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Is Soy Safe for Men?

With a Life Science background, Simon understands the science of nutrition and he wants to share information that can affect your health.

Soy Appears to Be Safe for Men

You may have heard that isoflavones in soy foods have a feminizing effect. Indeed, there have been reports in animals and in humans that they can lower testosterone and sperm. However, here is what we know now:

  • No effect of isoflavones on sex hormones (testosterone) in men (Good evidence)
  • There are many benefits of soy on men's health
  • Isoflavones can decrease arterial stiffness (Good evidence)
  • They can reduce the risk of prostate cancer (Weaker evidence)
  • They can reduce the risk of gout (Weaker evidence)

Taken together, we now know that soy and isoflavones do not appear to feminize men, and may even have additional health benefits, including lessening arterial stiffness and reducing the risk of prostate cancer and gout.

What Is Soy?

Soy foods like tofu, miso, soy milk and tempeh, are made from soybeans. Soybeans are a type of legume that grows in pods. Soy foods have been eaten for centuries in many Asian countries. More recently, it has gained popularity in Western nations for its nutritional and health benefits as well as an increased interest in plant-based diets. Soybeans are high in plant-based protein and fat. They are also rich in fibre, which can benefit your gut health.

What Are Isoflavones?

Soybeans are a rich source of isoflavones, which are chemicals that are structurally similar to the hormone estrogen (called phytoestrogens or "plant estrogens"). They can attach themselves to both estrogen receptors (alpha and beta) in your body and can potentially have weak estrogen or anti-estrogenic activity. Estrogen is a sex hormone that is important in maintaining the sexual and reproductive health of women in particular.

Isoflavone (Top) and Estrogen (Bottom)

Isoflavone (Top) and Estrogen (Bottom)

What Is the Concern of Isoflavones in Men?

Although there are health benefits of isoflavones, discussed more below, there are also some concerns about isoflavones in men. One of the biggest concerns is that they feminize men. There are a few rodent studies that seem to support the effect of isoflavones. Furthermore, a small number of human studies also show that they can decrease testosterone levels. Testosterone is a sex hormone responsible for male reproductive health. A more recent report of a young man who developed hypogonadism (low testosterone) and erectile dysfunction from allegedly consuming large quantities of soy. Another study in young men found that testosterone was reduced with soy supplementation. An association between soy and low sperm quality and concentration was also reported. Clearly, isoflavones have been linked to feminizing men.

What Do Studies Say About Soy and Male Sex Hormones?

A study that pulled the results of numerous other studies (a meta-analysis of 32 studies) in 2010 found that neither soy foods nor isoflavone supplements changed testosterone levels in men. It's also interesting to note that the soy protein and isoflavone consumption in most of these studies were much higher than in the typical Japanese diet. More recently (2021), another meta-analysis of 41 clinical studies came to the same conclusion that neither soy protein nor isoflavone affects testosterone or estrogen levels in men.

Based on these meta-analyses of interventional studies, the evidence is strong that soy or isoflavone does not significantly affect reproductive hormones in men. The discrepancy between these studies and the ones that show an effect on hormones may lie in the amount of isoflavone consumed. It is very possible that abnormally high amounts may alter male sex hormones, but normal amounts (30-50 mg/day) are likely safe. A serving of traditional soy food, such as 100 g of tofu or 250 ml of soy milk, has about 25 mg of isoflavones. Hence, a serving or two of soy food is unlikely to do much harm.

Can Soy Affect Arterial Stiffness?

Arterial stiffness, or the loss of arterial elasticity, is a predictor of coronary heart disease, hypertension, heart failure, stroke, chronic kidney disease, dementia and all-cause mortality. A meta-analysis of 8 clinical trials in 2021 showed that isoflavones reduce arterial stiffness. The evidence here is good as these were interventional studies and can show cause and effect.

Can Soy Affect the Risk of Prostate Cancer?

A meta-analysis of 30 studies found soy and isoflavones to be associated with a reduced risk of prostate cancer. All the studies were observational and not interventional experiments, thus lacking any conclusion on cause and effect. Regardless, this evidence provides strong support that soy may be protective against prostate cancer.



Can Soy Lower the Risk of Gout?

Gout is a type of inflammatory arthritis. It causes attacks of joint pain, often in the big toe. A 2018 meta-analysis of 19 studies showed that soy foods decrease the risk of gout. Like the prostate cancer meta-analysis above, all the studies included in this one were observational so we can't conclude a cause and effect of soy on gout. We can conclude that soy is associated with a lower risk of gout.

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This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.

© 2022 Simon Lam