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.
Findings from the LACE study also revealed that among postmenopausal women treated with tamoxifen, those who consumed the most soy were 60 per cent less likely to have their breast cancer recur compared with women with the lowest intakes.
Soy foods may protect from breast cancer in a number of ways. Isoflavones may decrease circulating estrogen levels and increase its removal from the body. Soy also contains folate, calcium, fibre, protein and many phytochemicals that, individually or together, may help combat cancer.
While it appears that moderate soy consumption is probably safe for breast cancer patients and survivors, keep in mind these findings are confined to soy foods, not soy supplements.
In fact, research suggests the effects of soy foods and supplements differ. In one study, soy milk reduced circulating estrogen levels but isoflavone supplements had no effect. And we don't have data on the long-term safety of these supplements.
In addition to adding soy to your diet, the following strategies may also help reduce the risk of breast cancer recurrence.
Soy protein in foods
The study published in today’s Journal of the American Medical Association found that consuming 11 grams of soy protein per day reduced the risk of breast cancer recurrence and death. Higher intakes did not confer extra protection.
Soy beans, canned, ½ cup 14 grams
Edamame, ½ cup 11 grams
Soy nuts, roasted, unsalted, ¼ cup 11 grams
Soy beverage, unflavoured, 1 cup 8 -9 grams
Soy yogurt, 1 cup 5 grams
Soy nut butter, 2 tbsp. 7 grams
Tofu, firm, ½ cup (125 g) 20 grams
Tofu, regular, ½ cup (125 g) 10 grams
Soy ground round, 1/3 cup 10 grams
Soy burger, 1 patty (14 g) 14 grams
Soy hot dog, 1 (46 g) 10 grams
Soy frozen dessert, ¾ cup 1 grams
Eat more vegetables and fruit
Strive for at least seven servings a day. Studies suggest that cruciferous vegetables (broccoli, Brussels sprouts, cabbage, cauliflower) and leafy greens are especially protective from breast cancer.
Research suggests it's prudent to reduce your intake of dietary fat, particularly if you've been diagnosed with ER-negative breast cancer. Choose lean meats, poultry breast, legumes and non-fat dairy products. Use high-fat spreads and salad dressings sparingly.
Control your weight
Breast cancer survivors who are overweight or obese are more likely to have cancer recur compared with leaner survivors. Women are advised to keep their body mass index (BMI) in the range of 21 to 23 and limit weight gain and increases in waist circumference.
Moderate to vigorous exercise has been shown to improve breast cancer survival; the evidence is strongest for women with ER-positive tumours and those diagnosed with advanced breast cancer. Regular exercise helps reduce breast cancer recurrence by preventing weight gain and reducing levels of circulating hormones.
Breast cancer survivors are advised to get at least 150 minutes (21/2 hours) a week of moderate intensity exercise (such as brisk walking) or 75 minutes a week of vigorous exercise (running, elliptical trainer, cardio machines).
Leslie Beck, a Toronto-based dietitian at the Medcan Clinic, is on CTV's Canada AM every Wednesday. Her website is lesliebeck.com.