soy
Meaning of soy: A bean whose plant compounds resemble estrogen closely enough to be interesting and loosely enough to behave quite differently.
Definition of soy
Soy is the edible bean of an Asian legume, grown as a staple food and processed into tofu, soy milk, tempeh, miso and protein isolates. It supplies protein containing all the essential amino acids, unsaturated fat and fiber, and it also contains isoflavones, plant compounds with a structure similar enough to estrogen that they bind weakly to the same receptors.
What are isoflavones and how do they behave?
They are plant compounds, chiefly genistein and daidzein, whose shape resembles that of estrogen. They attach to the same receptors but weakly, and they prefer one of the two receptor types over the other. Because the two receptor types are distributed differently around the body, the result is not a general estrogen effect but a patchy one, weakly estrogen-like in some tissues and mildly opposing in others. That is why blanket statements about soy and hormones tend to be wrong in one direction or the other.
Why do results vary between people?
Part of the answer is bacterial. One of the isoflavones is converted by gut bacteria into a more active compound, and only some people carry the bacteria that perform the conversion. The proportion who do is higher in populations with long dietary exposure. Two people eating identical amounts can therefore end up with different active compounds circulating, which helps explain why studies conducted in different countries have reached different conclusions.
What has been studied?
| Question | State of the evidence |
|---|---|
| Blood cholesterol | Small reduction in cholesterol from soy protein replacing animal protein |
| Menopausal hot flashes | Modest and inconsistent effect |
| Bone loss | Weak and inconsistent for osteoporosis |
| Breast cancer | No evidence of harm from dietary soy; observational data suggest neutral or favorable |
Why was there concern about hormone-sensitive cancer?
Isolated tumor cells in a dish grow faster when exposed to genistein, and rodents given large doses showed the same. Those findings drove a long period of caution about soy foods for people with an estrogen receptor positive tumor. Human studies did not reproduce it, and the doses involved in the laboratory work far exceeded what food provides. Concentrated supplements deliver quantities closer to those experimental levels and are not equivalent to the food.
Also known as
Used in a sentence
Soy protein replaced part of the animal protein in the study diet.
Good to know
Isoflavones bind estrogen receptors weakly and preferentially at one of the two receptor types, so their effect is tissue-dependent rather than uniformly estrogen-like, and concentrated supplements are not equivalent to soy foods.