The Confusion About Soy And Breast Cancer
Valerie Franc B.Sc., N.D.
Doctor of Naturopathic Medicine
There has been much controversy and confusion about the safety of eating soy if you have been diagnosed with breast cancer. Since Stephen and I focus on cancer prevention and treatment in our clinic, we felt it important to address this debate. Soy contains 'phytoestrogens', tiny components of the plant that act as weak estrogens in the body. Many breast cancer (as well as other types of cancer) tumors are estrogen positive - meaning that they grow bigger and faster when they come in contact with estrogen. As well estrogen has be shown to play a role in cancer formation. This has caused a concern for some oncologists as well as patients, who feel that plant based 'pseudo-estrogens' will greatly increase the likelihood of developing cancer. On the other hand, there are many doctors and researchers that promote the use of soy products. They feel that eating soy actually helps prevent breast and other types of estrogen related cancer. Who do you listen to?
Here's what we know:
Phytoestrogens can be split into two different types, isoflavones and lignans. Soy products contain isoflavones, specifically ones called genistein and daidzen (genistein is stronger than daidzen). Some herbs, legumes and sprouts also contain isoflavones. Flaxseeds are a common source of lignans, but they can also be found in pumpkin seeds, berries, some vegetables and many grains.
We are so used to hearing the term estrogen that we commonly think it is just one molecule, but it is not. There are 3 types of estrogen found in the body, each with different strengths. Estriol is the weakest acting form of estrogen, and has been associated with protection against cancer. Estrone and estradiol are much stronger acting forms of estrogen. These two forms are associated with cancer formation. It is interesting to note that almost all forms of hormone replacement therapy and birth control pills use a form of estrogen known as 17 beta-estradiol. This is the strongest estrogen and is known to promote cancer.
Phytoestrogens act as weak estrogens (similar to estriol). They bind to breast (and other estrogen sensitive tissues such as the uterus) cell receptors, and prevent other stronger estrogens (such as estrone and estradiol) from binding. As a result strong estrogens are unable to act on tissues and are excreted from the body, preventing them from initiating cancer growth (1). Although phytoestrogens are strong enough to bind to the receptors, it is important to note that they are too weak to affect the DNA of the cell. Therefore they are unable to promote or initiate breast cancer. Another benefit of phytoestrogens is that they increase the production of steroid hormone binding globulin (SHBG). SHBG is responsible for binding to estrogen and carrying it through the blood. As the number of these transporter molecules increases, the amount of free estrogen available to bind to tissues decreases. Therefore there is less estrogen available to promote cancer growth and formation. In short phytoestrogens can have an anti-estrogenic effect on breast and uterine tissues. At the same time, they still are able to have a weak estrogenic effect on other tissues such as bone (this is important in osteoporosis prevention and treatment).
Research has shown that soy isoflavones (particularly genistein) can help prevent as well as reverse breast cancer (2,3). They have been found to be involved in processes that help the cell to grow and divide normally. They also help protect cells from being damaged by free radicals (well known to accelerate aging as well as cause cancer). In this way they help prevent cancer (4). There is evidence that they are involved in a process (called apoptosis) that tells damaged cells to die. They have also been proven to prevent cancer cells from making new blood vessels and can turn off breast cancer genes (by shutting down protein tyrosine kinases). As a result phytoestrogens can help treat cancer as well. Studies of differing populations show that women in North America are 2/3 more likely to develop breast cancer than Asian women, who eat an average of 50 grams of soy per day (5).
The main argument against soy products is that all estrogens are bad estrogens whenever breast cancer is concerned. Better to be safe than sorry is a motto for some. Since the phytoestrogens found in soy look similar to and admittedly act as weak estrogens, they must act like strong estrogens too. This thinking however is incorrect, as it assumes that all estrogens weak or strong act the same.
Let's review some facts about estrogen. The estrogens in our bodies are not one but three different molecules, and each of the molecules have a particular strength. Estradiol is the strongest of the three, and is used in estrogen containing hormone replacement and birth control pill formulas. Estradiol has been proven in research to bind to receptors in estrogen sensitive tissue (such as breast and uterus) and stimulate growth. It can actually affect DNA in the cells, and therefore increases the likelihood of damage occurring. Estrone is weaker in strength than estradiol, but is still considered strong acting. Estrone and estradiol can be converted into each other in the body as well. Therefore estrone has the same cancer initiating and promoting effects as estradiol. Estriol, the third type of estrogen molecule has a weak acting effect on the body. This estrogen molecule looks similar to phytoestrogens, and acts very weakly on breast and uterus tissue. Research has actually shown this type of estrogen can help prevent cancer. The reasons for this are similar to those supporting the use of soy products in breast cancer treatment and prevention. Estriol binds to receptors and therefore prevents stronger acting estrogens from doing so. Therefore these stronger estrogens are unable to cause damage.
There is also new evidence supporting the thought that the three estrogens affect tissues differently. It has been found that weak acting estrogens are more active on tissues such as bone, and less active on tissues such as the breast and uterus. It is this understanding of the different forms and actions of estrogens that have produced a new type of drug called a SERM (selective estrogen receptor molecule), which is being used in the treatment of osteoporosis. (Note: this drug is currently being researched as an alternative to Tamoxifen in breast cancer prevention). Another important thing to remember is that estrogens can have a positive effect on our bodies as well. They promote bone growth, help prevent osteoporosis and it is suspected that they help keep arteries more flexible (therefore preventing heart disease and high blood pressure). Therefore it is too much of the strong estrogens (i.e. estradiol and estrone) that lead to cancer growth and development. Estriol is helpful in preventing breast cancer, and phytoestrogens (which have a similar action) are as well.
How Much Should I Be Getting?
In general 80 to 160 mg of isoflavones, or 35 - 60 grams of soy per day is recommended to help prevent breast cancer. In her book "A Call To Women: The Healthy Breast Program and Workbook" (this book is must have for every woman!), Sat Dharam Kaur N.D. recommends what she refers to as the "Fabulous Five".
1. Freshly Ground Flaxseed: 2 - 4 Tablespoons daily
2. Tofu and Soy Products: ½ cup tofu or 1 ½ cups soy milk daily
3. Raw Pumpkin Seeds: 1-2 Tablespoons daily
4. Clover Sprouts: 3 or more cups weekly
5. Mung Bean Sprouts: 3 or more cups weekly
Eating these five foods as recommended will make sure you get all the isoflavones and lignans you need. If these foods don't appeal to you, or you are having difficulty encorporating them into your diet, you can get isoflavones in a capsule form from your local health food store (be sure you get a good, reputable brand - see your local Doctor of Naturopathic Medicine for suggestions).
Best wishes to all,
Valerie Franc B.Sc., N.D.
Doctor of Naturopathic Medicine
1. Walker, Morton. Soybean isoflavones lower risk of degenerative disease. Townsend Letter For Doctors And Patients, Aug/Sept, 1994.
2. Ingra, D. K.Saunders, M.Kolybaba, D.Lopez. Case controlled study of phytoestrogens and breast cancer. Lancet, 1997:350 Oct 4:990-93
3. Ingra, D. K.Saunders, M.Kolybaba, D.Lopez. Phyto-oestrogens and breast cancer. Int. Clin. Nutr. Rev., 1998; 18(1):35-36
4. Constantinou et al. Untitled. Cancer Res, 1990; 50:2618-24
5. Kennedy, A. The evidence for soybean products as cancer preventive agents. J. Nutr., 1995; 125:733