Is soy good for building muscles?

building muscles with soy

The bodybuilding forums are full of messages about soy protein, one swears by it, the other says you should take whey protein better. Whatever you often hear coming back, athletes are afraid of the so-called phytoestrogens in soy protein, or the vegetable variant of human estrogen. And as we all know, you don’t want to have too much female estrogens in your body as a strength athlete! Is this fear justified? Does soy protein increase estrogen and at the same time lower male testosterone levels? With this article we are going to give a conclusive answer to this once and for all … Because soy protein could be a cheap alternative to whey …

What is soy protein?

Soy protein is extracted from soybeans. About 35% of a soybean consists of proteins, which is not bad for a plant. The unique feature of soy protein is the complete amino acid profile and the relatively high biological value, comparable to animal proteins. This makes soy protein one of the few vegetable protein sources that can compete with animal proteins and is therefore seen by many as a good vegetable alternative.

Amino acid profile whey protein versus soy protein

Proteins are the building materials of all kinds of body tissues. Proteins are also an important component of the immune system and muscles. Some amino acids are more important for building muscle tissue than others. The amino acids arginine, glutamine and leucine are especially needed in strikingly large quantities. Whey proteins contain a lot of leucine, but a lot less arginine and glutamine. Soy protein, on the other hand, contains a lot of arginine and glutamine. It is not for nothing that the combination of soy protein isolate and whey isolate would not be a crazy idea (Source: Journal of the American College of Nutrition).

It is also known that so-called BCAAs (Branched Chain Amino Acids) also allow the muscles to grow more easily compared to other amino acids. Both whey and soy protein contain BCAAs, whey more than soy.

See below the amino acid composition of whey protein isolate and soy protein isolate. Note, there is always variation per product, this table is therefore indicative.

infographic amino acid composition of whey protein isolate and soy protein isolate.

Amino acids with an * are BCAAs, with an (E) are essential amino acids. We also see that soy protein has a higher proportion of arginine, glutamine and glycine compared to whey. We also see that soy protein is a very low type of cystine, histidine, proline, serine and tyrosine. The latter is not a problem, because these are not essential amino acids, which means that the body can make these itself.

Does soy protein lower testosterone levels?

Soy protein contains phytoestrogens, so until recently people were rightly afraid that soy protein might lower your testosterone levels. Some “experts” also claimed at high and low that you absolutely should not take soy protein, because it would make you more feminine. Many (strength) athletes therefore avoided soy protein and it has therefore always remained in the damn corner. But were those athletes and experts right? All research on a lot, no, they were not right. Not if you have read the latest scientific studies in this area. A majority of the studies come to the same conclusion; yes, soy protein contains phytoestrogens, but these have a negligible effect on men’s testosterone levels, in fact, in some studies the testosterone levels actually got higher after prolonged intake of soy protein (Source: Eur J Clin Nutr. 2006) Dec; 60 (12): 1423-9.).

For example, a meta-study was conducted in 2009 into the effect of soy protein on testosterone level. A meta study is a collective study that bundles the results of all relevant studies in this field and compares them with each other. This meta study included 15 studies. All studies in this meta study were similar in design. The average study duration was 74 days. The protein intake varied from 30 to 70 grams of soy protein per day. The highest isoflavone intake was 450mg a day. The results of this can be seen in the image below.

soy meta study

Meta study that bundles the results of 15 studies into the effect of soy protein on testosterone level

The meta-study clearly shows that all of the studies shows that the testosterone level does not decrease, but even rises slightly as a result of soy protein consumption (Source: Fertil Steril. 2009 Jun 11.

The variations in effect may have to do with a certain amount of intestinal bacteria present. Some bacteria can make the isoflavones from soy protein harmless before they enter the body. But not everyone has the same number of bacteria. By the way, even if you don’t have many bacteria, the effect on testosterone is still negligible. Some animal studies even show that the body will produce more androgens in response to the phytoestrogens, the net effect of which is even positive on the testosterone level (Source: Reprod Biol. 2006 Nov; 6 (3): 275-90.).

Ok, so on average, soy protein does not lower the testosterone level, and if it does do so, long-term soy protein consumption appears to increase the testosterone level over time (Source: Mark McVey, Gerard Cooke, Ivan Curran. Increased serum and testicular androgen levels in F1 rats with lifetime exposure to soy isoflavones Reproductive Toxicology (2004), doi: 10.1016 / j.reprotox.2004.04.005.). Are there other reasons for not taking soy protein? Let’s take a look at the positive and negative properties of soy.

Positive properties of soy protein

( has listed the scientific literature on the positive effects of soy protein on training and health, with reference to the source.)

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Soy protein is cheaper than whey protein

The production and processing of soy protein is much cheaper and more efficient than that of whey protein. That is why soy protein is always cheaper than whey. In addition, enough studies have shown that strength athletes build muscle mass just as hard using soy as whey protein. (Source: J. Nutr. 137: 357-362, February 2007.)

Negative properties of soy protein

Is there anything negative to report about soy protein? Yes, a small number of studies (from the meta study) show that in some cases soy protein lowers testosterone levels. This can have various causes, such as the presence or absence of specific intestinal bacteria that break down isoflavones before it enters the bloodstream.

GMO and Non-GMO

These days, soy is in a negative position because of possible genetic modification (GMO) and the massive cultivation of soy, which displaces other crops (and small farmers). If you want to use soy protein isolate, make sure that it is produced in a sustainable way and not from GMO sources.


Given the overwhelming number of positive studies and the negligible effect on testosterone levels decrease (while in some studies it actually increases testosterone levels), we can only conclude that soy protein is a great plant-based alternative to whey protein and works just as well, even with a number of additional health benefits. So the answer to whether soy protein is suitable for strength athletes is a resounding YES. Hereby the following comments: The maximum dose used in the Meta study was 70 grams of soy protein per day. It is unknown what the effect of soy protein is at more than 70 grams. The effect of soy protein varies from person to person, although there is no reduction in testosterone levels in most men and, in some cases, an increase, there are also men who, due to their (likely) genetic predisposition, do react ‘negatively’ to soy protein. In other words, if you want to stay on the safe side, supplement less than 70 grams of soy protein per day. That is perfectly possible with protein supplements that consist of soy and whey. A trend that is increasingly emerging. Optionally, you can also make your own “hybrid” shake by both buying a pot of soy protein isolate and a pot of whey isolate and mixing it.

If you have any comments on this article and you can support this with scientific literature, we would like to see your response below in the comments field. Or if you have new insights into soy protein, then we would love to hear from you.

By S.van der Tap Pd.Internet.Marketer Founder Of digitalstico

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