Does Soy Protein Lower Testosterone?

What is Soy Protein?

Soy protein is exactly what is sounds like. A protein powder derived from soy beans. For vegans or dairy tolerant individuals, soy protein can be an enticing choice to top up protein intake easily without having to consume dairy, from whey or casein-based powders. Benefits of soy include being very low in fat and cholesterol. It is also one of the only vegan complete proteins. A complete protein is a food source containing all 9 essential amino acids, such as meat or fish. For vegans, it can be more difficult to obtain a full spectrum through diet, especially to optimise muscle protein synthesis.

Debunking the Myths

One of the biggest concerns people have about soy is that it can decrease testosterone in men.

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.

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?

What the research says

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.

Conclusion

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.

Alternatives

If looking for a vegan protein that does not contain soy, for example because of a food sensitivity, we would advise looking for a blend of sources. As discussed, soy is one of the few complete sources of protein for vegans. The best way to obtain a full spectrum of amino acids when following a vegan diet otherwise, is to combine proteins to 'fill the gaps' so to speak. We would suggest a product such as Gold Standard 100% Plant Protein or Natural Select Vegan Protein. We would replicate this approach with diet if avoiding soy, combining more than one protein per meal, such as rice and beans, to meet daily intake. 

References

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

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

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

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

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

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