Soy and Fertility: What Every Dietitian Should Know

Soy is widely recognized for its nutritional benefits, including its high protein content and versatility, but its impact on hormones has made it a topic of controversy, particularly in the context of reproductive health and fertility.

To help dietitians guide clients through the current evidence, this article will explore:

Where the controversy around soy and fertility stems from
The potential biological mechanisms
What the research shows on soy and fertility
Practical takeaways for dietitians

The Controversy: Why Soy and Fertility Remain a Contentious Topic

The debate around soy and fertility stems primarily from its phytoestrogen content, specifically isoflavones like genistein and daidzein, which can bind to estrogen receptors in the body. Animal studies initially raised concerns:

Sheep grazing on phytoestrogen-rich clover experienced fertility issues
Soy-fed rodents showed changes in reproductive organ development and function, especially when exposed to high doses

These animal studies, which often used unusually high doses of phytoestrogens or administration methods like injections, raised concerns about similar effects in humans.

In human studies, dietary intakes tend to be much lower and more consistent with typical dietary patterns.

Observational research on human populations generally shows less pronounced effects on fertility, and some studies even suggest potential benefits. One concern is that soy and its isoflavones may influence the menstrual cycle, with some studies suggesting a potential shortening of the luteal phase.

The luteal phase is critical for fertility, as it involves progesterone production necessary for uterine lining support and embryo implantation. A shortened luteal phase can reduce implantation chances, potentially impacting fertility outcomes. However, evidence from the literature suggests soy’s effects are modest and may not significantly disrupt fertility in most individuals.

Potential Mechanisms of Soy’s Impact on Fertility

Estrogenic and Anti-Estrogenic Effects:

Isoflavones in soy can act as weak estrogens due to their structural similarity to estrogen, allowing them to bind to estrogen receptors. This action can result in estrogenic or anti-estrogenic effects depending on the tissue type and level of endogenous estrogen.

When estrogen levels are low, phytoestrogens can act as weak estrogens, mimicking estrogen’s effects in the body
Conversely, when natural estrogen levels are high, phytoestrogens can compete with natural estrogen and have a balancing effect, potentially blocking excessive estrogenic stimulation

Cellular Pathways:

Beyond hormonal effects, phytoestrogens in soy may also interact with other cellular pathways . Some studies suggest that isoflavones might impact signaling proteins, which influence cellular processes involved in fertility, such as oocyte maturation and endometrial receptivity

What the Research Shows on Soy and Fertility

Effects on Assisted Reproductive Technology (ART)

Increased Live Birth Rates:

A prospective cohort study with 315 women undergoing ART found that soy isoflavone intake was positively associated with live birth rates. Women consuming the highest amounts of isoflavones (7.56–27.89 mg/day) were 77% more likely to achieve a live birth than non-consumers.
Protection Against BPA:

Another prospective cohort study of 239 participants found that soy consumption modified the negative effects of bisphenol A (BPA), an endocrine disruptor, on fertility. Among women undergoing IVF, those who had soy exhibited a stable live birth rate across increasing levels of urinary BPA concentrations, whereas non-consumers experienced a decline in success rates.
Endometrial Receptivity:

Research suggests that soy isoflavones may improve endometrial thickness, a critical factor for implantation during ART cycles. Improved endometrial receptivity contributes to higher implantation rates and better ART outcomes.
Luteal Phase Support:

A randomized control trial compared intramuscular progesterone alone to progesterone combined with oral phytoestrogens for luteal phase support in IVF-ET cycles. The group receiving phytoestrogens exhibited significantly improved pregnancy rates, suggesting that phytoestrogens may enhance luteal phase support in ART.

Effects of soy on ovulation

Studies on the impact of soy on ovulation are limited. In one small study including 36 women with secondary amenorrhea (or anovulation), 6 grams per day of black soybean powder over 6 months improved ovulation

Markers of Ovarian Reserve:

One cross-sectional study in women with a median age of 35 examined soy’s relationship with markers like:
- Antral Follicle Count (AFC): measures the number of small, fluid-filled follicles in the ovaries, with higher counts indicating a greater capacity to produce eggs.
- Anti-Müllerian Hormone (AMH): reflects the remaining egg supply, with higher levels suggesting better ovarian reserve and lower levels indicating reduced fertility potential.
- Follicle-Stimulating Hormone (FSH): released by the pituitary gland, stimulates follicle growth, and elevated levels can signal diminished ovarian reserve

They found no significant association between soy intake and AFC, FSH or AMH.

Gaps and Limitations in Current Soy and Fertility Research

There are significant limitations in the research on soy and fertility:

Many studies are observational, meaning they cannot establish causation
Sample sizes are often small, follow-up periods short, and results may be influenced by other dietary or lifestyle factors not accounted for in study designs
Most human studies on soy’s effects use typical dietary doses, which are generally safe, while some animal studies use very high doses that might not be applicable to typical human diets
Another key limitation is individual variability in metabolizing soy isoflavones. Some people produce equol, a metabolite of the soy isoflavone daidzein, which has stronger biological effects. Only about 30-50% of people, mostly in Asian populations, are equol producers. Equol-producing individuals may experience a more pronounced response to soy’s reproductive effects than non-producers, suggesting that genetic and microbiome differences could be crucial in determining soy’s impact on fertility

Practical Takeaways for Dietitians: How Much Soy is Recommended?

For dietitians advising clients focused on fertility, here are some practical takeaways from the current research:

Encourage Moderate Soy Intake: Current evidence supports moderate soy intake from whole foods, with 1-2 servings per day likely being both safe and potentially beneficial for fertility. Whole soy foods such as tofu, edamame, miso, and soy milk provide phytoestrogens along with fiber, protein, and other nutrients that support overall health
Focus on Whole Foods Over Supplements: High-dose soy supplements, which contain concentrated isoflavones, may not produce the same benefits as dietary soy and have inadequate data on safety in the context of fertility and overall health
Monitor Cycle Changes for Those With Luteal Phase Concerns: For clients with a history of luteal phase deficiency, dietitians might recommend monitoring menstrual cycles if they increase soy intake. Though dietary soy intake is unlikely to cause any concerns
Soy is a staple in many cultural diets, particularly in East and Southeast Asia. It is also a highly nutritious option for plant-based eaters, offering high-quality protein, essential nutrients, and potential health benefits, making it a valuable addition to culturally inclusive, value-based nutrition care

Conclusion

Soy can be part of a balanced, fertility-supportive diet when consumed in moderation as whole foods. Research suggests that moderate soy intake might support ART outcomes, protect against environmental toxins, and improve cycle regularity for some individuals. By understanding soy’s potential impacts, dietitians can help clients make well-informed decisions tailored to their fertility goals and unique responses.