The claim is everywhere in fitness forums, gym culture, and wellness influencer feeds: soy protein "feminises" men, lowers testosterone, raises estrogen, and disrupts hormones. It's repeated so often it feels like settled fact.
It isn't. The actual evidence — from large meta-analyses of dozens of randomised controlled trials, involving thousands of men — tells a very different story.
This is what the peer-reviewed science says, with citations you can check yourself.
Where the Soy-Hormone Fear Came From
The biological logic behind the worry is straightforward. Soy contains compounds called isoflavones — specifically genistein and daidzein — which are classified as phytoestrogens. The name tells you why people get nervous: "phyto" (plant) + "estrogen" (female hormone). The compounds can bind weakly to estrogen receptors in the body.
A small number of case reports, mostly involving men consuming extraordinarily high doses of isoflavone supplements — five to nine times the typical Japanese daily intake — described feminising effects, gynaecomastia, or lowered testosterone. These case reports went viral. They became the basis of an entire cultural narrative.
What got lost in translation is that case reports describe individual incidents at extreme doses, not population-level effects at normal intakes. To understand normal intakes, you need controlled trials. Many of them. And that's exactly what researchers have done.
What the Meta-Analyses Found
A meta-analysis is the gold standard of evidence synthesis — it pools results from multiple randomised controlled trials to give a statistically reliable answer. There are two key meta-analyses on soy and male hormones, and they reached the same conclusion.
Hamilton-Reeves et al. (2010), published in Fertility and Sterility, analysed studies up to 2008 examining the effects of soy protein and isoflavone intake on testosterone, sex hormone-binding globulin (SHBG), free testosterone, and free androgen index in men. The conclusion:
"Neither soy foods nor isoflavone supplements alter measures of bioavailable testosterone concentrations in men."
(Hamilton-Reeves JM et al. Fertility and Sterility, 2010; 94(3):997-1007. doi: 10.1016/j.fertnstert.2009.04.038)
Reed et al. (2021), published in Reproductive Toxicology, updated and expanded that work with all studies published through April 2020. The expanded meta-analysis pooled data from 41 clinical studies, measuring total testosterone in 1,753 men and free testosterone in 752 men, estradiol in 1,000 men, and SHBG in 967 men. Their finding:
"Regardless of the statistical model, no significant effects of soy protein or isoflavone intake on any of the outcomes measured were found. Sub-analysis of the data according to isoflavone dose and study duration also showed no effect. This updated and expanded meta-analysis indicates that regardless of dose and study duration, neither soy protein nor isoflavone exposure affects TT, FT, E2 or E1 levels in men."
(Reed KE et al. Reproductive Toxicology, 2021; 100:60-67. doi: 10.1016/j.reprotox.2020.12.019)
Two meta-analyses. Forty-one studies. Thousands of men. No effect on testosterone, no effect on estradiol, no effect on free testosterone, no effect on SHBG — at any dose, over any duration.
What About the Case Reports?
This is where intellectual honesty matters, because the case reports do exist, and they deserve a proper accounting rather than dismissal.
The most-cited cases involved men consuming 9 servings of soy milk per day for years, or isoflavone supplement doses of 360mg per day or more. For context: traditional Japanese soy food intake provides roughly 30-50mg of isoflavones daily, and a typical scoop of soy protein powder delivers around 25-50mg. The case-report doses were five to nine times higher than even the highest dietary intakes observed in soy-consuming populations.
A 2025 dose-response meta-analysis published in Food Frontiers (Rajaie et al.) acknowledged these case reports directly and noted they involved "exceptionally high amounts of soy protein or its isoflavone supplements." At normal dietary or supplemental doses, the population-level data shows no hormonal effect.
In other words: if you drink nine soy milks a day for years, something may shift. If you have a scoop of soy protein in your smoothie, the controlled trial data says you'll be fine.
What This Means for Protein Powder Choices
A few things follow from this.
First, soy protein powder used at normal doses doesn't disrupt male hormones. That's not opinion. That's what 41 clinical studies show.
Second, the "feminisation" claim is, on the evidence, false. It survives because it sounds plausible — phytoestrogen, female hormone, must lower testosterone — and because it gets repeated in fitness culture without anyone checking the underlying research. The case reports that started the panic involved doses no normal consumer would ever reach.
Third, choosing a non-soy protein for other reasons is entirely reasonable. Soy is one of the top 14 allergens. Soy farming has significant environmental and deforestation concerns when sourced from certain regions. Many soy protein powders are made from defatted soy meal that has been processed with hexane, a petrochemical solvent. Some people genuinely don't tolerate soy digestively. These are all legitimate reasons to choose something else — they're just not "soy will mess with your hormones," because that bit isn't supported by the evidence.
Why We Use Brown Rice Protein Instead
For full transparency: at That Protein, we don't use soy. We use organic brown rice protein blended with oats. That isn't because we believe the hormone myth — we don't. It's because brown rice protein:
- Is one of the lowest-allergen protein sources available
- Is gentle on sensitive guts and IBS sufferers
- Doesn't carry the deforestation footprint of unsustainably-sourced soy
- Can be produced without hexane extraction
- Tastes neutral, which lets us blend cleanly with cacao, peanut butter and other natural flavours
That's a sourcing and quality decision, not a hormone-fear decision. We think it's important to be honest about the difference.
The Bottom Line
If you've been avoiding soy because you read somewhere it would lower your testosterone or raise your estrogen, the science doesn't support that worry at normal intake levels. The 2021 Reed meta-analysis is unambiguous on this point and it pooled 41 studies to get there.
If you're avoiding soy for allergies, digestive issues, environmental reasons, or simply because you prefer plant proteins from other sources — those are all valid choices. We just think people deserve to make those choices based on what the evidence actually says, not on what got repeated until it sounded true.
Key Sources
- Reed KE, Camargo J, Hamilton-Reeves J, Kurzer M, Messina M. (2021). Neither soy nor isoflavone intake affects male reproductive hormones: An expanded and updated meta-analysis of clinical studies. Reproductive Toxicology, 100, 60-67. doi: 10.1016/j.reprotox.2020.12.019
- Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ. (2010). Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: Results of a meta-analysis. Fertility and Sterility, 94(3), 997-1007. doi: 10.1016/j.fertnstert.2009.04.038
- Messina M. (2010). Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertility and Sterility, 93(7), 2095-2104.
- Rajaie S et al. (2025). The Impact of Soy Products and Isoflavones on Male Reproductive Hormones: A Systematic Review and Dose-Response Meta-Analysis of Randomized Controlled Trials. Food Frontiers.
- Messina M, Nagata C, Wu AH. (2006). Estimated Asian adult soy protein and isoflavone intakes. Nutrition and Cancer, 55(1), 1-12.
This article is for general information only. Please consult a qualified healthcare professional for personal nutrition advice.