Soybean (Glycine max) intake is increasing in the U.S., but primarily through consumption of soybean oil, as well as soy protein isolate and soy lecithin in processed foods, and soy-based animal and fish feed. This webpage is intended to examine the impact of consuming relatively unprocessed soy foods on breast cancer risk.
Most of the population studies concerning the consumption of soy foods have been performed in Asian populations. This raises some question as to how applicable the results might be to non-Asian populations since Asians are more likely to convert soy isoflavones to potentially protective metabolites in the gut than whites (see the genistein and daidzein webpage).

Breast cancer-related effects of consuming soybeans

As shown in the brief summaries below, studies of the impact of consuming non-highly processed soy foods on breast cancer risk are somewhat contradictory, but generally have found a protective effect against breast cancer. Several studies have also found that high soy food intake during childhood or adolescence may reduce the risk of breast cancer in later life.
  • A case-control study designed to examine the association between childhood soy intake and breast cancer risk in U.S. women of Asian (Chinese, Japanese, and Filipino) descent found inverse associations with childhood soy intake in all three groups and women born both in Asia and the U.S. The authors concluded that early-life exposure to soy may influence breast cancer incidence.
  • A study designed to compare insulin-like growth factor-I (IGF-I) levels in Japanese women in Japan and Japanese and non-Hispanic whites in Hawaii with high and low soy intake found that average IGF-I levels were 11% lower among women in the highest tofu consumption category compared with the lowest, but the difference in IGF-I levels between tofu consumption levels was only significant among women in Japan. IGF-I is thought to be a link between diet and breast cancer risk since high circulating levels of IGF-I appears to increase cell proliferation and reduce apoptosis. The average IGF-I levels were 213 for Japanese in Japan, 257 for Japanese in Hawaii, and 255 ng/ml for whites in Hawaii. Tofu intake was higher in Japan than among either Hawaiian groups.
  • A case-control study designed to investigate the association between isoflavone intake and breast cancer risk among Japanese, Japanese Brazilians and non-Japanese Brazilians found an inverse association between isoflavone intake and breast cancer risk for Brazilians, whether Japanese or non-Japanese, but only a non-significant inverse association limited to postmenopausal women in Japan.
  • A German case-control study of premenopausal breast cancer found that the highest vs. lowest intake quartiles of dietary daidzein and genistein resulted in significantly reduced breast cancer risk. This protective effect was found to be associated with hormone receptor-positive tumors. High intake of other isoflavonoids such as formononetin and biochanin A, as well as total isoflavonoids, were not associated with a decrease in breast cancer risk. No impact on risk was found for total phytoestrogen intake. The authors concluded that the dietary intake of daidzein and genistein may have an important role in reducing breast cancer risk despite relatively low levels of intake.
  • A U.S. and Canadian case-control study designed to examine the associations between diet and premenopausal bilateral breast cancer found that total fat, monounsaturated fat, polyunsaturated fat, oleic acid, and linoleic acid intake each were found to be inversely associated with risk of premenopausal bilateral breast cancer. On the other hand, intake of carbohydrates and sweetened beverages were associated with increased risk. Reductions in risk were also observed for intakes of low-fat dairy products and tofu. The authors concluded that monounsaturated and polyunsaturated fats, as well as soy foods, might reduce the risk of premenopausal bilateral breast cancer.
  • An Australian study that measured urinary isoflavones, as well as serum and urinary sex steroids in postmenopausal women with recently diagnosed breast cancer (before surgery) and disease-free controls found that the breast cancer cases had lower 24-hour urinary daidzein compared to controls and somewhat lower urinary genistein excretion. Total testosterone was found to be higher in women with breast cancer compared with controls. The authors commented that the findings were in keeping with other observational data demonstrating a protective effect from phytoestrogens on breast cancer risk.
  • A large Japanese prospective study found no significant associations between consumption of soy foods (tofu, boiled soybeans, miso soup) and the risk of breast cancer, including for postmenopausal women as a group. The authors concluded that consumption of soy food has no protective effects against breast cancer.
  • A Japanese case-control study found high intake of soybean products to be associated with lower risk of breast cancer in premenopausal women. The authors concluded that they had found a statistically significant inverse association between tofu or isoflavone intake and risk of breast cancer among premenopausal women but not among postmenopausal women.
