Soy protein and its effects on testosterone

Soy protein excites fervour like few other nutritional items with opinion seemingly split evenly between those who tout it as a key protein for improving health and muscle mass, and those who regard it as the devil’s own concoction which will cause testosterone levels to plummet and estrogen to rise. In most cases these arguments are not based on real science and research but the biases and misunderstandings of extremists who assume that if a little of something is good (or bad depending on their view) then a load of it is the best thing sliced bread or will make your manhood and musculature shrivel to nothingness.

Soy protein is used widely in the food industry as a means to provide a better texture and extra protein, where it stands out as a protein source with a very high protein content compared to dairy proteins, and is high in BCAA’s, Glutamine in addition to a rapid digestion profile. It is considered by renowned nutrition guru Lyle McDonald as a high quality protein, with a high ranking from him of digestibility and protein content.


Soy Protein and Testosterone


The reason for its lack of popularity in bodybuilding circles though is due to the persistent belief that soy protein can increase estrogen and decrease testosterone levels. Is there any truth to this claim which, if it were true, would have serious consequences for anyone wanting to pursue a fitness lifestyle?

While some research has shown a reduction in testosterone being associated with soy consumption,
these studies suffer from one being performed on a vegan male with very high soy intake (1) while another often quoted study purported to show a 19% average decline in testosterone levels from soy consumption. This second study however only showed an actual decrease in testosterone in one man and this man started the study with a testosterone level over 200% higher than anyone else and 50% greater than the normal reference range. It is difficult to know why his values were so high to begin with and the researchers here provided no mechanism to explain this discrepancy. If we treat that one candidate as an outlier then no negative effect on testosterone was observed.

Research does exist showing that high levels of soy consumption are inversely correlated with testosterone levels (3) and that replacing meat with prodigious quantities of soybeans (4) leads to lower testosterone levels. This second study again uses an unrealistically high intake of soy and the possibility that removing meat rather than replacing with soy is the cause of the drop in testosterone certainly needs to be factored in, given that vegetarians are shown to have lower testosterone levels on average than meat eaters.

When we consider the empirical evidence, Asians who are known to have a history of higher soy consumption seem to have no issues with fertility or having children, which suggests that soy consumption at levels seen in Asia does not appear to have a negative effect on sex hormone levels.

A recent meta-analysis (5) looked at research conducted across 51 treatment groups and found no evidence for soy protein or soy isoflavones lowering testosterone or increasing estrogen levels.

A more relevant study than those looking at sex hormone levels is a recent study which examined the effects of different types of protein intake on body composition and sex hormones (6) and showed no significant differences on performance, lean body mass gains, body fat percentage or hormone status when comparing soy isolate, soy isolate plus whey and just whey.


Predator Blog - Soy Protein



Soy has been demonised in the bodybuilding community but with the exception of those who consume it to excess there is no evidence to support the theory that soy consumption can negatively impact on testosterone levels. While soybeans and isoflavones have some estrogenic activity, the amounts consumed in a normal diet are not sufficiently high enough to influence testosterone or estrogen levels in a negative way.

As with everything, this does not mean that users should look to soy protein as a sole source of protein but rather that its inclusion within a mixed food, varied diet does not deserve the drama and noise that some people would have you believe.


Author: Reggie Johal



1. Siepman (2011): Hypogonadism and erectile dysfunction associated with soy product consumption.

2. Goodwin (2007): Clinical and biological activity of soy protein powder supplementation in healthy male volunteers.

3. Nagata C (2000): Inverse association of soy product intake with serum androgen and estrogen concentrations in Japanese men.

4. Habito (2000): Effects of replacing meat with soybean in the diet on sex hormone concentrations in healthy adult males

5.  Hamilton-Reeves JM (2010): Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis.

6. Kalman D (2007): Effect of protein source and resistance training on body composition and sex hormones.

© 2012, Reggie Johal. All rights reserved.

About Reggie Johal

Reggie Johal is a former Great Britain American Football player and the founder of Predator Nutrition. He has a strong background in strength and fitness coaching with articles published in Flex and other leading online and print magazines.

9 thoughts on “Soy protein and its effects on testosterone

  1. There is an interesting article on the Reflex Nutrition site that I would appreciate feedback on:

    Apart from the fact about lowering testosterone (which seems to have been de-buffed above), there are still some unhealthy risks associated with soy protein.

