Does Soy Lower Testosterone?: The Myths and Reality

Written by Ben Bunting: BA, PGCert. (Sport & Exercise Nutrition) // British Army Physical Training Instructor // S&C Coach.


Around the globe, soy is one of most commonly consumed food sources. This food is thought to decrease testosterone due to its phytoestrogen contents. 

But does soy lower testosterone? 

This article will examine the facts, the myths, and the evidence about how this particular food can affect testosterone levels. We also answer some of the more important questions, such as if this type of food is harmful to your T-levels. This article covers: 

  • What is Soya?
  • Can it reduce testosterone?
  • Soy: What is it good for?
  • Is it possible to be allergic?
  • Last word and final conclusion

What is Soya?

The legume, soybeans, is a product that originates from East Asia. It's used in many products around the world. 

You can eat them whole, but they need to be cooked before eating as raw trypsin inhibitors make them poisonous. Edamame, for instance, are green young soybeans. This food is used in many products, including tofu and soymilk, as well as soups, meat substitutes, etc. 

Soy is not only found as a food ingredient, but also in many other foods, such as soybean oil, lecithin and textured vegetables protein. 

You can find it in a wide variety of foods, including Asian food, vegetables gum, cereals, and cookies. Bodybuilders may even use it to make protein bars, tuna in cans and peanut butter with low fat. 

Why does it get used so often? 

In 2012, 8.52 millions metric tons were consumed of soy oil. With the increase in GMO foods (genetically altered organisms), it's a cheap and convenient ingredient. The US produces 90% of soybean oil, which is used to make GMOs. 

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Soy and Testosterone

The key point is that soy appears in many different foods, including oils, lecithins, textured vegetables proteins, and hydrolyzed protein. 

Can soy reduce testosterone?

This is because the beans contain phytoestrogens called isoflavones. These are plants that mimic hormones like estrogen and estradiol.

This non-steroidal estrogen has been shown to reduce the activity of dihydrotestosterone, an androgen, and also inhibit prostate growth [1]. 

In fact, there are several studies which suggest soy has an estrogenic impact on the body. This in turn lowers T-levels. 

#Study 1: Dillingham et al [2] 

This study involved 35 men who consumed protein drinks with low or high soy content over a period of 57 days. Both the high and low dose groups reported a decrease in dihydrotestosterone and testosterone (DHT), and an increase of estradiol. 

This study found that soy proteins, regardless of their isoflavone contents, reduced testosterone and DHT, with only minor effects on the other hormones. It provides evidence of some of these effects. 

#Study 2: Chavarro et al [3] 

The study examined the possible association between consumption of soy products and isoflavones with parameters of semen quality, while taking into account personal characteristics. This study involved 99 male infertiles. 

Results showed an inverse relationship between soy intake and sperm count, with the highest soy consumption being associated with the lowest count. 

#Study 3: Siepman et al [4] 

The case report describes a diabetic 19-year-old man who developed sudden loss of libido after consuming large amounts of soy products as part of a vegan diet. Both his T and DHT were reduced. 

This case study was a metabolic patient who relied heavily on this particular food for his daily protein requirements. He consumed 9-10 times more than the average over a long period. 

After a year of reducing his consumption and stopping the vegan diet, all symptoms returned to normal. 

Does it reduce T levels then?

There's evidence that suggests that the T-levels are not reduced at all. 

First, let's look at the context. Many of the previous studies used large quantities of soy, far greater than the amount a 'typical person' would consume. 

Second, Messina's [5] large review evaluated clinically the concerns about isoflavones exposure in this food and supplements having a 'feminizing effect' on men. The study reviewed nine human and animal research studies to achieve this. 

This paper found that the clinical studies identified did not provide any evidence that exposure to isoflavones affects the circulating levels of estrogen in men. 

A meta-analysis (one of most reliable scientific methods of research) [6] also showed that isoflavones or soy products did not alter the bioavailable T levels in men. 

