Many breast cancer survivors avoid eating soy over worries it may raise the risk for their breast cancer to recur. Now, a new study finds just the opposite: that patients who eat a lot of soy have a lower risk of breast cancer recurrence and death.
The study, published in JAMA, the Journal of the American Medical Association, analyzed data from the Shanghai Breast Cancer Survival Study, a large study of more than 5,000 female breast cancer survivors in China.
Women aged 20 to 75 diagnosed with breast cancer between March 2002 and April 2006 were recruited and followed through June 2009.
After almost four years, the researchers recorded 444 total deaths and 534 recurrences or breast cancer-related deaths among the group. After looking at data on their eating habits, the researchers found that patients with the highest intake of soy protein had a:
- 29 per cent lower risk of death during the study period
- 32 per cent lower risk of breast cancer recurrence compared to patients with the lowest intake of soy protein.
The group of women who ate the lowest amount of soy protein had a 10.3 per cent death rate over four years, compared to a 7.4 per cent rate among those women with the highest soy protein intake.
As well, the breast cancer recurrence rate was 11.2 per cent among those who ate the lowest amount of soy protein and 8.0 per cent among those with the highest soy protein intake.
The link between high soy intake and low death and recurrence risk "was evident among women with either estrogen receptor-positive or -negative breast cancer and was present in both users and non-users of tamoxifen," the researchers write.
Soy foods are rich in substances called isoflavones, which can act as weak plant-based estrogen, called phytoestrogens. These estrogen-like effects, and the possiblity that isoflavones can interact with tamoxifen, have led to concern that breast cancer patients should avoid the foods.
But the authors of this study conclude that "soy food intake is safe and was associated with lower mortality and recurrence among breast cancer patients."
The authors note that no additional benefits on death rates and cancer recurrence were observed with women who ate more than 11 grams of soy protein a day -- about the equivalent of two glasses of soy milk a day.
"This study suggests that moderate soy food intake is safe and potentially beneficial for women with breast cancer," they conclude, adding that more research is needed on the effects of soy on different types of breast cancer.
Some other doctors, though, are noting there were limitations with the study.
Dr. Rachel Ballard-Barbash of the National Cancer Institute, Bethesda, Md., and Marian L. Neuhouser of the Fred Hutchinson Cancer Research Center, Seattle, write in an accompanying editorial that the follow-up time of the study was relatively short.
They also note there likely are differences in cancer screening rates in China compared with the U.S., and there may differences in the quality and type of soy food intake between China and the U.S.
Most significantly, they note that soy food consumption is much higher in China than in the U.S.