    • In brief:

      Protein Inhibition? Reference to a study demonstrating this at the amounts used?

      Phytates – A red herring if ever there was one. No evidence has been demonstrated again plus note you get Phytates in vegetables and grains (without negative consequences we may add).

      3. Testosterone – We discussed the study they reference. See our comments about this but basically only one person with an abnormally high test level saw any drop in the study. A review of that study considered it an outlier which the study failed to provide a rationale for. A meta analysis shows no issue and you may want to note that those who consume soy products most prolifically generally have less fertility issues than western males.

      4. Health effects – Soy associated with positive health effects in most studies and empirically, those eating a lot live longer.

  2. Many thanks for your thoughts Predator. It seems this is another example of a supplement manufacturer talking a bunch of sh!t.

    The more experience I am gaining about the supplement industry, the more I realise most companies will either talk sh!t, or use sh!t ingredients marketed to fool us into thinking its the ‘best’.

    Roll on independent industry regulation I say.

  3. Farhan – I wouldn’t say that as Reflex are a quality manufacturer but they probably looked at the abstract on that study and the rest looks as though it was not referenced.

    John – I wouldn’t go mad on it but 30-40g as a maximum in my opinion. Unless consuming a pure soy protein product multiple times a day it is unlikely to reach that.

    • Hi Predator,

      I would have to disagree. Up until recently I was a loyal customer of Reflex as I believed they were a ‘quality manufacturer’. I bought into their promises of only using the very best ingredients in their products. However I have been doing a lot of research into supplement companies recently and also into the ingredients they use.

      The main reason I have stopped using Reflex is because they have started to use rubbish ingredients in their products, which completely flies in the face of their own marketing:

      1. They have recently started to use Dextrose and Maltodextrin. These are simply pseudonym’s for Monosodium Glutamate (MSG) and are NOT HEALTHY. Even simple research on MSG will yeild very disturbing information indeed. The irony being MSG is now in 80% of the foods we eat (disguised under different names).

      2. My wife was using one of Reflex’s diet drinks only to discover one of its ingredients was Glucose Syrup, which is basically a cheaper substitute for sugar (!?!?). How the hell can you market a diet drink that contains sugar in it?????

      Before Reflex I was using Maximuscle, who are even worse. Sh!t ingredients and ridiculously over-priced.

      Going back to the research I have been doing on the supplement industry, the majority of Whey protein drinks contain the cheapest variety of protein. And yet these companies market them as top quality products. Optimum Nutrition being a prime example. I have discovered their Gold Standard Whey protein drink contains Whey Protein Concentrate, which is the cheapest and lowest quality form of whey protein. The whole production process of whey concentrate strips it of all of its valuable nutrients. You may be getting a shot of protein in your system, but your body is not getting QUALITY protein.

      My research has also indicated that there are very real health risks associated with too much consumption of cheap quality whey protein. Yet these companies are free to sell them, marketing them as high quality produce.

      This all goes back to your article and my query referring to the information that Reflex had on their website. Whilst they may not be telling lies outright, they are being very ‘blinkered’ in their viewpoint. A viewpoint that can very easily misguide people (which your article has proved). And this is the same for most of the companies I have had experience with. And there is nothing to stop them, as the whole industry is completely unregulated. Leaving companies to basically do what they want.

      Me personally, I am stopping whey protein drinks completely. I will instead be using Organic Hemp Protein which is much healthier. I have also started juicing fruits and veg recently and have a wicked recipe for a protein power smoothie.

      Don’t get me wrong, I am all for supplementation and am an avid body builder. However the information I have come across in the last 6 months leads me to believe that, far from helping its customers get fitter using quality ingredients, companies are doing the opposite. And all for extra profits. Like these supplements don’t cost enough as it is!

  4. I do not understand why people are so worry to have a reduction in testosterone levels. I mean you are a man anyway and a small reduction of testosterone actually help you to prevent prostate cancer. Body builders are so stupid, they want muscles they take protein powder and creatine and sometime they destroy their kidneys. Is this a model of man? if so I say thanks…I pass……

  5. I need to supplement at least twice a day with a protein shake, yet my body doesn’t tolerate whey.
    If I use soy, that would put me at 50-60 grams of soy a day. You advise that I shouldn’t go above 30-40 grams.
    What do you recommend?

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