Soy and Breast Cancer

Soybeans are the most widely used, least expensive, and least caloric way to get large amounts of protein with very little fat and no cholesterol. You can eat soybeans in many forms, including tofu, the beans themselves (also known as edamame), soy milk, miso, and tempeh.

Soy is often promoted as a healthy protein alternative for people who would rather not eat meat or include more plant foods in their diet, mainly because it has a lot of health-related phytochemicals (compounds found in plants).

These phytochemicals include:

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  protein kinase inhibitors, which help keep cell growth and activity normal 

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  phytosterols and saponins, which help regulate cholesterol

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  phenolic acid and phytates, which are antioxidants. Antioxidants are compounds that find and neutralize unstable molecules called free radicals. Some free radicals are natural byproducts of normal cell processes, but others are created when the body is exposed to toxins, such as smoke or ultraviolet light. Free radicals can damage DNA in cells, which can cause mutations that can lead to cancer and other diseases.

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  isoflavones, which are weak phytoestrogens (estrogen-like compounds found in plants). The main isoflavones in soy are genistein and daidzein. Although isoflavones function in a way that’s similar to estrogen, the effect is much weaker. Isoflavones are also antioxidants, and some studies suggest they may offer some protection against cancer, but more research is needed.

The bottom line is that eating soy doesn’t increase your risk of breast cancer.

Some older studies showed that large doses of isoflavones stimulated breast cancer cells to grow in petri dishes and in rats, but the same effect hasn’t been observed in people, who metabolize isoflavones differently.

Some people worry about the estrogen-like compounds in soy. But phytoestrogens don’t act exactly like estrogen. ¹ In some tissues and people, phytoestrogens may block estrogen’s actions. If soy’s estrogen-blocking happens in the breast, then eating soy could, in theory, reduce breast cancer risk because estrogen stimulates the growth of both healthy and cancerous breast cells.

Studies done so far have not offered a clear answer. Some research has shown eating soy may help reduce the risk of breast cancer while others show no links. ^{2  3  4  5} It appears that the effects of soy may vary depending on a woman’s menopausal status, the age at which she eats soy, and the type of breast cancer.

Women in certain Asian countries appear to get more breast cancer protection from soy than American and European women, but this may be because of the difference in the amount of soy Asian women eat.

The Shanghai Women’s Health Study followed more than 73,000 Chinese women for more than seven years and has been the largest and most detailed study of soy and breast cancer risk in a population that eats high amounts of soy. ⁶ The results found that women who ate the most soy had a 59% lower risk of pre-menopausal breast cancer than women who ate the lowest amounts of soy. There was no association between eating soy and post-menopausal breast cancer. Women who ate large amounts of soy when they were adolescents had a 43% lower risk of breast cancer. 

Eating soy also doesn’t appear to increase recurrence risk, and in fact may lower it.

A 2024 study on women who were either receiving breast cancer treatment or who had a history of breast cancer found that soy isoflavones reduced the risk of breast cancer coming back in post-menopausal women and women diagnosed with hormone receptor-positive breast cancer. Eating 60 milligrams of soy isoflavones per day seemed to offer the most risk reduction.

While this research is encouraging, it’s important to remember that soy isoflavones are not a breast cancer treatment. More research is needed to figure out the best amount to eat per day, and whether starting to eat soy after a breast cancer diagnosis has the same effect as eating it as a lifelong habit before being diagnosed.

It’s also important to remember that no food or diet can prevent breast cancer. Dietitians recommend creating a healthy eating plan that contains a wide variety of whole, unprocessed foods and limits processed foods and alcohol.