Soy and Breast Cancer - Research, Controversy and Your Diet

Does Soy Prevent Breast Cancer - or Make It Grow?

Soybeans. Photo © USDA, by Scott Bauer
Soy is one of those "wonder foods" that used to be sold only in health food stores or Asian markets in western countries. In the last several years, soy has been showing up regularly on the shelves of mainstream grocery stores, packaged in an amazing variety of products and flavors. At the same time, a controversy has been brewing – is soy healthy or risky for breast cancer patients and survivors?
Do soy foods protect you from cancer, or do they hasten its development? Before you throw out the tofu with the miso soup, or rush out to buy some soy supplements, let's take a look at soy foods and their health impact.

Soy Foods – More Than Just Tofu and Soy Sauce
Soy foods are made from soybeans – a crop that, until the 1980s, has been used in America primarily as livestock feed, but has been a part of the Asian diet for many generations. Soy is available as: edamame (green soy beans), tofu, soy milk, soy powder and flour, miso paste, tempeh, oil, and textured vegetable protein (TVP). Soy shows up in many meat analogue products – meatless meatballs, "burger" style crumbles, and even bacon-like strips and chicken-shaped nuggets.

Benefits of Soy Foods
Products based on soy can make great entrees for those on a vegetarian diet, and some products are even suitable for vegans. Tofu and tempeh can be cooked as part of an Asian meal and combined with just about any flavoring.

Soy is high in protein, helps lower cholesterol and blood pressure, and may help with menopause symptoms and osteoporosis. The right amount of soy in your diet helps guard your heart health by lowering LDL cholesterol and triglycerides. The FDA has officially ruled that 25 grams of soy protein consumed daily is considered beneficial.

They also note that consumers must read the labels carefully to see whether the right amounts of soy have been added to products.

Soy Chemistry Controversy - Isoflavones
Soybeans contain all the amino acids essential to human nutrition. Soy foods contain isoflavones (phytoestrogens). These isoflavones have powerful antioxidant properties, and may be able to prevent cell damage (oxidation) caused by free radicals. Soy isoflavones can act like weak estrogens, and may block estrogen receptors, similar to the way that tamoxifen works to prevent a recurrence of estrogen-sensitive breast cancer. Sounds great, doesn't it? But there may be a problem of "too much of a good thing." Just as an excess of natural estrogen may fuel the growth of a breast tumor, too much of the soy isoflavone genistein, in concentrated form in many over-the-counter nutritional supplements, may set the stage for tumor development. But what about Asians who grow up on tofu? Let's look at their rates of breast cancer.

The Well-Polished Chopstick – A Lifetime of Soy and Green Tea
Japanese women typically consume soy beginning in childhood, which may be a key to prevention of breast cancer.

In April of 2008, a Japanese study was published on soy consumption and rates of breast cancer. In this study, Dr. Iwasaki and his team recruited 24,226 Japanese women aged 40 to 69. Their study covered 10.6 years, and the women in the study did not keep a food journal, which is sometimes an unreliable component of such a study. The researchers used blood and urine samples to measure isoflavone levels. Women who had the most consistently high levels of genistein (isoflavone from soy) had the lowest rates of breast cancer.

Dietary Soy Versus Soy Supplements
The isoflavones found in soybeans, sesame seeds, and legumes are about one hundredth as powerful as natural female estrogens. If you're getting your isoflavones from dietary sources, you'd have a hard time overdosing yourself, unless you went on an all-soy diet. So wouldn't those capsules containing soy isoflavones that are sold as hormonal support and bone health protection be safe? The answer is: it depends. Pills with isolated soy isoflavones may cause trouble – not enough studies have been done yet on people to determine whether or not high concentrations of those isoflavones may encourage the growth of breast cancer. If you're taking soy supplements to help with menopausal symptoms, speak with your doctor about what level of isoflavones may be safe for you.

When Should You Avoid Soy?

Reasons to be Shy About Soy
While soy may help relieve your hot flashes, researchers caution postmenopausal women against having too high a dose of soy, particularly in the form of supplements that contain high amounts of soy isoflavones. And if you've had estrogen-sensitive breast cancer, and are taking a selective estrogen receptor modulator, such as tamoxifen, or an aromatase inhibitor, such as Aromasin, it's a good idea to refrain from soy.

The soy isoflavone genistein may counteract estrogen suppressors - and that would make your post-treatment medication less effective. After you've completed a full course of estrogen suppressors (usually 5 years) you can start including soy in your diet again, in modest amounts. If you still want the benefits of isoflavones, try dining on legumes, whole grains, and nuts. On the other hand, a good reason to avoid soy altogether is if you know that you're allergic to it. You should also skip soy if you have a thyroid disorder or goiter.

The Bottom Line on Soy and Breast Cancer
You may get the most benefits from consuming soy isoflavones such as genistein, if the isoflavones come from food – not from nutritional supplements. The American Cancer Society says that concentrated extracts of soy isoflavones may encourage tumor growth, and should be avoided. Women in the Japanese study who had the lowest rates of breast cancer had consumed soy from childhood, or at least from pre-puberty.

Post-menopausal women should not overdo soy products, because the powerful isoflavones mimic natural estrogen, which fuels 80% of all cases of breast cancer. Adults who start a diet that includes 25 grams of soy foods daily will experience some benefit from soy isoflavones (lower cholesterol, better heart health) but will not gain the same protection from cancer as people who have eaten soy regularly over a lifetime.

The Soy and Breast Cancer Controversy

American Cancer Society. Soybean. Revised: 07/12/2007.

American Cancer Society. Soy May Counteract Tamoxifen Used by Breast Cancer Patients. Published: 06/20/2007.

U. S. Food and Drug Administration. Soy: Health Claims for Soy Protein, Questions About Other Components. FDA Consumer, May-June 2000. John Henkel.

J Clin Oncol. 2008 Apr 1;26(10):1677-83. Plasma isoflavone level and subsequent risk of breast cancer among Japanese women: a nested case-control study from the Japan Public Health Center-based prospective study group. Iwasaki M, et al.


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