Eating soy may protect against the detrimental effects of endocrine disruptors such as bisphenol A (BPA) on women's fertility, according to a new study.
BPA is widely found in consumer products like plastic water bottles and the lining of tin cans. Over 90% of people in the US have BPA in their urine, suggesting exposure to this chemical, and a wealth of data from epidemiological studies point to adverse health effects resulting from BPA exposure, including reproductive disorders.
While animal studies have shown that soy might mitigate these harmful effects, this is the first study to show an interaction between soy and BPA in humans.
"We found that among women who were undergoing infertility treatment with assisted reproductive technologies, urinary BPA levels were related to nearly 50% lower chances of live birth among women who did not consume soy, but [BPA levels] had no impact on live births among women who did consume soy," commented first author Jorge E Chavarro, MD, ScD, associate professor at Harvard TH Chan School of Public Health, Boston, Massachusetts.
"Our study highlights the need to consider the possibility that the health effects of environmental chemicals can be modified by lifestyle factors such as diet," he emphasized.
Dr Chavarro and colleagues published their work January 27 in the Journal of Clinical Endocrinology & Metabolism.
Asked to comment, Heather Patisaul, PhD, of North Carolina State University, Raleigh, told Medscape Medical News that these results are "particularly compelling" because they are consistent with past studies in mice. Much of the work showing an interaction between soy and BPA in animals came from Dr Patisaul's lab.
However, she stressed that this does not mean that women should immediately start consuming large amounts of soy products in the hope that it improves fertility. This is because soy isoflavones are estrogenic and can also be endocrine disrupting, so too much soy could have the opposite effect to that intended, with adverse effects on ovulation, she stressed.
More Than 70% of Women Studied Ate Soy Products
The research by Dr Chavorro and colleagues involved 239 women who participated in the prospective Environment and Reproductive Health (EARTH) study, examining the role of environment and nutrition in fertility. Between 2007 and 2012, participants went through 347 in vitro fertilization (IVF) cycles at the Massachusetts General Hospital Fertility Center. The women had a median age of 35 years, and 74% (n = 176) ate soy.
They completed lifestyle surveys that included questions about how frequently they consumed 15 soy-based foods. They also provided urine samples during each IVF cycle for assessment of BPA levels. IVF outcomes were identified from electronic medical records.
In women who did not eat soy, live births (P for trend = .03), clinical pregnancy rates (P for interaction = .03), and implantation rates (P for interaction = .02) decreased as urinary BPA levels increased.
In contrast, there was no relationship between increasing urinary BPA levels and less successful IVF outcomes in women who ate soy (P for trend = .35).
Women who did not eat soy and had the highest levels of urinary BPA had live birth rates of 17%, compared with a rate of 49% in women who ate soy and had the highest levels of urinary BPA.
"We found suggestive evidence that BPA might relate to lower implantation, clinical pregnancy, and live birth rates only among women who did not consume soy foods but was unrelated to these outcomes among women who consumed soy foods," the researchers say.
They note one limitation, however — that women may have misreported their food consumption on the self-reported dietary questionnaires — and they point out that the group of women who did not eat soy was quite small (n = 67).
They also caution that the results may not apply to women who are trying to get pregnant without IVF.
What Are the Mechanisms?
Although the study could not look at the mechanisms underlying the interaction between BPA, soy, and IVF outcomes, at least two studies in rodents have shown that soy could offset BPA-induced changes in DNA methylation, Dr Chavarro told Medscape Medical News.
"In other words, BPA was able to switch on and off certain genes, and soy prevented BPA from doing so," he explained. But "additional work will be required to further understand how soy and BPA may interact with each other," he noted.
Dr Patisaul said studies in mouse oocytes have shown that BPA can increase the risk of abnormal chromosomes and that soy can decrease this risk.
As an endocrine disruptor, BPA binds to several estrogen receptors. While soy is also hormonally active, whether or not that property plays a role in its interaction with BPA cannot be determined from this current study in women undergoing IVF, she continued.
"It is tempting to conclude that, because soy can be estrogenic, the isoflavones in soy are responsible for this mitigating effect, but that has not been demonstrated experimentally," she explained.
"One possibility is that soy alters the metabolism and uptake of BPA. There is some evidence for this in rats, but it requires confirmatory investigation."
She also noted that women in this study ate low to moderate levels of soy. People metabolize soy isoflavones differently, so this is also an important consideration when thinking about the potential risks and benefits of soy.
"It is important to point out that soy isoflavones are estrogenic and thus also endocrine disrupting. Women should not rush out and add a bunch of soy to their diet hoping it will improve fertility," she advised.
"It is well-known that overdoing it can actually produce the opposite effect and shut down the ovulatory cycle."
The studies conducted by Dr Patisaul and colleagues have also shown that soy can decrease the effects of BPA on the brain and behavioral outcomes in rats, suggesting that the effects of soy on BPA might apply to many organ systems.
The study authors and Dr Patisaul report no relevant financial relationships.
J Clin Endocrinol Metab. Published online January 27, 2016. Abstract
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Cite this: Soy May Protect Against Fertility Harms of Endocrine Disruptors - Medscape - Feb 01, 2016.