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April 5, 2021

Soy, testosterone, and male fertility


Does soy affect male fertility?

Soy is well known to be a plant-based, nutrient-rich source of protein. It also happens to be a controversial food in the nutrition world. If you search for "soy” and “men," you'll find tons of articles and related searches that suggest soy is harmful, that it increases estrogen and impacts male fertility hormones like testosterone.

Doctors and scientists are still trying to understand the short-term and long-term effects that soy may have on male fertility, so the current evidence is a mixed bag. However, if tofu and soy milk are staples in your household, you may find that it's not necessary to scrap these from your diet entirely. Here, we dive into what we currently know about soy and male fertility.

What’s the concern around soy and male fertility?

Many people claim that soy contains estrogen. While that's technically not false, it's also a broad and somewhat misleading statement.

Understanding phytoestrogens

Soy contains phytoestrogens, naturally occurring plant compounds. Phytoestrogens may act in a similar way to estrogen that's produced in the body, but with weaker effects.

Phytoestrogens are not unique to soy products, and can be found in many foods including fruits, like pears and berries; vegetables like garlic, onions, beans, and cabbage; and hops and grains. Soybeans contain the highest amount of phytoestrogens compared to other foods. As a result, soy products like tofu, tempeh, soy sauce, and soy milk have a high concentration of phytoestrogens.

Even if you were to eliminate soy from your diet (which may be tricky, since many processed food products like cereal, crackers, and energy bars contain soy), it would still be difficult to entirely eliminate phytoestrogens from your diet, since they naturally occur in many vegetables and fruits.

Health effects of phytoestrogens

The concern is that, when consumed, phytoestrogens can bind to estrogen receptors in the body and potentially cause weak estrogenic activity. Estrogen is often thought of as a female hormone, but it’s actually an essential male fertility hormone, as well. However, too-high levels of estrogen in men can affect libido (sex drive) and fertility.

Interestingly, studies show that phytoestrogens can sometimes have an anti-estrogenic effect in the body—because they bind to estrogen receptors, blocking actual estrogen from binding. We also know that phytoestrogens in soy do not seem to impact testosterone levels, as research has shown that testosterone levels are not correlated with soy intake.

Studies have also shown that phytoestrogens, in moderation, can provide you with health benefits, such as lower cholesterol levels, reduced risk of prostate cancer, and a decreased risk of osteoporosis.

Soy and male fertility: complications of research

The biggest reason why it's so difficult for medical professionals to land on a definitive statement is that soy behaves differently in the body, based on multiple factors. Most studies on soy don't holistically address all these factors, including:

  • Ethnicity. Soy may be processed by the body differently in different ethnic groups. This could be why some individuals from certain regions appear to benefit more from a soy-rich diet. For example, certain Asian populations consume high levels of soy foods in comparison to Western countries, and do not experience a negative effect on male fertility.
  • Hormone levels. The effects of soy may vary depending on an individual's baseline level of hormones. Since no two people are 100% alike, determining exactly what soy does to the human body can be difficult. For example, research shows that post-menopausal women may experience more estrogenic effects from soy than premenopausal women who have higher natural levels of estrogen—in which case, soy may actually behave like an anti-estrogen. 
  • Type of soy. Not all soy behaves the same. Whole soy foods like tofu and soybeans may behave differently than processed soy foods, like soy protein powders. Then, there's also fermented and fresh soy foods—each category may behave differently.

With so many factors in motion, it's difficult to determine whether soy is "good" or "bad" when it comes to male fertility. The best anyone can do is understand why these factors matter when reading scientific research. For example, if you consume a soy protein powder regularly but don't eat whole soybeans or tofu, you might want to focus on studies that observe the effects on the specific type of soy food you consume. 

Another very important factor to consider is to look for studies that involve humans, not animals. Animals may metabolize soy differently than humans, so animal studies may not give you the most accurate information.

Soy's impact on sperm health and fertility

While the research in this space may not be conclusive as to whether soy negatively impacts male fertility, the results are promising, indicating that soy may not be as harmful as feared.

One study observed 99 male partners of subfertile couples over a three month period. Men in the highest category of soy intake had sperm concentrations about 32% lower than men who did not consume soy foods at all. However, sperm concentration was well above normal even in the groups that ate the most soy. Additionally, the study noted that soy food and soy isoflavones (a type of phytoestrogen) were unrelated to other sperm quality parameters like motility, morphology or ejaculate volume.

On the flip side, another study found that high soy intake may actually have a positive effect on sperm motility, an important sperm quality parameter. It was found that nearly 50% of men who consumed no soy foods had low sperm motile count, versus 25% of men who consumed some soy foods or lots of soy foods. As always, studies on diet are complicated, and it’s possible that men who eat more soy foods are also more likely to eat more vegetables (especially if they follow a vegan or vegetarian diet). We know that fruits and vegetables are beneficial for sperm health.

Another study also found that a man's intake of soy foods was unrelated to in vitro fertilization (IVF) treatment outcomes. Consumption of soy before IVF treatment was unrelated to fertilization rate, implantation or pregnancy—which suggests that soy may have little to no impact on a man's overall sperm quality.

Should men avoid soy products?

You don't necessarily need to eliminate soy entirely from your diet—especially when research can’t definitively say soy is bad for male fertility. And, since some studies show that soy can actually be beneficial to sperm health, it may be counterproductive to completely eliminate soy if it's a staple in your diet.

Instead, if you're trying to conceive or hoping to improve your sperm health, moderating your soy consumption may grant you some peace of mind.

Which foods should you avoid?

If you want to improve your sperm health, a healthy diet is a great way to do that. Before we dive into what you can supplement your diet with, it's just as important to cover what you should avoid. There are some foods that are associated with poorer sperm health:

  • Fried foods
  • Refined grains
  • Added sugars
  • Red meat
  • Processed meats

Even more so than soy, these foods have been shown to negatively impact sperm concentration, count, motility, and morphology. If you want to eliminate foods that could potentially harm your fertility, these should rank the highest on your priority list.

What foods should you eat for improved male fertility?

A diet rich in fruits and vegetables, seafood, and nuts may improve sperm health. If you want to incorporate more of these nutrient-rich foods in your diet, you may want to consider these superfoods for male fertility.


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