estrogenic impact as it apparently contains real estrogens: estrone and estradiol. the total amounts are not very high and apparently they are closely related to the fat fraction of the milk."
  • How's soy milk compare to regular milk? - Started By Someguy150
    "I've been using Silk Enhanced (more vitamins) soy milk for weeks instead of regular milk now. It tastes similar or better to me, and the nutritional value (to skim milk) is also very similar."

Arnold's Exact Words On Milk

In the movie pumping iron, Arnold replied to a spectator about milk. The spectator asked, "Do you drink milk?" Arnold replied, "No I drink no milk." "Milk is for babies, when you grow up you have to drink beer." So, as Arnold said, Milk is for babies.


Honestly, Soy sounds like a bodybuilder's best friend to me, based on all these nutrient values and benefits.

  • Low sodium AND high potassium which equates to a good pre-contest food. Interestingly, soy milk has just about the perfect balance/ratio of these two electrolytes.
  • Low caloric value. A cup of light plain soy milk has 70 calories per cup.
  • Low glycemic AND insulinemic index means one will stay in a fat-burning state with lower insulin levels and higher glucagon levels.
  • High protein source, dare I say more about this being important to bodybuilders?
  • Heart health. Often, bodybuilding is associated with bad health and for good reason. With the unfortunate widespread use of anabolic steroids, diuretics, and testosterone prohormones, it is no surprise that bodybuilding has sort of gone back into the closet.

In fact, one recent study (Brown et al., 2006) found prohormones do not even produce any ergogenic or anabolic effects in men, but it found that the use of these supplements can actually raise the risk for certain negative health risks. However, with the advent of natural bodybuilding, bodybuilding can be one of the healthiest endeavors one can partake.

If done the right way, one can eat healthy with variety, balance, and training that includes ever-important cardio. The logic to natural bodybuilding is: You are what you eat AND you are what you do. It is not, you are what you take.

The answer to this debate is quite simple; Everything in moderation. In my personal opinion, when we receive exogenous testosterone (anabolic steroids), this sends the signal to the body that there is an abundant supply of testosterone coming from the outside, so the body shuts off endogenous testosterone production.

This is why many former anabolic steroid users shrink in body size so substantially, that you can't even recognize them later in life. Their bodies stop producing testosterone naturally, which by the way confirms the common side effect of shrinkage of the testes.

On the other hand, when there are low levels of testosterone in the body, this sends the signal for the body to actually increase testosterone production. Thus, one may deduct that if soy products actually incur minor reductions in testosterone, this may actually lead to an increase in endogenous production of testosterone! Who would have thought, huh!?

The key to this entire riddle is everything in moderation, which leads to balance (homeostasis). Personally, whey and casein are the top protein sources one can consume, lean red meat and dairy products have many benefits and I personally consume these food sources on a regular basis.

However, the point of this article was to understand that soy protein has several benefits and can be included as part of a regular diet. So the next time you read or hear of someone saying soy halts gains in the gym, just smile to yourself and continue walking with that impeccable truth of knowing you are creating variety in your diet (via not blogging about how it sucked to eat tuna every other meal) and you are doing your body and health good while still winning bodybuilding shows.

And finally, I'd like to revise Arnold's notion and finish with, "Milk is for babies, when you grow up you have to drink soy milk."


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Copyright © Ivan Blazquez, 2007. No part of this publication may be reproduced or transmitted in any form without the prior written permission of the copyright holder and author of this publication.