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Does Soy Affect Fertility?

The popularity of soybeans in the United States has increased dramatically over the last 20 years as more scientific evidence found them a healthy, meat-free source of protein that also lowers cholesterol, decreases menopausal symptoms and the risk of osteoporosis, as well as breast cancer and heart disease.

Soy foods (as well as chick peas and other legumes) contain a type of phytoestrogen (plant-derived estrogen), known as isoflavones. Two primary isoflavones found in soy are genistein and daidzein. Some of these isoflavones act as a mild estrogen in the body, while others act as antiestrogens, reducing the activity of estrogen. It is this estrogen property that benefits menopausal women and protects others from breast cancer; however, scientists have discovered that soy's estrogen component may decrease fertility.

The Science

The field of study on the effects of soy on fertility is fairly new, so there have been only a handful of studies dedicated to this topic.

In 1994, the American Journal of Clinical Nutrition reported that high levels of soy can increase menstrual cycle length by an average of 2.5 days, decrease FSH (follicle-stimulating hormone) and decrease LH (luteinizing hormone). However, the amount of soy necessary to achieve these results is equivalent to drinking three 12-ounce glasses of soy milk (60 g soy protein is equivalent to 45 mg of isoflavones) every day for one month, an amount that exceeds most regular consumption.

In more research performed at King's College London and reported in 2005, scientists studied the effects of the isoflavone genistein on human and mouse sperm. They found that human sperm were more than three times as likely to lose their acrosomes (caps that enable them to penetrate the egg) an hour after genistein exposure than before exposure.

However, other studies have found evidence that suggests soy does not have any negative effect on fertility. Some reports point to the fact that in countries where soy products are consumed in large amounts, such as China and Japan, the birth rate is comparable to that of countries where soy is not a part of the daily diet.

In 1998, the American Journal of Clinical Nutrition reported on a study conducted at the Department of Pharmacology and Toxicology, University of Alabama at Birmingham in which researchers injected prepubescent rats with genistein. They did not find any significant alterations to fertility, number of male or female offspring, body weight, menstrual cycle, or follicular development.

In another study conducted at Wake Forest University Baptist Medical Center and Emory University School of Medicine, and reported at the 2004 meeting of the American Society for Reproductive Medicine, researchers found that a soy supplement containing twice the level of plant estrogen consumed by Asian women did not alter any aspects of the menstrual cycle or ovarian function in monkeys. The monkeys who were fed the soy supplement did not exhibit any changes in the characteristics of the menstrual cycle, including length, amount of bleeding, or hormone levels. The researchers chose monkeys because they have menstrual cycles similar to those of female humans. Lead researcher Jay Kaplan, Ph.D. believes that consumption of a diet rich in soy probably would not compromise fertility, although further research is necessary to evaluate effects of soy on placental function and on the fetus.

The Bottom Line

Science has yet to agree on a conclusive recommendation regarding soy and fertility. Some research has suggested that women with certain risk factors for infertility may be more sensitive to soy than others, which would explain the contradictory research results. However, if you are experiencing any difficulties conceiving, it's probably a good idea to avoid soy, especially around ovulation, to boost your chances as much as possible. Soy foods differ somewhat in their concentration of isoflavones, but all of the traditional soy foods, such as tofu, soymilk, tempeh and miso, are rich sources of isoflavones, providing about 30 to 40 milligrams per serving. Soy sauce does not contain any phytoestrogens, so you can continue to use this in cooking and as a condiment.