soybean
Meaning of soybean: A legume unusual for supplying complete protein, and unusual again for containing plant compounds shaped like a human hormone.
Definition of soybean
The soybean is the seed of Glycine max, a legume native to East Asia and now among the world's major crops. It is unusual among plant foods in supplying all the amino acids the body cannot make for itself. Soybeans also contain isoflavones, plant compounds with a weak action resembling that of estrogen.
What is in a soybean?
Roughly a third of the dry weight is protein and a fifth is oil, a combination no other common bean matches. The protein contains adequate amounts of every amino acid humans cannot build for themselves, which is rare in a plant and explains why soy foods feature so heavily in diets without meat. The oil is mostly unsaturated. Soybeans also supply fiber, potassium, iron and folate. They contain compounds that block digestive enzymes as well, which is one reason the raw bean is not eaten and cooking or fermentation always comes first.
What are isoflavones?
These compounds account for most of the interest in soy and most of the argument about it. Their shape lets them bind the receptor that estrogen normally occupies, although they act far more weakly, and their effect differs between tissues, sometimes imitating the hormone and sometimes blocking it. That dual behavior is why simple statements about soy and hormone-sensitive conditions tend to be wrong in one direction or the other. Studies of hot flushes around menopause have given mixed results with modest benefit at best, and studies of bone have not shown that soy prevents osteoporosis.
How is it eaten?
| Food | How it is made |
|---|---|
| Tofu | Curdled soy milk pressed into blocks |
| Tempeh | Whole beans fermented into a firm cake |
| Miso and soy sauce | Fermented, used mainly as seasoning |
| Edamame | The immature bean, cooked in the pod |
| Soy protein isolate | Extracted protein added to processed foods |
What does the evidence show on heart disease and cancer?
Replacing animal protein with soy protein produces a small reduction in cholesterol, an effect accepted well enough to be permitted as a health claim in some countries, though its size is modest. For cancer the picture is unsettled. Populations in East Asia, where soy is eaten from childhood and in larger amounts, show lower rates of several cancers, but those diets differ in many other ways at the same time. Current evidence does not indicate that soy foods raise risk in breast cancer, and concentrated isoflavone supplements are a separate question from whole foods.
Also known as
Used in a sentence
The dietary history recorded soybean products as the main source of protein.
Good to know
Isoflavones can imitate or block estrogen depending on the tissue, so findings about whole soy foods and about concentrated supplements are not interchangeable.