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Soy Products and Breast Cancer


There is probably no more confusing area related to the health effects of soy than the relationship between soy intake and breast cancer risk. Soy is a rich and primary dietary source of the phytoestrogen class known as isoflavones. Isoflavones are present in relatively large amounts in virtually all soy foods. The primary isoflavones found in soy, genistein and daidzein, are structurally like human estrogen and can weakly mimic its effect in the body. But, phytoestrogens (isoflavones) genistein and daidzein, though structurally and functionally like 17 β-estradiol and estrone, are less bioactive. Concern over the possibility that the phytoestrogens in soy could stimulate breast tumor growth has led to much confusion among oncologists, health professionals and breast cancer patients.

Soybeans originated in Southeast Asia and were first domesticated by Chinese farmers around 1100 BC. By the first century AD, soybeans were grown in Japan and many other countries. Today the soybean is economically the most important bean in the world, providing vegetable protein for millions of people and ingredients for hundreds of chemical products. More than 2 billion people live in China, Japan, Korea and southeast Asia - and they consume on average more soy than the average American - every single day of their lives. However, the soy they consume is primarily fermented and fermented soy has been a part of their diet for thousands of years.

The soybean is one of the richest and cheapest sources of protein and is a staple in the diets of people and animals in numerous parts of the world. In East Asia the bean is extensively consumed in the forms of soy milk, a whitish liquid suspension, and tofu, a curd somewhat resembling cottage cheese. Soybeans are also sprouted for use as a salad ingredient or as a vegetable and may be eaten roasted as a snack food. Young soybeans, known as edamame, are commonly steamed or boiled and eaten directly from the pod. Soy sauce, a salty brown liquid, is produced from crushed soybeans and wheat that undergo yeast fermentation in salt water for six months to a year or more. Soy sauce is a ubiquitous ingredient in Asian cooking. Other fermented soy foods include tempeh, miso, and fermented bean paste.

Initially the low rate of breast cancer in these soy-consuming Asian countries led to the hypothesis that soy might reduce breast cancer risks. The initial support for soy’s preventive effect came from a 1990 study on rats, which found that 50% of soy-fed rats developed fewer mammary tumors.1 A second study in 1991 found that a soybean protein diet, low in methionine content, fed 5 weeks after carcinogen exposure significantly repressed mammary tumor progression.2 Since then, numerous subsequent research has shown that phytoestrogens in soy foods have been linked to lower incidences of breast and other cancers, improved heart health, and potentially lowered risk of osteoporosis.3, 4, 5, 6, 7, 8, 9, 10

What we know at this point is isoflavones have been shown to be estrogen blockers. In other words, they may block estrogen from reaching receptors and, therefore, potentially protect women from developing breast cancer. A 2006 study showed that dietary soy isoflavones antagonized estradiol (E2) effects on breast proliferation in a dose-dependent manner, altered estrogen receptor activity in the breast, and resulted in lower serum E2 concentrations. They concluded that a diet rich in soy isoflavones may have an estrogen-sparing effect in the postmenopausal breast, potentially limiting cancer risk associated with higher estrogen concentrations.11

Although Asians have consumed soy products for centuries, it is primarily used as a condiment and usually in fermented form, such as natto, miso, tempeh, gochujang, douchi, doenjang, and soy sauce. In Japan, Korea and China, the average person eats about an ounce of fermented soy each day, much less than the amounts of unfermented soy in American diets. Soy products promoted in the U. S. are generally unfermented, such as edamame, tofu, and soy milk, and as such have not undergone chemical changes that neutralize some potentially harmful compounds within soybeans. Few Americans regularly eat natto or its healthy fermented cousins - miso and tempeh.

Research has shown that soy proteins become more digestible with fermentation. The friendly bacteria or probiotics found in fermented soy help nourish the gut and digestive flora, boosting digestion and the absorption of nutrients. Because much of the immune system resides in the intestinal tract, these beneficial soy products also aid immune function. A significant percentage of soy proteins get broken down into shorter protein strands (called polypeptides) or even into single amino acids during the process of fermentation. These protein forms require less chemical activity in our digestive tract and are much better prepared for digestion than whole, intact proteins. Perhaps one of the most beneficial characteristics of fermented soy is the presence of vitamin K2. Vitamin K2 is an important vitamin, and it is not uncommon for people to be deficient in it.

Unfermented soy products like tofu and soy milk contain the following compounds that are disturbing to the body:

  • Lectins and saponins that are linked to altered bowel permeability, as well as other gastrointestinal and immune problems;

  • Oligosaccharides that abnormally affect the intestinal microbiome and cause gas;

  • Oxalates that are known to promote kidney stones;

  • Protease inhibitors that interfere with protein digestion and have caused malnutrition, poor growth; digestive distress and pancreatitis;

  • Phytates that block mineral absorption, causing calcium, zinc, and iron calcium deficiencies.

Cooking tofu does not reduce the level of phytates and other antinutrients. What does reduce these antinutrients is fermentation.

Soy is a widely planted genetically modified (GM) crop that is used to produce numerous genetically modified foods. Alarmingly, over 90% of the soy in the U.S. is genetically modified. There exists worldwide controversy over both health safety and environmental concerns of GM crops. That topic is too immense for through discussion here. In brief, as GM foods are increasinglyintroduced into our diet