Fact Checked By Dr Mark Watson M.D.
In the realm of sports nutrition and dietary controversies, few topics have sparked as much debate as the impact of soy on testosterone levels. Soy, a humble bean that has been a dietary staple in many Asian countries for centuries, is consumed in various forms worldwide.
However, concerns have been raised about the potential negative effects of soy on testosterone levels, specifically in men. In this comprehensive article, we will delve into the scientific evidence to determine whether there is any truth to these claims. To be honest, there's an awful lot of misinformation on this, with people often simply shouting down the soy lowers testosterone arguement before properly looking at what's going on here. And there is actually something going on, it's just not what you may think and can be easily resolved for most people without cutting soy.
Why Testosterone is Important
Testosterone, a crucial sex hormone, plays a vital role in various aspects of men's health, including muscle mass, sexual function, strength, and overall well-being. Maintaining optimal testosterone levels is essential for overall health and avoiding conditions such as obesity, type 2 diabetes, metabolic syndrome, and heart problems. While numerous factors influence testosterone regulation, diet is believed to be a key determinant in testosterone levels.
Soy: A Nutritional Powerhouse And Why We Use It In Protein Powder
Soy is a versatile and affordable source of high-quality protein, making it a popular dietary choice for many individuals. Apart from being consumed in its whole bean form, soy can be found in an array of products, including soy milk, tofu, tempeh, and imitation meat. Soy is also a common ingredient in protein powders due to its emulsifying properties and more importantly soy is it’s the only vegan protein that contains all of the essential amino acids your body needs to build muscle.
So, Does Soy Actually Lower Testosterone Levels?
The answer is most likely to be no, at least for most people. Although it should be noted that consuming too much soy like all things isn’t going to be great for you, so it’s ideal that if you are following a plant based diet to get your protein from a variety of sources, and not solely rely on soy despite the aforementioned amino acids.
There have been a couple of studies that suggest that soy lowers testosterone [1,2], but far more that suggest it hasn’t.
Studies Where Soy Lowers Testosterone.
There have been a couple of studies (some of them as small as 12 men)  that have shown that daily consumption of 20-40grams of soy protein can lower testosterone levels in trained men.  However, it should be noted that there are many more robust studies that prove the contrary. Currently the consensus is that soy doesn’t lower testosterone levels, but it is possible that large volumes of soy protein daily could be bad for your health.
In general consumption of more than 100mg of soy isoflavones per day has been shown to potentially cause issues for both men and women. For example, it has been linked to lowered ovarian function.  For context It’s around 3.5mg of isoflavones per gram of soy protein. Meaning that you may want to steer clear of more than 30 grams a day. But, there seems to be secondary reason for all of this, and that is iodine.
Is iodine the culprit?
This is where things get interesting, and ironically soy could be worse for vegans than for meat eaters. If you consume a lot of soy protein, it seems that said protein reduces the body’s uptake of iodine. 
And this is where the vegans come in, generally speaking they don’t much iodine in their diet and are already prone to deficiency as it typically comes from fish, meat, eggs and dairy, although seaweed is rich in it.
And to top it off, if you already have a thyroid issue, soy consumption can exacerbate it. 
So, what’s all this got to do with testosterone?
That’s quite simple, the side effects from thyroid issues often include low mood, weight gain, low energy and other symptoms related to low testosterone. 
Having a low amount of iodine whilst consuming soy can also lead to thyroid issues in otherwise healthy individuals.  And to top it off, problems with the thyroid can cause, you guessed it, low testosterone. 
Whilst this wouldn’t affect huge numbers of people, it could be where some of the anecdotal evidence for soy causing testosterone issues comes from.
In short, most of the negatives can be dealt with by making sure you’re not deficient in iodine as this can restrict thyroid function, leading to a lot of the same issues that you would experience with lowered testosterone levels.
So, is soy going to cause you any harm, most likely not, but it is worth checking iodine levels. A supplement would certainly resolve this especially for vegans. Although surprisingly, this isn’t actually where the idea that soy lowers testosterone comes from.
The Myth of Soy and Testosterone
The belief that soy consumption can decrease testosterone levels has been perpetuated by the presence of phytoestrogens,  or plant estrogens, in soy. Phytoestrogens are compounds that, when consumed, can mimic or interact with estrogen receptors in the body. 
Consequently, concerns have been raised about the potential feminizing effects of soy, including increased body fat, reduced muscle mass, impotence, and lower sperm count.
The Scientific Evidence
Several studies have examined the effects of soy on testosterone levels, with the majority of them dispelling the notion that soy negatively impacts testosterone in men. A comprehensive analysis of over 30 studies involving more than 900 men concluded that neither soy foods nor isoflavone supplements, which are rich in phytoestrogens, affect testosterone concentrations (Hamilton-Reeves et al., 2010). This finding was supported by another study that examined 41 additional studies published between 2010 and 2020, which also found no link between soy consumption and testosterone levels (Reed et al., 2021). 
In fact in many cases soy can actually reduce several things that can lower testosterone levels like LDL cholesterol,  and that supplementation with soy protein has been linked reducing obesity faster than other dietary restrictions  on obese patients, and obesity lowers testosterone.
