In a study of estrogen receptor-binding by flavonoid molecules, the authors state:

“Numerous reports have implicated flavonoid phytochemicals as possessing hormone-disrupting activity, in particular acting as environmental estrogens. The endocrine-disrupting effects of flavonoids are seen in examples of sheep grazing on flavonoid-rich clover, and cheetahs fed soy-rich diets have presented with infertility, reproductive abnormalities, and tumors.”19 Yikes!

The isoflavones in soy are also well-known endocrine disruptors in both humans and animals.20,21,22,23 In men, consumption of soy is associated with increased rates of infertility and poor sperm quality.24 The authors of a review paper examining the potential risks of polyphenols note:

“High intakes [of isoflavones] have been associated with reduced fertility in animals and with anti-luteinizing hormone effects among premenopausal women. Furthermore, concerns have been expressed regarding sexual maturation of infants receiving very high levels of isoflavones in soy-based infant formula. This is of particular importance for baby boys, who normally exhibit luteinizing hormone secretion between birth and 6 mo of age.”25 For both males and females, disruption of hormonal signaling by plant compounds is clearly a very bad thing! In both sexes, proper balance between estrogen and testosterone is crucial for libido and the health of hormonally responsive tissues in the breasts, prostate, ovaries, and testicles. Even small doses of endocrine disrupting compounds from plants or the environment can cause things to go awry. As we’ll see later in this chapter, resveratrol has also been found to have hormone-disrupting effects by activating the estrogen receptor. Do we really want to be ingesting plant compounds that can mess with our precious nether regions?

Soy is a really bad actor here, and isoflavones like genistein contained in it also affect hormonal processes negatively at the level of the thyroid:

“Furthermore, a reduction of thyroid peroxidase activity was observed in rats fed a diet supplemented with genistein. These effects of genistein on thyroid function are more pronounced in cases of iodine deficiency. This is of particular concern for babies exposed to particularly high doses of isoflavones through soy feeding.”25 Thyroid peroxidase is one of the enzymes needed to make active thyroid hormones. Reducing or inhibiting its activity leads to a drop in these hormones and other negative effects throughout the body. Catechins in tea have also been connected with thyroid abnormalities as well as decreasing levels of thyroid peroxidase and other enzymes needed for thyroid hormone synthesis in animal models.26 Inadequate levels of thyroid hormones cause fatigue, depression, weight gain, cold intolerance, brain fog, and many other symptoms that make it impossible to lead a radical life.

Soy and the flavonoid compounds within it also appear to worsen our inflammatory response to pathogens. In a recent study, 250 individuals were given a bacterial cell-wall component known to trigger the immune system. The resulting inflammatory markers, including IL-1, IL-6, and TNF-alpha were all higher in those eating the most soy-containing foods.27 More research is needed here, but studies like this suggest that flavonoids and other compounds in soy are not playing nice with our immune system.