Should Soy Be a Part of Your Muscle Building Diet?

Soy derived protein is currently being sold as active ingredients in many bodybuilding supplements. You’re probably even taking soy protein as part of your muscle building diet without knowing it.

Not sure? Here’s a list of food sources where soy is commonly found in considerable concentration: tofu, vegetable soups, veggie meats, some sauces, baked goods, desserts, smoothies and shakes, some protein bars, and of course some bodybuilding supplements.

But the thing is, there’s this really confusing issue about soy.

First it’s touted as a miracle food having complete protein, then next it’s bashed as bearer of highly toxic chemicals that will reduce your sperm count and shrink your balls. Then it’s back to being the answer to preventing all sorts of diseases and ailments, only to be accused again of being carcinogenic!

So what’s the real skinny on soy?

Should soy protein be a part of your muscle building diet, or are you better off leaving soy on the shelves?

Considering the fact that there are seemingly reputable researches shouting from either side of the 2 camps, how do you know whose “research” to believe?

Let’s consider what each camp is saying, then we can evaluate for ourselves.

Soy – a Sneaky Muscle-killing Poison in Our Food Supply?

So here are the cases against soy that point to it being taken off as part of your muscle-building diet:


  • Soy isoflavones and Soy protein isolate don’t have GRAS (Generally Recognized as Safe) status. Archer Daniels Midland (ADM) recently withdrew its application to the FDA for GRAS status for soy isoflavones following an outpouring of protest from the scientific community. The FDA never approved GRAS status for soy protein isolate because of concern regarding the presence of toxins and carcinogens in processed soy.
  • Soy foods contain trypsin inhibitors that inhibit protein digestion and affect pancreatic function. In test animals, diets high in trypsin inhibitors led to stunted growth.
  • Phytic acid in soy foods results in reduced bioavailabilty of iron and zinc which are required for the health and development of the brain and nervous system.
  • Megadoses of phytoestrogens in soy formula have been implicated in the current trend toward increasingly premature sexual development in girls and delayed or retarded sexual development in boys.
  • A British government report concluded that there is little evidence that soy foods protect against breast cancer or any other forms of cancer. In fact, soy foods may result in an increased risk of cancer.
  • Soy consumption enhances hair growth in middle-aged men, indicating lowered testosterone levels. (Read: lowered testosterone levels = decreased muscle building capacity)

The truth is, soy is the last place protein source to consider if you are a bodybuilder looking for a good muscle mass gaining diet. Why, you may ask? First of all, you have to think of not only how well protein sources are digested, but how they are also assimilated and utilized after they are digested by the body. Did you know that when matched up against each other, a hydrolyzed soy isolate would still fail to deliver against the protein from a plain ol’ glass of milk? According to an illegal study, hydrolyzed soy protein was given a biological value of 87% with a net protein utilization of about 78%. While they may appear decent numbers, simple milk concentrate has a 95% biological value rate and a net protein utilization of 85%.

One of the negative effects of soy protein, particularly soy isolate, would be its ability to slow down the rate of protein synthesis and its inhibitory effect on myoblast production, maturation and activity5. When the protein synthesis rate is reduced, it lowers the amount of protein accumulated as well as the rate of muscle growth. Myoblasts, however, are the ones that actually lay down the newly formed proteins and create the tissue.

In addition, soy has a suppressing effect on your body’s immune system, destroying the immune tissue and lowering the T-lymphocyte production by over 80%. It also shuts down production of the T4 thyroid hormone that contributes to an efficient metabolism, making it an all in all bad choice of protein supplement for bodybuilders.

To further back these facts up, a study published online by Human Reproduction (July 24, 2008), Europe’s leading reproductive medicine journal, shows men who consume an average of half a serving of soy food a day have lower concentration of sperm than men who do not consume soy foods.

This association was marked especially in overweight or obese men. Spearheaded by Dr. Jorge Chavarro, 99 men, who attended a fertility clinic for evaluation of their sub-fertility between years 2000 and 2006, had to take 15 soy-based foods which were then analyzed by the Dr. Chavarro and his colleagues after. The foods were comprised of tofu, tempeh, tofu or soy sausages, bacon, burgers and mince, soy milk, cheese, yoghurt and ice cream, and other soy products like roasted nuts, drinks, powders and energy bars. The study found that men who ate the most soy food had 41 million sperm per millilitre less than men who did not consume soy products. Note that the “normal” sperm concentration for men ranges between 80-120 million/ml.

Soy – the Miracle Food

On the other hand here’s what the PRO-Soy camp are saying:

In a study offered at the American College of Sports Medicine (ACSM) Meeting last May 2004, which was sponsored by the US National Dairy Council, soy and milk proteins are both effective in supporting muscle building, suggesting that soy is a good post-workout nutritional drink. According to the study, soy improves athletic performance, and the isoflavones found in soy protein generate antioxidant effects, which hasten up recovery period and reduce muscle soreness and inflammation.

According to Douglas S. Kalman, M.S, R.D., Director of Nutrition & Applied Clinical Research at Miami Research Associates:

“We believe that this study validates that soy protein is safe and just as effective as whey protein in helping exercising males achieve their fitness goals and supports the development of lean muscle mass.”

As another result of the study, it was established that soy, like whey protein, does not have a negative impact on the testosterone levels in men. In addition to that, the study found that soy provides a full set of amino acids that benefit the exercising muscles, making them even larger and stronger. Such findings contradict the previous ones I’ve mentioned above.

Actually I can cite loads and loads of “researches” done that state exactly the opposite of what the Anti-Soy camp are saying above, so I’m not gonna bother.

So who are we going to believe?


Now I know this could sound lame but I think in this case, I would advise not to believe everything you read. As always, use your common sense, and I think the best way really is to listen to your body. I’ve found that as I ate less processed foods and stuck to wholesome, simple, freshly prepared  meals, and as I practiced more awareness I started to intuitively know/sense how certain foods affect me – my moods, my health and my energy balance.

For example, I personally react to soy milk (it’s known to be a highly mucous-forming food, and this is the case with me), so I avoid it. When I go out with friends and I eat crap stuff like fast food, I almost always get a headache from all the MSG or an upset stomach.

One could easily say I’m a wuss for being too sensitive, but I actually regard having that sensitivity as a good sign because I immediately receive feedback about what’s good or bad for my body.

So my personal opinion about this whole soy debate? Consume it in very little amounts, or just leave it altogether – just in case!

For a list of bodybuilding supplements I recommend, click here.




Founder & Fitness Nerd

Founder of Muscle4Hardgainers. Loves to experiment and teach others the best ways to reach their physical goals.

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