Introduction
Phytoestrogens are compounds derived from plants and mimic the effects of oestrogen in the body. They are found in a wide range of food products such as soy, flaxseed and certain fruits and vegetables. We will explore further potential benefits and drawbacks of the phytoestrogens on healthy male testosterone levels.
Asian populations historically have lower rates of cardiovascular disease, diabetes and obesity compared to western populations and was originally theorised that Soy (a food high in phytoestrogens) reduces the risks of these diseases. However, due to their structural similarity to oestrogen, there is concern that these compounds may interfere with endocrine function, particularly in males where testosterone plays a crucial role in muscle mass, libido, mood, and overall reproductive health.
Mechanisms of Action
Oestrogen Receptor Binding
Phytoestrogens bind to oestrogen receptors α and β. They have:
- Higher affinity for ER-β, which is more abundant in peripheral tissues (e.g., prostate, bone).
- Weaker potency compared to endogenous estradiol.
Effects on the Hypothalamic–Pituitary–Gonadal (HPG) Axis
Phytoestrogens may influence:
- Gonadotropin-releasing hormone (GnRH) production in the hypothalamus
- Luteinizing hormone (LH) secretion from the pituitary
- Leydig cell activity in the testes, which produce testosterone
In theory, excess oestrogenic activity could suppress this axis, reducing testosterone synthesis.
Foods containing phytoestrogens
- Soy and soy-based products including soy milk, tofu and tempeh
- Nuts and seeds, including flaxseeds
- Fruits, including apples, grapes and pomegranates
- Dairy milk and dairy products
- Herbs
- Legumes, including beans and lentils
- Vegetables (broccoli, carrots and potatoes)
- Whole grains
Evidence from Animal Studies
Many rodent studies have shown that high doses of phytoestrogens can lead to:
- Decreased serum testosterone
- Reduced testicular weight
- Impaired sperm production
However, these studies often use doses far exceeding typical human consumption and there are clear specie differences (e.g., rodent vs. human metabolism) which limit the direct applicability of results.
Human Studies
Clinical Trials
Numerous randomised controlled trials (RCTs) have evaluated soy isoflavone supplementation in men:
- Hamilton-Reeves et al. (2007) [2]: No significant change in testosterone, SHBG, or LH after 12 weeks of soy protein intake.
- Messina et al. (2010) [3]: Meta-analysis of 15 studies found no significant effects of soy/ isoflavones on total or free testosterone levels.
- Zhang et al. (2021) [6]: Systematic review reaffirmed the lack of consistent evidence for testosterone suppression from soy or isoflavones in adult males.
Observational Studies
Some epidemiological studies suggest:
- No link between soy intake and testosterone levels in general populations.
- Cultural dietary patterns (e.g., in East Asia) with high soy consumption show no adverse effects on male fertility or hormone levels.
Controversial Cases
Case Reports
- A few isolated case reports have suggested feminising effects (e.g., gynecomastia) in men with extremely high soy intake (>12 servings/day for several months).
- In most cases, symptoms resolved upon discontinuation of soy.
Athlete and Bodybuilder Concerns
Despite a lack of clinical evidence, anecdotal reports persist within fitness communities that soy or flax reduce testosterone. These may stem from:
- Misinterpretation of animal data
- High emphasis on optimising androgen levels
- Bias toward animal protein and anti-soy narratives
Other Influences on Testosterone
It is critical to note that many factors affect testosterone levels including age, sleep, stress, exercise and nutrition (especially calorie and fat intake), as well as alcohol, endocrine disruptors, medications
The effect of moderate phytoestrogen consumption is likely negligible when these other variables are controlled.
Conclusion
Current scientific evidence suggests that moderate dietary intake of phytoestrogens does not significantly affect testosterone levels in healthy adult men.
- Animal studies show hormonal changes at high doses, but these are not directly translatable to typical human diets.
- Clinical trials consistently show no adverse hormonal effects from soy or isoflavone supplementation.
- Rare case reports of hormonal effects usually involve extreme, sustained overconsumption.
Therefore, phytoestrogens—especially from whole foods like soy, flaxseed, and legumes—can be part of a balanced diet without negative consequences for male testosterone in most individuals.