The Role of Phytoestrogens
While soy does contain phytoestrogens, it is important to understand that their effects on the body are complex and not fully understood. Phytoestrogens can act as both estrogen agonists and antagonists, depending on various factors such as dosage, timing, and individual differences. [10,15]
Additionally, the impact of phytoestrogens on testosterone levels may differ between men and women. Thus, it is essential to interpret the scientific evidence in the context of these nuances.
Hidden Sources of Soy
It's important to note that soy can be found in unexpected places, making it challenging to avoid entirely. Soy lecithin, derived from soybean oil, is commonly used as an emulsifier in various products, including whey protein powder, baked goods, candy bars, and even soap. Moreover, soybean meal, a byproduct of soybean oil extraction, is widely used in animal feed worldwide. Thus, even those deliberately trying to eliminate soy from their diets may find it challenging to do so completely.
Other Foods That Influence Testosterone Levels
While soy consumption has been largely debunked as a significant factor in testosterone regulation, it is worth noting that various other dietary factors that can affect testosterone levels. Understanding these factors can provide a more comprehensive view of testosterone regulation and perhaps help you find the culprit if it is in fact not soy protein.
Mint and Testosterone Levels
Some research suggests that certain types of mint, such as spearmint and peppermint, may have the potential to influence testosterone levels in women specifically. There isn’t anything to suggest that it would have this effect on men, however and is unlikely to as the mechanisms appear to be related to womens hormone production center the ovaries. [16,17]
Licorice Root and Testosterone Levels
Licorice root, commonly used as a sweetening agent, has been found to influence hormone levels, including testosterone. Studies have shown that licorice root consumption can lead to a decrease in testosterone levels in both men and women.
However, it is important to differentiate between licorice root and licorice candy, as the latter does not typically contain licorice root. [19,20]
Flaxseed and Testosterone Levels
While flaxseed is a nutrient-rich food with numerous health benefits, some research suggests that it may decrease testosterone levels. Flaxseed contains lignans, plant compounds that can bind to testosterone and promote its excretion from the body. [21,22]
Many men supplement with Flaxseed for it’s omega content, however, the type of omega 3 AHA is not actually usable by the body and we use over 90% to convert it into the usable type, so not only is it bad for test, but it’s bad as an omega supplement too.
Whilst some studies reported that flax seed oil reduced testosterone levels due to its omega 3 and omega 6 content,  further studies have shown that it’s the omega 6 that has this effect and that fish oil (rich in DHA and EPA, the other omega 3 oils) even raised testosterone levels. 
Trans Fats and Testosterone Levels
Trans fats, which have been banned in many countries due to their negative health effects, have also been linked to decreased testosterone levels. Regular consumption of trans fats from processed foods has been associated with lower testosterone levels, reduced sperm count, and impaired testicular function. [24.25]
Alcohol and Testosterone Levels
Excessive alcohol consumption has been shown to have a negative impact on testosterone levels. While the evidence is not entirely conclusive, moderate to heavy alcohol intake has been associated with decreased testosterone levels in some studies. However, further research is required to fully understand the effects of different alcohol dosages on testosterone levels. 
Nuts and Testosterone Levels
While nuts are generally considered a healthy food, some studies suggest that certain types of nuts may decrease testosterone levels. Walnuts and almonds, for example, have been shown to increase levels of sex hormone-binding globulin (SHBG), a protein that can bind to testosterone and reduce its availability in the body.  So you may want to reach for the soy milk after all if you’re looking for a plant based option and to optimize testosterone levels.
Conclusion: Does Soy Protein Lower Testosterone?
In conclusion, the belief that soy consumption negatively affects testosterone levels in men has been largely debunked by more robust scientific research. Although we can see why this myth has persisted, as anecdotally people have seen effects that they would assume to be low T after eating a lot of soy protein. However, the side effects of too much soy consumption are likely to be related to iodine deficiency rather than soy itself which can be easily resolved.
And considering many vegans should look to supplement iodine anyway, this shouldn't be a big issue.
While soy does contain phytoestrogens, the evidence suggests that they do not significantly alter testosterone concentrations.
Other food items, such as mint, licorice root, flaxseed, trans fats, alcohol, and certain nuts, may have some influence on testosterone levels, but their effects are complex and require further investigation for things to be 100% clear. Although there are far stronger cases for them causing lowered testosterone than soy.
Maintaining healthy testosterone levels is crucial for overall well-being, and diet plays a significant role in achieving this goal. By consuming a balanced diet rich in whole foods, including a variety of fruits, vegetables, lean proteins, and healthy fats, individuals can support optimal testosterone levels and promote overall health.
It is important to consult with a healthcare professional before making any significant changes to your diet or lifestyle, especially if you have specific concerns about testosterone levels or other hormonal issues.
About Fact Checker
Dr Mark Watson is the founder of Center TRT, having graduated from stamford more than 20 years ago, he is an expert in the field of supplemental health, focusing on long term benefits of complimenary treatment. View Profile
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