Phytoestrogens are a special group of phytonutrients that occur naturally in most plant foods. The two major classes are isoflavones in soy, and lignans in seeds, whole grains, legumes, fruits, and vegetables.
They’re called phytoestrogens because they interact with estrogen receptors but they’re not estrogen. In fact, they bind so weakly to estrogen receptors that they effectively block estradiol and are therefore better categorized as anti-estrogen.
For example, it’s long been observed that phytoestrogen crops such as red clover suppress the estrogen and fertility of livestock. It’s even been proposed that plants evolved phytoestrogens to reduce the fertility of female herbivores and prevent overgrazing.
In a chapter called “Agriculture and Selection for High Levels of Estrogen,” evolutionary biologist Grazyna Jasienska makes the case that ancient humans evolved higher levels of estrogen as a way to adapt to the anti-estrogen effect of agriculture (ie. adapt to phytoestrogen-rich plant food). It could thus be said that women with agrarian ancestors are hormonally calibrated to a relatively high intake of phytoestrogens to shelter us from our own high estrogen.
In other words, it’s fine to eat phytoestrogens like legumes and seeds because they’re part of our traditional diet and our hormonal system is adapted to them.
How phytoestrogens can benefit women’s health and hormones
For women of reproductive age, phytoestrogens have a beneficial anti-estrogen effect and help to promote the healthy metabolism or detoxification of estrogen. Food-based phytoestrogens may even reduce the risk of some hormone-sensitive cancers.
Heavy periods. In general, phytoestrogens make periods lighter but very high doses can cause heavier periods if they suppress ovulation and progesterone. That’s why (in part) a vegan or exclusively plant-based diet can cause period problems.
Endometriosis. Phytoestrogens are generally beneficial for endometriosis but soy, in particular, appears to worsen endometriosis in some women. That may be the result of an adverse immune reaction. Read Immune treatment for endometriosis.
Hypothalamic amenorrhea. Phytoestrogens cannot correct the estrogen deficiency of hypothalamic amenorrhea. The treatment for hypothalamic amenorrhea is to promote ovulation by eating more. Read How to increase estrogen.
Menopause. During menopause, when estrogen is low, phytoestrogens can have a mild pro-estrogen effect, which has led to the investigation of soy as an alternative to menopausal hormone therapy. Unfortunately, several large research studies have concluded that soy isoflavones don’t do much, if anything, for menopausal symptoms.
Thyroid disease. Concentrated extracts of soy isoflavones may suppress thyroid function. Food-based soy is probably fine as long as you also consume enough iodine.
In conclusion, phytoestrogens generally have a beneficial anti-estrogen effect in women of reproductive age. They have a mild pro-estrogen effect in menopausal women, which may be beneficial, and a pro-estrogen effect in men and children, which may be detrimental at a high dose.