Soy and Fertility: Do Phytoestrogens Affect Sperm Count or Ovulation?

Short answer: eating soy foods in normal amounts is unlikely to harm fertility for most people. The worry comes from two places: animal studies showing that high isoflavone exposure can reduce fertility, and one small 2008 study that found lower sperm concentration in men who ate the most soy. On the other side, a 2021 meta-analysis of 41 clinical studies found that soy protein and isoflavones did not change men's testosterone, estrogen or related hormones, regardless of dose or study length. If you eat tofu, tempeh, edamame or soy milk as part of a varied diet, there's no strong reason to cut them out while trying to conceive.

Why soy gets flagged

Soybeans contain isoflavones, plant compounds often called phytoestrogens because they can bind to estrogen receptors. They're far weaker than the body's own estrogen and don't behave identically, but "plant estrogen" sounds alarming to anyone thinking about hormones and fertility.

The concern isn't invented. High isoflavone intake has been linked to reduced fertility in animal studies. The question is whether that translates to people eating soy foods in ordinary amounts, and that's where the human data come in.

The study behind "soy lowers sperm count"

In 2008, Jorge Chavarro and colleagues at Harvard published a study in Human Reproduction of 99 men who were partners in couples seen at the Massachusetts General Hospital Fertility Center. The men reported how often they ate 15 soy-based foods over the previous three months, and the researchers compared that with their semen analyses.

That's a real finding, but look at its shape:

• 99 men is a small sample, and the confidence interval is wide.
• They were already attending a fertility clinic, so they're not a typical population.
• It's observational. Men who ate more soy may have differed in other ways.
• Only one semen measure moved. Motility, morphology and volume showed no association.
• It didn't measure pregnancy or time to conceive.

The bigger picture for men: hormones

If soy were acting like estrogen in men, you'd expect to see it in their hormone levels. A 2021 meta-analysis by Reed and colleagues in Reproductive Toxicology tested exactly that. It pooled 41 clinical studies in which adult men consumed soy foods, soy protein, isoflavone extracts or red clover extracts and had their hormones measured.

• Total testosterone was measured in 1,753 men and free testosterone in 752.
• Estradiol was measured in 1,000 men, estrone in 239 and SHBG in 967.
• No significant effect of soy protein or isoflavone intake was found on any of these outcomes, regardless of the statistical model used.
• Analyses by isoflavone dose and study duration also showed no effect.

Hormones aren't the same as sperm, so this doesn't fully cancel out the 2008 finding. But it removes the most common explanation for why soy would harm male fertility. Taken together, the evidence suggests that ordinary soy food intake isn't a meaningful fertility risk for most men, with a reasonable question mark around very high intake.

What about women and ovulation?

The research on soy and ovulation in women is smaller and mostly observational. Much of the concern traces back to animal studies and to discussions of isoflavone supplements at doses well above what a food-based diet provides. We didn't find good human evidence that soy foods eaten in usual amounts impair ovulation.

Soy foods also bring things that fit a fertility-friendly pattern: plant protein, fiber, and in many products calcium and iron. For plant-based eaters, tofu and soy milk are often central sources of protein and choline; see our vegan and vegetarian TTC checklist. Replacing some red and processed meat with soy and other legumes is consistent with the broader dietary research summarized in our fertility diet guide and conceiveguide.com's evidence-based fertility diet.

What "realistic intake" looks like

There's no official upper limit for soy foods. A practical approach, based on the evidence above:

• Everyday amounts are fine. A serving or two of tofu, tempeh, edamame or soy milk a day fits a varied diet.
• Favor whole and traditional soy foods over highly processed soy-protein bars and isolates when you have the choice.
• Men with a borderline semen analysis who eat soy many times a day, or rely on soy protein powders, might reasonably moderate and see whether the next analysis changes. That's a cautious step, not a proven fix.
• Don't swap soy for something worse. Replacing tofu with processed meat to "protect fertility" runs against the rest of the diet research.

Myths vs. evidence

How to read soy headlines

Soy is a good case study in how nutrition headlines go wrong. A few habits help with any "food X harms fertility" story:

1. Check who was studied. Rodents, men at a fertility clinic and healthy volunteers can give very different answers.
2. Check the exposure. Soy foods, soy protein isolate and isoflavone pills are not interchangeable.
3. Check the outcome. A hormone level, a sperm measure and a pregnancy are three different things.
4. Check the design. An association in one group of people is a starting point. Consistent findings across trials and meta-analyses carry more weight.
5. Check the size of the effect. Statistically significant isn't the same as practically important, and wide confidence intervals mean uncertainty.

Applied to soy, those questions explain why one small observational study made headlines while a much larger body of hormone research got less attention.

Where to put your effort instead

If you're worried about hormone-disrupting exposures, the stronger evidence points at chemicals rather than beans: certain plastics, pesticides and personal-care ingredients. Our endocrine disruptors guide covers which ones have the most research. For men, the basics in our male fertility diet plan and fertilestart.com's male fertility lifestyle guide matter more than soy. And howtohaveababy.com's foods to avoid when TTC covers the foods with clearer reasons for caution.

When to see a doctor: if you've been trying for 12 months (or 6 months if the female partner is 35 or older), ASRM recommends an infertility evaluation for both partners. A semen analysis will tell you far more than adjusting soy intake.

Sources

• Chavarro JE, Toth TL, Sadio SM, Hauser R. Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic. Hum Reprod. 2008;23(11):2584–2590. PubMed 18650557
• Reed KE, Camargo J, Hamilton-Reeves J, et al. Neither soy nor isoflavone intake affects male reproductive hormones: an expanded and updated meta-analysis of clinical studies. Reprod Toxicol. 2021;100:60–67. PubMed 33383165
• ASRM Practice Committee. Definition of infertility: a committee opinion (2023). asrm.org (PDF)