Phytoestrogens are weak plant-derived estrogens that are structurally similar to estrogen hormones produced by the body.  The phytoestrogens contained in soy-based products are called isoflavones.  Because they have estrogenic activity, soy isoflavones have long been touted as a treatment for menopausal symptoms, although these clinical studies have yielded mixed results.  

States of estrogen deficiency and falling estrogen levels, as occur during the postpartum period or the transition to menopause, have been associated with increased risk for depressive illness, an observation which has led many to hypothesize that estrogen has antidepressant or neuroprotective effects.  While using supplemental estrogen as an antidepressant may have significant side effects, plant-based phytoestrogens may carry less risk and may have a positive impact on mood.  

A recent study from Japan, including 1745 pregnant women, has assessed the impact of soy product intake on risk of depression during pregnancy. The researchers found that higher intake of total soy products (including tofu, tofu products, fermented soybeans, boiled soybeans, miso soup) was related to a lower prevalence of depressive symptoms during pregnancy, as compared to women with lower intake of soy products.  The magnitude of the effect was large, ranging from about 26% to 43%, depending on the type of soy product evaluated.  

A significant positive exposure-response relationship was observed between miso intake and depressive symptoms during pregnancy. No relationship was observed between soy milk intake and depressive symptoms during pregnancy.

This is a really interesting study, but it is preliminary and needs to be repeated in other, less homogeneous, populations.  What is exciting here is the magnitude of the effect; a reduction in risk of 25%-35% is pretty impressive.  They did not assess blood isoflavone levels in the participants, so one might wonder if eating more soy products is a marker of a healthier diet.  (We have seen a few studies which suggest that diets higher in fats and processed foods are associated with higher risk for depression).

While there are dietary sources of soy isoflavones, some might find it more convenient to turn to a bottle of nutritional supplements.  However, some of these products contain very high doses, as well as other additives, and we do not know if these excessive doses may pose risk to the pregnancy.  So for now, it seems reasonable to increase levels of soy-based products in one’s diet but we would not recommend the high isoflavone doses found in some supplements.  

 

Ruta Nonacs, MD PhD

Miyake Y, Tanaka K, Okubo H, Sasaki S, Furukawa S, Arakawa M. Soy isoflavone intake and prevalence of depressive symptoms during pregnancy in Japan: baseline data from the Kyushu Okinawa Maternal and Child Health Study. Eur J Nutr. 2016; Oct 15.

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