Soy and Cancer Risk
Consuming moderate amounts of soy foods as part of a healthy diet does not increase cancer risk and may even be beneficial for certain cancers, but high-dose soy supplements should be avoided, especially by breast cancer survivors.
Understanding Soy and Cancer Risk
General Relationship Between Soy and Cancer
Soy contains several phytochemicals, including isoflavones that have weak estrogenic activity. According to the American Cancer Society guidelines, soy-derived foods are an excellent source of protein and a good alternative to meat 1. The relationship between soy consumption and cancer risk has been studied extensively, with the following findings:
- Traditional soy foods (like tofu and soymilk) may be associated with decreased risk of certain cancers, particularly breast and prostate cancers 1
- The most recent meta-analysis (2024) found that high consumption of total soy products, especially tofu and soymilk, was associated with reduced overall cancer risk 2
- There is no evidence that moderate consumption of soy foods increases cancer risk 1
Specific Cancer Types and Soy
The relationship between soy and specific cancer types varies:
- Breast Cancer: Despite initial concerns about soy's weak estrogenic effects, recent studies show that soy food consumption is not adversely related to breast cancer prognosis 3. In fact, higher soy intake has been associated with decreased risk of death and recurrence in breast cancer survivors 4
- Hormone-Dependent Cancers: While animal studies suggest protective effects against hormone-dependent cancers, human studies comparing Asian and Western populations show differences in cancer risk that may be due to factors other than soy consumption 1
Recommendations for Soy Consumption
General Population
- Incorporate moderate amounts of soy foods as part of a healthy plant-based diet 1
- Focus on traditional soy foods like tofu and soymilk rather than processed soy products or supplements 2
- Soy foods can be used as a healthy alternative to red meat, which may help reduce cancer risk 1
Special Populations
- Breast Cancer Survivors: Should consume only moderate amounts of soy foods as part of a healthy plant-based diet 1
- Everyone: Should avoid high-dose soy supplements or concentrated sources of soy isoflavones 1
Important Caveats and Considerations
Soy Supplements vs. Whole Soy Foods
- There is no convincing data that soy supplements are beneficial in reducing cancer risk 1
- There is a theoretical possibility that high doses of soy might increase the risk of estrogen-responsive cancers in certain individuals 1
- Avoid soy-containing pills, powders, or supplements containing isolated or concentrated isoflavones 1
Quantity Considerations
- The 2024 meta-analysis found that a 54g per day increment of total soy products reduced cancer risk by 11%, a 61g per day increment of tofu reduced cancer risk by 12%, and a 23g per day increment of soymilk reduced cancer risk by 28% 2
- Moderate consumption appears safe and potentially beneficial, while very high intake or supplement use should be avoided 1
Research Limitations
- Many studies comparing soy intake and cancer risk are observational and compare Asian and Western populations, whose differences in cancer risk may be due to factors other than soy consumption 1
- More prospective cohort studies are needed to confirm the causal relationship between soy product consumption and cancer risk 2
In summary, moderate consumption of traditional soy foods as part of a healthy diet appears safe and may even be beneficial for cancer prevention, while high-dose soy supplements should be avoided, particularly by those with a history of hormone-sensitive cancers.