  • A Chinese study of 5,042 breast cancer survivors found that soy food intake was inversely associated with breast cancer-related death and recurrence. Risks of recurrence and death both were found to be approximately 30% lower for women with the highest fourth of soy protein consumption compared to the lowest quartile. The results held for both estrogen receptor positive (ER+) and estrogen receptor negative (ER-) breast cancer.
  • A Chinese case-control study found that both premenopausal and postmenopausal women in the highest quartile of total isoflavone intake had reduced risks of all receptor status subtypes of breast cancer. A stronger protective effect of high isoflavone intake was found for breast cancer tumors with concordant rather than discordant receptor status; i.e., those with ER+/PR+ or ER-/PR- status.
  • A large prospective study of ethnically Chinese women in Singapore found that breast cancer risk was significantly reduced in association with high soy intake for postmenopausal women (but not for premenopausal women). The protective effect was stronger in women with above median body mass index than in leaner women.
  • A large Chinese prospective study designed to investigate the impact of soy intake on risk of breast cancer found an inverse dose-response relationship for the intake of all the individual soy food items evaluated in the study, including tofu, fresh soybeans, dried beans, soy products, soy bean sprouts, and soy milk. The inverse association was stronger among pre-menopausal women. When adolescent soy consumption was considered, pre-menopausal women with the highest soy protein intake during both adolescence and adulthood had a more than 50% lower risk compared to women with the lowest soy intake in both periods.
  • A Korean case-control study comparing the diets of breast cancer patients with a healthy control group found that the breast cancer patients consumed significantly fewer eggs and a significantly lower quantity of bean curd (tofu), onion, garlic, green pepper, sweet pepper, kale, cucumber, seasoned bean sprouts, sesame leaf, zucchini, radish, mushroom, crown daisy, red pepper paste, bean paste, spicy bean paste, orange juice, grape juice, and tomato juice than the controls. On the other hand, the breast cancer patients consumed significantly greater quantities of cooked rice, noodles, deep fried chicken, satsuma mandarin, Korean melon, kimchi and coffee than the controls.
  • Another Korean case-control study designed to examine the relationship between fruit, vegetable, and soy intake and the risk of breast cancer found that, in premenopausal women, high tomato consumption was associated with lower breast cancer risk. In postmenopausal women, high grape, green pepper, and soybean consumption were each found to be associated with lower breast cancer risk.
  • On the other hand, a small Korean study reported that soybean and soy isoflavone consumption was found to be associated with increased risk of recurrence among women with HER2 overexpressing (HER2+) tumors, whereas it reduced risk of relapse in women with HER2- disease.
  • In addition, a 2018 study reported that soy milk consumption was associated with increased breast density, a strong breast cancer risk factor, among premenopausal women.
Soybeans are a good dietary source of the lignan enterolactone, as well as coumestrol, both of which have been shown to have chemopreventive properties.
On the other hand, soybeans incorporate some copper (approximately 0.1 mg per ounce of cooked edamame), which could contribute to angiogenesis and metastasis of breast cancer, especially in aggressive forms of breast cancer such as inflammatory (IBC), triple negative (ER-/PR-/HER2-), and HER2 overexpressing (HER2+) disease. Although copper is a vital nutrient, women with breast cancer who are not anemic probably should not exceed the RDA (recommended dietary allowance) of approximately 0.9 mg.

Additional comments

Soybeans should be avoided during radiation treatment because genistein has been shown to protect cells against cell death caused by radiation damage, raising the possibility that consuming soybeans will lessen the cytotoxic impact of radiation on breast cancer cells. Soybeans should also 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.
Edamame are immature soybeans, typically prepared by boiling them whole in their green pods. Tempeh is made by the controlled fermentation of cooked soybeans.
Soybeans have relatively high levels of phytic acid, which binds to some ingested minerals such as calcium, magnesium, iron, and zinc in the intestinal tract, and thereby reduces the amounts available in the body. This can be undesirable for people with a low intake of essential minerals (especially infants and young children).
Soy milk brands listing carrageenan (a potential breast carcinogen) as an ingredient should be avoided.

Sources of information 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, including less recent studies, please click on soybeans.