Soy is a type of legume that contains plant compounds called phytoestrogens. These compounds share similarities with the hormone estrogen, which plays a role in the development of certain breast cancers. For this reason, scientists have investigated whether soy may affect breast cancer development and progression.

Breast cancer is the most common cancer globally. Though treatment advances are improving survival rates, dietary changes may also play an important role in breast health.

Experts generally agree that soy consumption does not have a negative effect on breast cancer risk and that it may, in fact, have some protective effects.

This article outlines the link between soy consumption and breast cancer risk, including whether it increases or decreases the risk of breast cancer development or recurrence. We also provide guidelines on how much soy a person should eat.

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The current scientific consensus is that soy does not increase a person’s risk of developing breast cancer.

The myth that soy increases breast cancer risk comes from the discovery that soy contains plant compounds called phytoestrogens. These compounds share similarities with the hormone estrogen.

While estrogen can sometimes trigger the growth of breast cancer cells, there is no evidence that phytoestrogens have this effect.

According to the Dana-Faber Cancer Institute, a person should consider the following regarding soy:

  • Phytoestrogens do not turn into estrogen when consumed.
  • Phytoestrogens are structurally different and much weaker than human estrogen.
  • Moderate consumption of soy does not increase cancer growth.

Does it increase risk?

According to a 2016 review, studies over the past 25 years consistently show that phytoestrogen intake does not adversely affect breast cancer risk.

However, not all research fully agrees. According to a 2017 study, soy-containing products have both positive and negative effects on breast cancer cells. The researchers noted that many studies tested the effects of phytoestrogens on breast cancer cells in vitro, which does not necessarily indicate how the cells would respond in animal models or humans.

They also noted that studies used different amounts of soy that was derived from different sources, making cross-study comparisons difficult.

Primary and secondary sources of soy

When considering whether soy increases the risk of breast cancer, it may be important to differentiate between primary and secondary sources of soy. Primary sources include tofu, tempeh, and edamame. Secondary sources refer to products that contain soy, such as soy-based meat derivatives and meat products with added soy protein.

According to a 2017 study, secondary sources of soy contain significantly more phytoestrogens, which may affect breast cancer risk. The researchers noted that females in China who consume large amounts of primary soy products showed a lower risk of developing breast cancer.

The American Cancer Society notes that soy consumption from primary sources may lower the risk of breast cancer. Overall, they state that soy foods are both healthy and safe.

Does it decrease risk?

Some evidence suggests that consuming soy may decrease a person’s risk of developing breast cancer.

A 2016 review mentions that observational studies show that higher soy consumption is associated with an approximate 30% reduced risk of developing breast cancer in Asian women. However, the review mentioned that current evidence suggests that consumption must occur early in life for soy to reduce breast cancer risk.

According to the breast cancer organization Susan G. Komen, soy seems to have a protective effect against breast cancer in Asian countries where people begin consuming soy products earlier in life and in higher quantities. They note that there is a significant difference in soy consumption in the United States and Japan.

The average daily intake of soy in the U.S. is 1–3 milligrams (mg), while the average daily intake in Japan is 25–50 mg.

Overall, findings suggest that the amount of soy a person consumes affects the reduction in their breast cancer risk. It appears that soy has protective effects if a person consumes it in high enough quantities.

The consensus is that a person who has a diagnosis of breast cancer can safely consume soy products.

Products containing soy as a food additive in the form of soy lecithin and soy oil are also generally safe for people living with breast cancer. These products do not contain any phytoestrogen.

However, the Dana-Farber Cancer Institute points out that many doctors recommend that people with hormone-sensitive cancer minimize their intake of soy protein powder supplements or soy protein isolate.

Anyone who is considering making significant changes to their diet should speak with a doctor before doing so.

Evidence suggests that consuming high levels of soy may help reduce the risk of breast cancer recurring.

A meta-analysis from 2012 investigated breast cancer survival among women who began consuming soy following a diagnosis of breast cancer. The analysis found that women who consumed 10 mg or more of soy daily had a 25% reduced risk of cancer recurrence compared to those who consumed less than 4 mg of soy daily.

A 2019 meta-analysis also found that the consumption of soy isoflavones both before and after diagnosis was associated with a reduced risk of breast cancer recurrence.

There is currently insufficient evidence to determine whether soy supplements affect breast cancer.

It is important to note that the Food and Drug Administration (FDA) does not regulate the supplement market in the same way as it does pharmaceuticals. This means that different soy supplements may contain higher or lower concentrations of soy-based products.

As mentioned above, many healthcare professionals advise those with hormone-sensitive cancers to minimize their intake of soy protein powder supplements. A person should speak with a doctor about their risk factors before starting a soy supplement.

It is not clear how much soy a person should eat to experience any beneficial effects against breast cancer. Research among Asian populations suggests that eating between 25–50 mg of soy a day may provide a positive preventive effect.

A person with breast cancer can consult their medical team if they have questions regarding their diet and whether they should increase their soy intake. They should avoid significantly increasing their soy intake, especially through dietary supplements and protein powders, unless advised to do so.

The University of California San Francisco Health lists the following common soy foods along with their soy protein content range:

  • 3 ounces (oz) of water-packed tofu: 6–13 grams (g)
  • 3 oz of silken tofu: 6 g
  • 1/2 cup tempeh: 16–22 g
  • 2/3 cup edamame: 6 g
  • 8 oz of plain soy milk: 3–10 g
  • 1/4 cup soy nuts: 12 g
  • 1/2 cup canned white soybeans: 13 g
  • 1/2 cup canned black soybeans: 11 g
  • 2/3 cup green or sweet soybeans: 7–9 g
  • 2 tablespoons soy nut butter: 6–8 g
  • 1 soy burger: 10 g
  • 1/2 cup rehydrated textured vegetable protein: 12 g

Soy contains plant compounds called phytoestrogens, which share similarities with the hormone estrogen. Because of these similarities, scientists have investigated whether soy consumption affects the risk of breast cancer development or recurrence.

Eating primary sources of soy may have a positive impact on breast cancer prevention and survival. However, further studies are necessary to help determine the amount of soy a person needs to eat to experience these beneficial effects.

Experts generally agree that soy consumption is safe following a diagnosis of breast cancer. However, people should speak with a doctor before making drastic changes to their diet, particularly if they are receiving treatment for or recovering from cancer.