Trying to conceive: Can soy adversely impact your fertility?

An article I read stated that soy products should be avoided while trying to conceive and during early pregnancy because they contain natural estrogens. This could interfere with the production of progesterone needed to support the pregnancy. Another article stated that they are an excellent source of protein during pregnancy. Which is true?

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Sue Gilbert

Sue Gilbert works as a consulting nutritionist. For many years she worked with Earth's Best Organic Baby Food, integrating nutrition and... Read more

To a certain extent, both are true. Tofu and soy are excellent sources of protein during pregnancy. Soy foods contain phytoestrogens, literally meaning plant-derived estrogen. The type of phytoestrogen in soy is known as isoflavones. These isoflavones come in different forms and have different effects. Some may function similarly to estrogens in the body, producing a very weak effect compared to the real thing. And others act as antiestrogens, to reduce the activity of estrogen. Evidence is growing that these phytoestrogens may help with menopausal symptoms, as well as inhibit cancer cell growth.

A few studies suggest that high levels of soy protein may decrease fertility. According to a report in The Journal of Clinical Nutrition (Am J Clin Nutr 1994 Sep;60(3):333-40), a small number of studies have shown that high levels of soy can increase menstrual cycle length, decrease follicle-stimulating hormone and decrease leutinizing hormone. The high levels of soy generating this effect were equivalent to drinking three 12-ounce glasses of soy milk (60 g soy protein equivalent to 45 mg of isoflavones) for a month. Typically, most people do not consume this much soy.

An abstract from the journal Natural Toxins (1998;6(2):51-9) stated that evidence from studies of various animal species has shown that eating high levels of phytoestrogens can have adverse effects on fertility. It qualifies the findings by saying that there is no current data to suggest that consumption of phytoestrogens at the levels normally encountered in the diet are likely to be harmful.

A more recent review reported in The American Journal of Clinical Nutrition (1998 Dec;68(6 Suppl):1400S-1405S) adds to this information by reporting that reproductive and developmental toxicity studies did not find significant alterations to fertility from soy phytoestrogens.

From these reports to date, it seems logical to conclude that normal intake of soy proteins will not adversely affect your chances of becoming pregnant or maintaining a pregnancy. As with any food, it is wise not to overconsume. Eating a food in moderation allows you to avoid any potential harm, as well as leaving room in your diet for variety. The more variety in your diet, the more likely you are to get all the important nutrients that your body needs.

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