- Raising Vegetable Eaters
- The Reasons We Eat
- Seafood Choices for Your Health & a Healthy World
- Five Ways to Green Your Kitchen on a Budget
- What a "Natural" Label Really Means
- Cutting Grocery Costs without Cutting Nutrition
- Reduce Food Waste by Eating Smarter, Not Clean Plate Club Membership
- Is Fast Food the Problem?
- Whole Grains: More Than You Think
- Probiotics for Cancer Prevention?
- What's Your Vegetable-to-Meat Ratio?
- Nutritional Gatekeeper: How Big a Role?
Cancer Risk and Cancer Survivors
Soy and Breast Cancer: Where Are We Now?
The pendulum of advice about soyfoods has swung steadily during the last 15 years. Initial hope that soy could lower breast cancer risk soon turned to fear that it might promote its development. A new analysis of soy’s relationship to breast cancer seems to suspend the pendulum in the middle of these two extremes, although further research is clearly needed.
Early speculation that soy might be protective stemmed from population studies that showed lower incidence of breast cancer in Asia (where soyfood consumption is common) than in Western countries like the U.S. Researchers theorized that natural soy compounds called isoflavones, which have weak estrogen-like effects, could lower breast cancer risk by binding to estrogen receptors in breast tissue and blocking the cancer-promoting effects of the hormone. But many experts wondered: If isoflavones have estrogen-like effects, might they actually promote breast cancer?
Many of the known risk factors for breast cancer involve increases in total lifetime estrogen exposure. Research consistently shows that after menopause, estrogens produced within the body promote growth of estrogen-sensitive cancerous (and pre-cancerous) cells. After menopause, when ovaries are no longer producing estrogen, greater body fat boosts estrogen levels and breast cancer risk.
Concern regarding the hormone’s potential effect heightened when a major study found that postmenopausal women treated with combined estrogen plus progesterone showed a 26 percent increase in breast cancer risk. Consequently use of hormone replacement therapy dropped sharply. (Today research suggests that treatment with only estrogen for less than 10 years shows little – if any – breast cancer risk.)
Many theories about soy’s effects on breast cancer risk are based on laboratory experiments and Asian population studies. Both pose challenges for interpretation.
While soy isoflavones do function as weak estrogens in animal and test tube studies, most of these experiments use large amounts of isoflavones – equivalent to five to sixteen times the amount commonly consumed in Asia. As a result, soy’s effects on cell growth in a lab may be difficult to replicate with amounts consumed from food.
Population studies pose other challenges. Although women in Asia who consume at least one serving of soyfoods a day are about 30 percent less likely to develop breast cancer than women who eat soy only occasionally, this may be the result of a lifetime exposure to soy. In fact, research suggests that early exposure to soyfoods may offer protection, while consumption of soy in middle age or later likely has little effect. Researchers hypothesize that adolescence may be a time when soy isoflavones convey a unique structural change to breast cells, making them more resistant to cancer later in life.
The new analysis, published in June’s Nutrition Journal, concluded that soy isoflavones’ estrogen-like effects are probably too weak to have any significant consequence on breast tissue in healthy women – even breast cancer survivors. But many women still seek guidelines for soy consumption. The following is a synopsis of the current state of the evidence:
- Soy consumed at normal dietary levels – one or two servings* daily (up to 100 milligrams of isoflavones) – is probably safe for most women.
- As a precaution, women receiving anti-estrogen treatments should minimize soyfoods and avoid isoflavone supplements.
- Evidence does not support adding soy to your diet hoping that it will help prevent breast cancer. Do note, however, that soy is still a healthful choice. It is low in saturated fat, high in nutrients, fiber and antioxidant phytochemicals.
* One serving of soyfoods equals 8 oz. of soymilk, 4 oz. of tofu or tempeh, or a half-cup of green soybeans (edamame). Soybean oil and soy sauce do not contain isoflavones.