There's no argument about whether soy beans are nutritious: They're packed with protein, very low in saturated fat, and offer fibre, vitamins, minerals and antioxidants.
But the consumption of soy foods by women at high risk for breast cancer and by breast-cancer survivors is controversial.
The concern: Natural compounds in soy called isoflavones resemble human estrogen in chemical structure and therefore could stimulate the growth of hormone-sensitive breast tumours. (Doctors refer to hormone-sensitive cancers as estrogen- and progesterone-receptor-positive cancers. Estrogen fuels the growth of cancer cells in estrogen-sensitive tumours.)
The drugs tamoxifen and anastrozole are commonly used to treat hormone-sensitive breast cancer by lowering the amount of estrogen in the body. It's unclear whether soy isoflavones enhance or decrease the drugs' beneficial effects.
According to a study published this week in the Canadian Medical Association Journal, eating more soy significantly reduced the risk of breast-cancer recurrence among postmenopausal women.
The study, by researchers at the Cancer Hospital of Harbin Medical School in China, involved 524 women who'd had surgery for breast cancer and were followed for five to six years afterwards.
Postmenopausal women who consumed the most soy isoflavones (more than 42 milligrams per day) were 33 per cent less likely to have their cancer return than women who consumed the least (less than 15 milligrams daily).
Soymilk, tofu and soy flour were the main sources of isoflavones. A daily intake of 42 milligrams of isoflavones is equivalent to roughly three-quarters of a cup of firm tofu, one-and-a-half cups of soy milk, half a cup of cooked soybeans or one-third of a cup of soy flour.
Among women with estrogen- and progesterone-receptor-positive breast cancer, the recurrence rate was 13 per cent lower among high consumers of soy compared with low consumers.
Among women receiving anastrozole, those whose diets contained the most soy isoflavones had a 19 per cent lower recurrence rate than women who consumed the least. Researchers speculate that the drug and the isoflavones work synergistically to block estrogen production.
There was no association between soy intake and risk of breast-cancer recurrence in pre-menopausal women.
While large clinical trials are needed to provide more data, there's growing evidence that soy isoflavones may protect against the development and progression of hormone-sensitive breast cancers. Some researchers speculate that the isoflavones block estrogen receptors on tumours, preventing a woman's own estrogen from fuelling cancer growth.
That said, I do advise against the use of isoflavone supplements and soy protein powders, which offer highly concentrated doses. We don't have long-term safety data on the use of these supplements.
SOY FOODS
Soybeans:. Buy them canned or dried. Add cooked mature soybeans to salads, soups, casseroles, chili and curries.
Each half-cup serving contains 149 calories, 14 grams of protein, 8.5 grams of carbohydrate, 5.2 grams of fibre and 45 milligrams of isoflavones
Edamame: These green young soybeans, still in the pod, can be found in the frozen-food aisle of most grocery stores. Boil or steam them and add hulled beans to salads and rice dishes or enjoy them hot or cold as a snack.
A half-cup serving has 127 calories, 11 grams of protein, 10 grams of carbohydrate, 4 grams of fibre and 115 milligrams of isoflavones
Soy nuts: They're made from whole soybeans that have been soaked in water, then baked until crisp and brown. If you have high blood pressure, look for unsalted soy nuts.
A quarter-cup serving has 194 calories, 17 grams of protein, 14 grams of carbohydrate, 3.5 grams of fibre and 63 milligrams of isoflavones.
Soy flour: Soybeans that are defatted and finely ground become soy flour. Replace up to half a cup of all-purpose wheat flour with soy flour in baking recipes.
Two tablespoons of soy flour contain 44 calories, 6 grams of protein, 5 grams of carbohydrate, 2 grams of fibre and 20 milligrams of isoflavones
Soy beverages: Unflavoured products contain much less added sugar and more protein than flavoured varieties; unsweetened brands have no added sugar. Use on cereal, in smoothies, in coffee, in soups and in cooking and baking.
One cup (unflavoured) has 100 calories, 7 grams of protein, 8 grams of carbohydrate and 26 milligrams of isoflavones
Tofu: Made from soybeans, water and a curdling agent, tofu has a mild flavour that takes on the tastes of spices and marinades.
A half-cup of regular, firm tofu has 183 calories, 20 grams of protein, 5.4 grams of carbohydrate, 3 grams of fibre and 28 milligrams of isoflavones
Silken tofu has a soft consistency and is best suited for salad dressings, sauces and desserts. Each 84-gram slice has 52 calories, 5.5 grams of protein, 2 grams of carbohydrate, 0.1 grams of fibre and 36 milligrams of isoflavones.
Leslie Beck, a Toronto-based dietitian at the Medcan Clinic, is on CTV's Canada AM every Wednesday. Her website is lesliebeck.com.
Leslie Beck