The research covered a wide range of studies, including 15 control groups and 32 reports. 

It seems the risk is not as obvious as we first believed. And if large-scale clinical studies prove to be accurate, then there's no danger at all. 

This food may also provide a variety of other benefits... 

Important Point: Evidence is not conclusive. In large clinical studies, soy does neither increase nor decrease estrogen or testosterone. If eaten in a healthy diet it is not that bad. 

What are its benefits?

We can now look at some of the benefits that you may get from including soy in your diet. 

One cup of soybeans cooked will provide you with around 22g protein, which is about the same amount as one tuna filet.

Studies have shown that consuming vegetable protein instead of animal proteins may reduce the risk of heart diseases[7].

Research suggests soy consumption may reduce prostate cancer risks in men. This protection could be related to the quantity and type of soy products consumed. It is therefore important that you choose healthy soy food sources. [8]

Is it possible to be allergic?

This is very unlikely. 

A soybean allergy affects 0.4% of all children, but most have outgrown the condition by age 10 [9]. Most allergic reactions tend to be mild. However, in rare instances more serious reactions like anaphylaxis can occur. 

Conclusion: Does soy reduce testosterone?

Soy contains isoflavones that can act as phytoestrogens, leading to concerns that an abundance of soy/isoflavone intake could adversely impact hormone levels, particularly testosterone.

A 2010 meta-analysis reviewed over 35 clinical studies and concluded that soy/isoflavone intake does not reduce testosterone.

The same lack of effect was also noted when other hormones such as SHBG and FT were examined.

Recent research corroborates the 2010 meta-analysis findings.

For instance, Bosland et al1 conducted an investigation on the effects of soy and casein protein supplements on hormone levels; their study demonstrated that neither soy protein or casein protein decreased testosterone or estradiol circulating levels.

Instead their ratio of IGF-1 to IGFBP-3 (which measures insulin-like growth factor 1) was 5.1% higher in the soy group than it was in casein group.

This research not only validated the findings from 2010 meta-analysis but also demonstrated that soy protein has no negative effect on LHCGR expression in Leydig cells; indeed, immunohistochemistry demonstrated an upregulation in its expression following soy consumption.

Soy/isoflavones should not pose an issue for hard training men looking to maximize hormone and test levels, while egg whites (rich in amino acids), lean beef, and pumpkin seeds (which contain zinc). All these sources should be included as part of an overall healthy diet plan.

When compared to larger analyses of clinical data, there is no evidence that soy consumption leads to reduced testosterone. 

Men should be encouraged to include soy in their diet due to its many benefits. 

To maintain a healthy life style, it is essential to make smart food choices.

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  1. Lund, TD et al. Equol is a novel anti-androgen that inhibits prostate growth and hormone feedback. Biol Reprod. 2004; 70(4): 1188-95
  2. Dillingham, BL et al. Soy protein isolates of varying isoflavone content exert minor effects on serum reproductive hormones in healthy young men. J Nutr. 2005; 135(3): 584-91
  3. Chavarro, JE et al. Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic. Human Reprod. 2008; 23(11): 2584-2590
  4. Siepmann, T et al. Hypogonadism and erectile dysfunction associated with soy product consumption. Nutrition. 2011; 27(7-8): 859-62
  5. Messina, M et al. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertil Steril. 2010; 93(7): 2095-104
  6. Hamilton-Reeves, M et al. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010; 94(3): 997-1007
  7. Hilleboe HE. Some epidemiologic aspects of coronary artery disease. J Chronic Dis. 1957;6: 210-228
  8. Yan, L et al. Soy consumption and prostate cancer risk in men: a revisit of a meta-analysis. Am J Clin Nutr April 2009; 89(4): 1155-1163
  9. Savage JH, Kaeding AJ, Matsui EC, Wood RA. The natural history of soy allergy. J Allergy Clin Immunol, 2010;125:683-86
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