I’ve never been a huge fan of soy. I don’t like soybean oil because it contains mostly omega 6 fatty acids, which promote inflammation. I don’t like processed soy protein because it impairs nutrient absorption. I don’t like how much pesticide is sprayed on soybeans.
Inflammation, impaired nutrient absorption, pesticides — that’s why industrially processed soy is bad for hormones. But what about whole bean, organic, and fermented soy? Does it affect hormones? Yes, but possibly not in the way you think.
Is Soy An Estrogen?
Soy contains phytoestrogens or “plant estrogens,” which sounds a bit scary at first. Who wants to ingest a foreign estrogen? But in reality, we ingest a lot of foreign estrogens (also called xenoestrogens).
Xenoestrogens are not estrogen. They’re chemicals that are molecularly similar to estrogen, so they interact with estrogen receptors and with the pituitary-ovarian feedback loop.
We take in a frighteningly large array of man-made xenoestrogens such as pesticides, plastics, and ethinylestradiol (the synthetic estrogen in the birth control pill). Those xenoestrogens are the most potent and concerning xenoestrogens because they cause hormonal problems such as PCOS, endometriosis, infertility, and breast cancer.
Phytoestrogens, on the other hand, are more benign. They’re naturally present in many different foods including flaxseeds, lentils, oats, peanuts, cashews, garlic, cabbage, fennel, apples, coffee, beer, and even dairy and meat. It’s pretty safe to say that at least some level of phytoestrogens has long been part of our traditional diet. We’re adapted to them, and phytoestrogens appear to play a beneficial role in regulating and metabolizing estrogen.
In women, phytoestrogens protect and buffer estrogen receptors from estradiol (our strongest estrogen). They also promote the healthy detoxification of estrogen. So when estrogen is high (as in menstruating women), phytoestrogens have a net anti-estrogen effect. They make periods lighter and reduce high-estrogen symptoms such as PMS and heavy periods . They also seem to be protective against breast cancer in large population studies .
Soy’s anti-estrogen effect is so strong that it can stop periods and impair fertility. Period suppression is the biggest negative effect from soy that I see in my hormone clinic, and that’s why I listed soy as one of the “hidden causes of polycystic ovarian syndrome” in my 4 Types of PCOS post.
What about men, children, and post-menopausal women? When estrogen is low, phytoestrogens have a net pro-estrogen effect. That’s probably okay at low dose, but men and children should avoid too much soy. Post-menopausal women may benefit from phytoestrogen’s pro-estrogen effect. The jury is still out.
Thyroid suppression is second biggest negative hormonal effect from soy. Soy’s isoflavones inactivate thyroid peroxidase, which is the enzyme that makes thyroid hormone . Thyroid suppression is less likely to be a problem if you have sufficient iodine in your diet.
There is really no need to eat a lot of soy because there are so many other more nutrient-dense foods to enjoy (such as meat and vegetables). But you need not fear small amounts of whole-bean soy such as tofu and soy sauce. You might even benefit from them if you suffer from PMS or heavy periods.
You will certainly benefit from small amounts of traditionally fermented soy such as tempeh, miso, tamari (good soy sauce), and natto because they are a great source of vitamin K2.
You should strictly avoid industrially processed soybean oil, flour, and protein powder.
Natural health evangelist, hormone expert, and author of Period Repair Manual, Lara Briden first worked as an evolutionary biologist at the University of Calgary. She went on to graduate as a naturopathic doctor from the Canadian College of Naturopathic Medicine (CCNM) in Toronto. Her love of science and the natural world has informed the way she practices medicine. During her nearly twenty years of practice, thousands of patients have entrusted her with their hormone stories. She shares what she’s learned at larabriden.com.