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The Skinny On Soy!

For the past few years I have taken upon my self to learn as much as possible about the human body, both from a structural and a bio-chemical stand point. Here's a look into soy!

As I have stated in the past there are a few things that seem to linger around longer than they should in the bodybuilding world, things that seem to have more than one side to the story. Soy is one of those things that we can add to the list.

First Off

First its bad for bodybuilding, then its good for bodybuilding, but then its good for health, but humans don't assimilate plan protein well. And it goes on and on, back and forth. Well I have taken the time to piece together yet another stack of research and studies to come to a final conclusion.

What is that conclusion you ask? My conclusion is that for the bodybuilder looking to take advantage of the vast advancements in protein science to pack on as much muscle as possible, Soy is the last place to look. But as always, with any claim I make, let me explain.

Firstly when we talk about protein sources we must look at not only how efficiently our body can digest it, but also how well we can assimilate and utilize what we do digest. That's sounds simple, but it seems that every week there is a new method of measuring these factors. The biological value, the protein efficiency ratio, the protein corrected digestibility amino acid score, and the chemical score.

Each of these does have a slightly different focus on protein metabolism and each does serve a purpose. Most recently, the PCDAAS is the method of choice. However, it does come with inherent flaws. The biggest being that it looks at the protein content of the fecal matter.

This is bad because the gut tube is regenerating at an extremely rapid rate and dumping dead cells (protein matter) and this will invariably affect the end results. So for the simple fact that the PDCAAS overestimates the body's ability to utilize protein, I will not follow this method.

I feel that what is important is the biological value and the net protein utilization value. And secondly, these measurements need to be drawn in the end of the lower intestine so we can see exactly how much has been absorbed at the end of the digestion process. This portion of the intestine is called the Ileum and it is the part where food material resides just before entering the large intestine.

The Rate

When this method of assessment is used, recent research has demonstrated that previous research was a tad off the mark. Even when the highest quality of soy proteins was matched against the lowest of dairy proteins, soy still came up a far cry short. That's right, a hydrolyzed soy isolate couldn't stand toe-to-toe with the protein from a plain ol' glass of milk.3 You want numbers, huh?

In an Illegal assessment, hydrolyzed soy protein was given a biological value of 87%, which isn't bad. But humans can only utilize about 78% of it. Whereas simple milk concentrate hits the BV at 95% and a net protein utilization of 85%. Now keep in mind that this is ordinary run of the mill milk protein, we haven't even gotten into high quality whey yet!

Why is this important? For the Bodybuilder looking to pack on muscle mass, protein intake is of the highest concern. However, the quality of that protein is as important as the amount. It's not just a matter of how much you can choke down, but how well your body can use it. So that's one in the loss column for soy.


Next on the list are the Isoflavones found in soy. And yes, soy protein powders do maintain their isoflavone content. In fact, soy protein powders, isolates in particular, have the highest concentrations of them! It has been documented that the various isoflavones have a rather significantly negative impact on muscle development.

Soy appears to slow down the rate of protein synthesis and thus lowering the amount of protein accumulation and accretion (muscle growth). Secondly soy has an inhibitory effect on myoblast production, maturation and activity5. So this is really a triple death for muscle growth. Myoblasts are the "grunts" of muscle building. They are the things that actually lay down the newly formed proteins and create new tissue.

So low protein synthesis and accumulation with slowed myoblast activity and development. That's like trying to build the Buckingham palace with 1 lazy construction worker who doesn't have enough bricks to finish the job in the first place. Now, that's 2 in the loss column for Soy.

Still on the subject of the isoflavones is the matter of the hormonal effects. Or, indirect effects in this case. If you have by chance read Big Cat's articles on prohormones, then you've heard of the enzymes beta-hydroxysteroid dehydrogenase ( both 3B-HSD, and 5B-HSD), and 5 alpha reductase. That's right, Soy has a negative impact on these as well.

These Two Enzymes

These two enzymes are responsible for efficient testosterone production and metabolism. This could put a damper on overall muscle development and could very well put a plug on any prohormone cycle. And that's a lot of money wasted! Did I say could? I take that back…It WILL. What's even worse, is that these isoflavones in Soy seem to create a "Down regulation" of androgen receptors.

So we have a Double-Death this time. Poor conversion and metabolism of testosterone or prohormones and less androgen receptors to pick up what little there maybe floating around to begin with.

And If you are wondering, yes, this has been seen in human subjects as well. In a recent study human subject were fed 2 different diets. One who's protein was from lean meat and the second was an equal amount ( g of protein) of soy. And only in the meat containing diet were increases in bioactive testosterone seen.

By as much as 10% above baseline And the soy diets showed significantly lower levels of the biologically active form. That's right, soy actually lowers the amount of free testosterone in your blood. And this all adds up to a Triple-Death for muscle growth. Lowered anabolic hormones, lack of enzymatic production and down regulation of androgenic receptors. Chalk one more in the loss column for Soy.

Soy's Suppressing

Lastly, Soy has one hell of a suppressing effect on the immune system. In both animal, and humans, soy literally destroys immune tissue and decreases T-lymphocyte production by more than 80%. , It also shuts down production of the T4 thyroid hormone, which is critical in an efficient metabolism .

This could be a very undesirable effect when trying to get lean, especially for a contest. I think that makes 2 more in the loss column for soy. This is starting to get ugly, so I think I'll call it a day.

I don't see what all the confusion over soy is, the data is rather conclusive to me. Unless you are a woman entering menopause or live on an island where only soy foods are produced, I would steer clear.

For the bodybuilder looking for the next level of development, soy has no place the list of useful supplements. Any product that contains soy, or isoflavones, I would take a minute to think about what you are doing.

So be on the look out for those "bargain brand" weight gainers, any methoxy product, ipriflavone and of course any soy food/supplement products. If you've been using any of these, you have truthfully been doing your muscles, health and body as a whole an injustice.


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Evans BA, Griffiths K, Morton MS. Inhibition of 5 alpha-reductase in genital skin fibroblasts and prostate tissue by dietary lignans and isoflavonoids. J Endocrinol. 1995 Nov;147(2):295-302.

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Effects of replacing meat with soybean in the diet on sex hormone concentrations in healthy adult males. Br J Nutr.84;(4):557-63,2000. ellayi S, Naaz A, Szewczykowski MA, Sato T, Woods JA, Chang J, Segre M, Allred CD, Helferich WG, Cooke PS. The phytoestrogen genistein induces thymic and immune changes: a human health concern? Proc Natl Acad Sci U S A. 2002 May 28;99(11):7616-21.

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Klein M, Schadereit R, Kuchenmeister U. Energy metabolism and thyroid hormone levels of growing rats in response to different dietary proteins--soy protein or casein. Arch Tierernahr. 2000;53(2):99-125.