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Does Soy Lower Testosterone?: The Myths and Reality

Soy protein is one of the most abundantly eaten food sources around the world. Many have suggested that due to phytoestrogen content, this food may decrease testosterone- this has led to many discussions about the benefits and dangers of soy in the diet.

But does soy lower testosterone?

In this article we weigh up the evidence, myths and realities about the effects of it on testosterone levels, and answer the bigger questions regarding this food, and if it really can damage your T-levels. We cover:

  • What is soy?
  • Does it decrease testosterone?
  • What are the benefits of soy?
  • Can you be allergic to it?
  • Final word and conclusion

What is soy?

Soybeans are legumes that originated from East Asia and are now used worldwide in a variety of products.

They can be eaten whole – although they must be cooked first as they are poisonous when raw due to trypsin inhibitors. Edamame beans for example are young, green soybeans. A number of products this food as an ingredient- tofu, soy milk, soups and meat substitutes are all examples.

We don’t just find soy from the food source itself – it is found in all sorts of foods as an additional ingredient – soybean oil, soy lecithin, textured vegetable protein (TVP) and hydrolyzed vegetable protein (HVP) to name just a few. 

This means that you could find it in any number of products including Asian cuisine, vegetable gum, cereals and cookies. The bodybuilding fraternity might even find it in the more regularly used protein bars, canned tuna and low-fat peanut butter.

Why is it used so much?

8.52 million metric tons of soybean oil were consumed in 2012, and with an increase in GMO (genetically modified organism) foods, it is cheap additionally ingredient. In fact in the US 90% of the amount produced goes into GMOs.


Soy-and-Testosterone


Key point: Soy can be found in all sorts of foods- oils, lecithin, textured vegetable protein and hydrolyzed vegetable protein to name just a few.


Does soy decrease testosterone?

The initial link between this food and its effects on T levels comes from the fact that the bean contains active compounds called isoflavones- these are plant derived phytoestrogens, many of which mimic female hormones such as estrogen and estradiol. 

It is reported that once isoflavones are metabolized, the gut produces equol- this non-steroidal estrogen has previously been reported to inhibit prostate growth, as well as reduce dihydrotestosterone (DHT) activity [1]- an important androgen and form of testosterone.

There are a number of studies that suggest that soy does indeed have an estrogenic effect on the body, which in turn decreases T levels-

#Study 1: Dillingham et al [2]

In this study 35 men consumed a range of protein isolate drinks (including low and high content soy isolates) over a 57 day period. The results reported both decrease in testosterone and dihydrotestosterone (DHT) and an increase in estradiol in both the low and high doe groups.

The study concluded that this food, regardless of isoflavone content, decreased testosterone and DHT with minor effects on other hormones, providing evidence for some effects of soy protein on hormones

#Study 2: Chavarro et al [3]

This study investigated the potential association between soy foods and isoflavones intake with semen quality parameters while adjusting for personal characteristics. The study was conducted using 99 infertile males.

The results found an inverse association between soy food intake and sperm concentration with those eating the most soy having the lowest count.

#Study 3: Siepman et al [4]

This case study reported a 19 year old diabetic man with sudden onset of loss of libido and erectile dysfunction after the ingestion of large quantities of soy-based products in a vegan-style diet. His T and DHT levels were both reduced.

It is worth mentioning though that this case study had an existing metabolic disease, and relied on large amounts of this food to satisfy his protein needs- 9-10 times the norm for an extended period of time.

All symptoms normalized after a 1 year period of reduced his intake of it and once the vegan diet had ceased.


So it does decrease T levels?

It might not be as clear cut as that- there’s also evidence to suggest that it doesn’t reduce T levels at all:

The first defense of this is context- many of the studies mentioned previously used large amounts of soy– far more than what the ‘typical’ person would eat.

Secondly, a large review paper by Messina [5] clinically evaluated concerns that isoflavone exposure from this food or supplements had ‘feminizing’ effects on men. To do this the study used research from 9 human and animal studies.

The paper concluded that there is essentially no evidence from the nine identified clinical studies that isoflavone exposure affects circulating estrogen levels in men.

Additionally, a meta-analysis [6] (one of the most robust scientific research methods) also suggested that neither soy foods nor isoflavone supplements alter measures of bioavailable T concentrations in men. 

This was an analysis of 15 control groups, 32 reports and 36 treatment groups, so again the research covers a broad range of independent studies.

So it seems that the risks are not as clear as first thought- and if these large clinical studies are to go by, there is no risk at all.

There are also a number of benefits that this food might provide too…


does soy lower testosterone myth

Key Point: The evidence is far from conclusive – In very large amounts soy has been seen to lower testosterone, however in larger clinical studies soy doesn’t decrease testosterone or increase estrogen. When eaten as part of a balanced diet, it may not be that bad.


What are the benefits?

Now that we’ve seen that soy is not the demon it’s made out to be, we can start to look at the potential benefits of including it in your diet.

  • It has a high protein content – one cup of cooked soybeans will give you around 22g of protein- about the same as a tuna fillet.
  • May reduce your risk of coronary heart disease – studies have found that ingestion of vegetable proteins in place of animal protein appears to be associated with a lower risk of heart-related illnesses[7].
  • It could be anti-carcinogenic – Some research suggests that consumption of soy foods is associated with a reduction in prostate cancer risk in men. However this protection may be associated with the type and quantity of soy foods consumed- so obviously its important to ensure your soy sources are healthy. [8]

Can you be allergic to it?

It’s very unlikely.

Around 0.4% of children have a soybean allergy, although evidence would suggest that most outgrown it by the age of 10 [9]. Typically any allergic reactions are mild, although in very rare cases more severe reactions such as anaphylaxis may occur.


Final word – Does soy lower testosterone?

While some independent studies have claimed strong links between soy consumption and reduced testosterone, larger clinical analyses have found no link at all.

Data at present suggests that neither soy foods, nor isoflavone supplements alter measures of bio-available T concentrations in men, and that due to the many health-related benefits that come with soy, mean should not be afraid to include it in their diet.

What is important however, is that in order to maintain a healthy lifestyle, clever choices regarding food choices need to be made.


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References

  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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