Soy Food Intake and Breast Cancer
| Soy foods are rich in isoflavones, a group of phytoestrogens that have been suggested to be protective against breast cancer. Others, mostly in the popular press, have expressed concerns that soy might increase the risk of breast cancer. A new large study in China addresses this issue and gives good evidence that soy may actually decrease the risk of breast cancer.
The Shanghai Breast Cancer Survival Study includes 5,033 women who have all survived breast cancer surgery. The study followed these women and also looked at lifestyle factors linked to good health, including diet and especially how much soy they ate. During the next four years 444 deaths and 534 cancer recurrences occurred. Among those women eating the most soy (top 25 percent) compared to those eating the least, breast cancer recurrence or death dropped by nearly one-third (29 percent decrease in death rates from breast cancer and recurrence, and a 32 percent decrease in recurrence) after adjusting for other lifestyle and known risk factors, such as weight, activity level, income level, meat intake, tea intake, cruciferous vegetable intake, and other known risk factors.
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This is good news, because some studies in the laboratory showed that adding genistein (a soy isoflavone) to breast cancer cells in a test tube promoted proliferation of cancer cells. However, in living humans, soy actually reduces the risk of breast cancer from occurring.
An interesting finding in this study was that soy food intake was dose response related (the more soy eaten the greater the benefit) up to an intake of about 11 grams of soy protein daily. Intakes above that had little or no additional benefit. This means that you don't have to eat a lot of soy to lower your risk of breast cancer. A moderate amount, at least 11 grams of soy protein per day, will have maximal benefit.
Foods that will give you at least 11 grams of soy protein daily include:- 1 1/2 cups of soy milk
- 1/2 cup cooked soybeans
- 1 oz of dry roasted soy nuts1/2 cup of tofu (1/4 block)
- 1/3 cup soy miso
- 1/3 cup soy tempeh
Another interesting and valuable finding was that soy foods (at least 11 grams of soy protein per day) were as effective as tamoxifen in protecting against breast cancer recurrence. In fact, women who had the highest level of soy food intake (11+ grams of soy protein/day) and who did not take tamoxifen had a lower risk of mortality and a lower recurrence rate for breast cancer than women who had the lowest soy food intake and used tamoxifen.
If soy helps reduce the risk of breast cancer from reoccurring in high-risk women (women who have already undergone surgery and treatment for breast cancer) it is very likely that it will also have a similar protective effect on primary prevention of breast cancer in the general public. More studies need to be done in this area. It is well known that higher levels of estrogen increase the risk of breast cancer. Soy foods help reduce the amount of estrogen in the body and researchers feel this may be one of the ways soy benefits cancer prevention. Other suggested benefits may be the antioxidant properties of soy, and soy may boost immune response.
The researchers concluded their study by saying, "Among women with breast cancer, soy food consumption was significantly associated with decreased risk of death and recurrence."
Reference:
Shu X, et al. Soy food intake and breast cancer survival. Journal of the American Medical Association. 2009 Dec9;302(22):2437-2443.
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