Straight Talk About Soy

a variety of soy foods, including: soybeans, edamame, soy sauce, tofu, tempeh, soy milk
The Takeaway: Soy is a unique food that is widely studied for its estrogenic and anti-estrogenic effects on the body. Studies may seem to present conflicting conclusions about soy, but this is largely due to the wide variation in how soy is studied. Results of recent population studies suggest that soy has either a beneficial or neutral effect on various health conditions. Soy is a nutrient-dense source of protein that can safely be consumed several times a week, and is likely to provide health benefits—especially when eaten as an alternative to red and processed meat.

Soy is exalted as a health food by some, with claims of taming hot flashes, warding off osteoporosis, and protecting against hormonal cancers like breast and prostate.

At the same time, soy is shunned by others for fear that it may cause breast cancer, thyroid problems, and dementia.

Whether reading a popular press article or a well-designed clinical study about soy, some debate remains. As a species within the legume family, nutrition scientists often label soy as a food with potential for significant health benefits. However, due to contrary research that suggests possible negative effects of soy in certain situations, there has been a hesitancy to wholeheartedly promote soy.

Part of the uncertainty is due to the intricacy of soy’s effects on the body. Soy is unique in that it contains a high concentration of isoflavones, a type of plant estrogen (phytoestrogen) that is similar in function to human estrogen but with much weaker effects. Soy isoflavones can bind to estrogen receptors in the body and cause either weak estrogenic or anti-estrogenic activity. The two major soy isoflavones are called genistein and daidzein. Soy isoflavones and soy protein appear to have different actions in the body based on the following factors:

  • Type of study. Is it being examined in a study with animals or humans? Soy may be metabolized differently in animals, so the outcomes of animal studies may not be applicable to humans.
  • Ethnicity. Soy may be broken down and used by the body differently in different ethnic groups, which is why individuals from some countries who eat a lot of soy appear to benefit from the food.
  • Hormone levels. Because soy can have estrogenic properties, its effects can vary depending on the existing level of hormones in the body. Premenopausal women have much higher circulating levels of estradiol—the major form of estrogen in the human body—than postmenopausal women. In this context soy may act like an anti-estrogen, but among postmenopausal women soy may act more like an estrogen. Also, women with breast cancer are classified into hormone type—either hormone positive (ER+/PR+) or hormone negative (ER-/PR-) breast cancer—and these tumors respond differently to estrogens
  • Type of soy. What type of soy is being studied: Whole soy foods such as tofu and soybeans, processed versions like soy protein powders, or soy-based veggie burgers? Fermented or unfermented soy foods? If supplements are used, do they contain isoflavones or soy protein?

Thus, there are many factors that make it difficult to construct blanket statements about the health effects of soy. 

That said, aside from their isoflavone content, soy foods are rich in nutrients including B vitamins, fiber, potassium, magnesium, and high-quality protein. Unlike some plant proteins, soy protein is considered a complete protein, containing all nine essential amino acids that the body cannot make which must be obtained from the diet. Soy foods are also classified as fermented or unfermented (see table with examples, below). Fermented means that the soy food has been cultured with beneficial bacteria, yeast, or mold. Some believe that fermenting soy improves its digestibility and absorption in the body, as this process partially breaks down soy’s sugar and protein molecules.

Unfermented soy foods Isoflavone content (mg) Protein (g)
soy milk, 1 cup 6 7
tofu (bean curd), soft, 3 ounces 20 8
soybeans, mature, boiled, ½ cup 55 15
soybeans, dry roasted, 1 oz. 40 11
edamame, boiled, ½ cup 16 11
soy cheese, 1oz. 2 4
soy burger, 1 patty 5 14

 

Fermented soy foods Isoflavone content (mg) Protein (g)
miso, 3 oz. 37 10
natto, 3 oz. 70 14
tempeh, cooked, 3 oz. 30 13
soy sauce, 1 tbsp 0.02 0

Research on Soy and Disease

Learn more about the research on soy and specific diseases or other conditions:

References

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