It’s largely a myth. A comprehensive meta-analysis pooling data from 41 clinical studies and more than 1,700 men found that neither soy protein nor soy isoflavone intake had any significant effect on total testosterone, free testosterone, or estrogen levels in men, regardless of the dose or how long the studies lasted. Despite this, the idea that soy “feminizes” men remains one of the most persistent claims in men’s health circles.
The myth isn’t entirely made up out of nothing. Soy does contain compounds with weak estrogen-like activity, which gives the concern a surface-level biological plausibility. But when researchers have actually measured hormone levels in men consuming soy, the concern hasn’t held up.
What the Research Shows
The most rigorous evidence on this question comes from an updated and expanded meta-analysis published in 2021, which searched clinical databases for all available studies on soy or isoflavone intake and testosterone in men. Across 41 included studies measuring total testosterone in 1,753 men and free testosterone in 752 men, the researchers found no significant effect of soy protein or isoflavone intake on either measure, and sub-analyses by dose and study duration also showed no effect (PubMed).
This wasn’t a one-off finding. An earlier version of the same meta-analysis, published in 2010 and covering more than 50 treatment groups, reached the same conclusion: neither soy foods nor isoflavone supplements meaningfully altered testosterone, free testosterone, or the free androgen index in men (Physicians Committee for Responsible Medicine).
A more recent 2025 dose-response analysis added some nuance. It found that at very high isoflavone intakes, roughly five to nine times typical Japanese dietary intake, there were small, nonlinear changes in estradiol and sex hormone-binding globulin (SHBG). However, even at these unusually high doses, the analysis found no significant effect on testosterone or free testosterone specifically (Wiley/Food Frontiers). In other words, even the study most likely to find a soy effect on hormones still didn’t find one on testosterone itself.
Where the Myth Comes From
The concern about soy and testosterone isn’t purely fabricated. It traces back to a few specific sources:
- Soy’s isoflavone content. Soy contains isoflavones, plant compounds classified as phytoestrogens because they can weakly bind to estrogen receptors in the body. This gives the theoretical concern some biological basis, even though the effect in practice hasn’t matched the theory.
- Isolated case reports. A small number of published case reports have described feminizing symptoms, including reduced testosterone or elevated estrogen, in men consuming very unusual amounts of soy. These reports typically involve intake far beyond what a typical soy-containing diet would provide.
- Extrapolation from animal and cell studies. Some early concerns were based on laboratory or animal research using isoflavone doses and delivery methods that don’t reflect normal human dietary soy consumption.
What This Means in Practice
For the vast majority of men, including those eating soy regularly as tofu, edamame, soy milk, or soy protein supplements, the evidence does not support any meaningful concern about testosterone suppression. The typical soy intake even in populations that consume it heavily, such as in parts of East Asia, falls well within the range studied in these meta-analyses without any measurable hormonal effect.
Soy also comes with its own set of established health benefits, including associations with reduced risk of certain cancers and improved cardiovascular health markers. Men avoiding soy specifically out of concern for their testosterone are likely giving up a nutritious protein source based on a claim that hasn’t held up under scientific scrutiny. As always, extremely high intake of any single food, well beyond typical dietary patterns, is a different question, and it’s the extreme end of that range where the 2025 dose-response data found small hormonal shifts, in estrogen and SHBG rather than testosterone itself.
The Bottom Line
Soy lowering testosterone is largely a myth. Multiple meta-analyses spanning decades of research and thousands of men consistently find no significant effect of soy or isoflavone intake on testosterone levels, even at the higher end of typical consumption. Men can reasonably include soy as part of a healthy diet without concern for their testosterone levels.