Soy protein isolate is produced by removing most of the fat and carbohydrates from defatted soy flour (made by grinding soybeans). Soybeans have a protein content of approximately 40%, whereas soy protein isolate has a protein content of at least 90%. Soy protein isolates vary widely in isoflavone content — it can be substantial.
Soy protein concentrate is a product similar to soy protein isolate which is produced by removing the water soluble carbohydrates from defatted soy flour. Soy protein concentrate has a protein content of approximately 70% and also contains some carbohydrate, ash and oil. Textured soy protein is made by forming a dough from defatted soy flour plus water, extruding it through a die, and drying it. The extrusion process alters the structure of the soy protein, resulting in a meat-like texture. Textured soy protein contains approximately 50% soy protein. It is used as a meat replacement (e.g., in veggie burgers). Unlike soy protein isolate and soy protein concentrate, textured soy protein is a good source of fiber (in addition to soy isoflavones).
This webpage focuses on soy protein isolate, which is a component of many processed foods, including baked foods, breakfast cereals, soy-based infant formula, and protein shakes and protein bars. Soybean oil, soy lecithin and soy protein isolate in processed foods are the primary forms of soy consumed by many U.S. residents. Separate webpages cover soybeans, soybean oil, soybean paste and tofu. We attempt to untangle the conflicting findings regarding breast cancer risk and soybean isoflavones in the genistein and daidzein webpage.
Soy protein has been shown to improve glycemic control in diabetic postmenopausal women and to improve insulin sensitivity in female ovariectomized monkeys as well as in male monkeys. Dietary intake of soy protein isolate has been found to reduce circulating insulin levels in rats. Dietary soy has also been found to reduce atherosclerotic plaque development in male monkeys. Several studies have found that rats with carcinogen-induced colon cancer that were fed soy protein isolate had reduced tumor incidence compared to control diets. The reduction in colon tumor incidence in soy protein isolate-fed rats was found to be associated with reduced circulating insulin and leptin in one study. One population study of soy consumption and thyroid cancer risk conducted in the San Francisco Bay Area found that consumption of traditional and nontraditional soy-based foods were associated with reduced risk of thyroid cancer. However, consumption of "Western" foods with added soy flour or soy protein did not affect thyroid cancer risk. Soy protein isolate consumption has been found to have potentially beneficial impact on prostate cancer progression in some studies.
The lower estrogen levels associated with menopause typically result in bone loss. Diets supplemented with soy isoflavones or isoflavone-rich soy protein isolate have not been found to prevent bone resorption in postmenopausal women. However, in a study comparing the effects of diets with casein, soy protein isolate, whey protein hydrolysate and rice protein isolate on bone in intact growing female rats and in ovariectomized rats showing signs of sex steroid deficiency-induced bone loss, soy protein isolate was found to have the greatest positive effects in both groups of rats. Soy protein isolate increased serum bone formation markers but suppressed bone resorption markers. It is possible that consumption of soy protein isolate low in soy isoflavones may improve postmenopausal bone health but more studies are required to determine whether this is the case.

Breast cancer-related effects of consuming soy protein isolate

Evidence concerning the impact of soy protein isolate on breast cancer risk consists of animal studies and human experiments. Population studies concerning any possible association between soy protein isolate and risk of breast cancer are not available, which is especially unfortunate since experimental studies have produced conflicting findings.
Feeding soy protein isolate to female rats has been shown in several studies to decrease carcinogen-induced mammary tumor incidence. The female offspring of the rats in one of these studies were found to have smaller mammary fat cells, lower abdominal fat mass and lower overall body weight. Another study found that while rats fed a soy protein isolate diet had a lower tumor incidence than rats fed a control diet, the tumors had more invasive characteristics. The authors ascribed the increased invasiveness to a higher ratio of progesterone receptor A to progesterone receptor B in the soy protein isolate group, accompanied by increased levels of HER-2/neu oncogene. In addition, a 2020 study reported that a soy protein isolate diet resulted in significantly higher rates of tumor development than a casein (milk protein) diet in obese mice. This is the first study to take into account the interaction between obesity and a soy-based high protein diet.
Another study found that soy protein isolate stimulated estrogen positive breast tumor growth and increased uterus weight in ovariectomized mice. Yet another study found that a high soy protein diet increased mitosis (leading to cell division) in the mammary glands of rats. The change in proliferation was associated with increased estrogen receptor α (ERα) and decreased estrogen receptor β (ERβ) expression.
A one-year study of the effects of soy protein isolate consumption found that prolonged consumption of soy protein isolate with a defined level of genistein had little effect on postmenopausal women. However, a stimulatory effect on the breasts of the premenopausal women was observed, characterized by increased secretion of breast fluid, the appearance of hyperplastic epithelial breast cells, and increased levels of plasma estradiol. The authors note that these findings did not support their a priori hypothesis that soy protein isolate consumption would be protective against breast cancer.

Additional comments

Soy protein shakes and powders and other products that list carrageenan (a breast carcinogen) as an ingredient should be avoided. Soybean protein isolate should be avoided during treatment with tamoxifen, since soy foods have been shown to weaken the effects of this treatment in premenopausal women with luminal A breast cancer. In addition, a 2021 study demonstrated that genistein at a dietary level reduced the treatment effects of cisplatin chemotherapy in a mouse model of breast cancer. Another 2021 study reported that habitual consumption of soybeans, soy isoflavones or soy products has the potential to interfere with the effectiveness of Taxol (paclitaxel) chemotherapy.

Sources of information provided in this webpage

The information above, which is updated continually as new research becomes available, has been developed based solely on the results of academic studies. Clicking on any of the underlined terms will take you to its tag or webpage, which contain more extensive information.
Below are links to 20 recent studies concerning this food and its components. For a more complete list of studies, please click on soy protein isolate.