In the past several years, soy foods have been showing up regularly on grocery store shelves. And it's not just tofu. Packages of soy nuts, burgers, drinks, cereals, yogurt and desserts as well as edamame have become mainstream grocery items.
At the same time, there's been controversy about the use of soy, especially for breast cancer survivors. The concern is that isoflavones, compounds that occur naturally in soy, could increase the risk of the cancer returning.
But according to a study published in Wednesday's issue of the Journal of the American Medical Association, enjoying a soy latte or soy burger causes no harm – and can reduce the risk of breast cancer recurrence.
Foods made from soy beans are high in protein and contain isoflavones, phytochemicals that compete for the same place on breast cells that estrogen does. By acting like weak forms of the body's own estrogen, some experts have worried that soy isoflavones could possibly promote cancer growth.
That's because certain risk factors for breast cancer, such as beginning your menstrual period before age 12 or starting menopause after 55, are related to the length of time breast cells are exposed to the body's own circulating estrogen. It's thought that estrogen can promote the growth of breast cancer cells.
Women taking tamoxifen, a drug used to reduce the risk of breast cancer recurrence, are often advised to avoid soy foods because soy isoflavones can bind to estrogen receptors on breast cells just like the medication does. The fear is that soy may reduce the effectiveness of tamoxifen. (Tamoxifen works by blocking estrogen in the body from attaching to estrogen receptors on breast cancer cells, slowing the growth of tumours and killing tumour cells.)
In the new study, the largest conducted to date on breast cancer survival, researchers examined the safety of soy food consumption among 5,042 breast cancer survivors, aged 20 to 75, in China. Women were recruited into the research effort – known as the Shanghai Breast Cancer Survival Study – six months after breast cancer diagnosis
After four years, women with the highest soy consumption – measured as soy protein intake – had a 29 per cent lower risk of death and were 32 per cent less likely to have breast cancer recur compared with women with the lowest intake.
The protective effect of soy was observed for women with either early or late stage breast cancer and in women with estrogen-receptor (ER) positive and ER negative breast cancer. (Doctors test breast cancer cells to determine whether they have hormone receptors. If breast cancer cells have estrogen receptors, the cancer is said to be ER positive. If they do not, it is called ER-negative.)
Soy food consumption was associated with improved survival regardless of tamoxifen use. Among women who consumed the most soy, both users and non users were 35 per cent less likely to have breast cancer recur.
Women who consumed the most soy and did not take tamoxifen had a lower risk of cancer recurrence and death than tamoxifen users with low soy intakes. This suggests that high soy intake and tamoxifen use may have a comparable effect on breast cancer survival.
A daily intake of 11 grams of soy protein offered the most benefit, an amount found in about 11/2 cups of soy beverage, one soy burger, 1/2 cup of edamame (young green soy beans) or 1/2 cup of tofu. Higher intakes did not offer greater protection.
Only one previous study has explored soy intake and the risk of breast cancer recurrence. The Life After Cancer Epidemiology (LACE) study, published in November, followed 1,954 northern California early stage breast cancer survivors for six years and found that postmenopausal women with a high soy isoflavone intake – compared with none – had a lower risk of their cancer returning.
