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Soy, Phytoestrogens, And Cancer – A Bad Combination?

Written by Wellness Club on August 5, 2009 – 4:12 pm -

By Dr. Myatt


Cancer, diet, hormones, drugs – individually these are incredibly complex subjects, and when one has to consider them all together – well, then things get really complicated!

This looked at first sight to be a fairly straightforward question, but the answer actually required several hours of intensive research and fact-checking. Now you, dear reader, are the beneficiary of that! 

I have a question about Cal-Mag Amino supplements.  I have just purchased and received this item for the first time (since my old supplements are no longer available).  After opening the first bottle, I noticed on the label under “other ingredients” that the supplement tablets contain soy.  Under ordinary circumstances, this would not be a problem for me.  However, I have a history of estrogen 3+ / progesterone 1+ positive,  tubular breast cancer.  I am currently taking Arimidex and have completed 3 years of adjuvant therapy.  I was unaware that the Cal-Mag Amino contained soy, and now that I have several bottles I am concerned with how much soy is in the product.  I have an appointment with my medical oncologist in August and I would like to discuss this with her.  It would be helpful to know how much soy is in the product so that I may discuss this with my doctor. Could you please address this question for me? 
Thank you,

Dr. Myatt’s Answer:

Phytoestrogens and Breast Cancer

“Phytoestrogens” (literally, “plant estrogens”), are substances found in many foods and plants including flax seeds, soy and soy products (tofu, etc.), sesame seeds, garlic, apricots, squash, green beans and more. Here is a list of common phytoestrogen-containing foods.

Phytoestrogens are not true estrogens and cannot be converted in the body into estrogens. Because of molecular similarities between human estrogens and phytoestrogens, the phytoestrogens are able to bind to estrogen receptors where they have weak estrogenic effects.

Because of these weak estrogenic effects, some people theorize that phytoestrogens should be avoided in the treatment of hormone-responsive cancers such as breast cancer. In my opinion, this hypothesis is partly correct and partly incorrect. Here’s why.

First, phytoestrogens are widespread in plants. In order to avoid all phytoestrogenic substances, one would need to stop eating a wide variety of foods, including such things as flax seed which have proven anti-cancer effects (1-3).

Soy isolates including MSG and “hydrolyzed protein” are not necessarily listed on food labels — they are “stealth ingredients” — which means that if you eat ANY processed foods, you are likely consuming phytoestrogens. The best advice is to avoid processed foods, for this and many other reasons.

Second, there are studies which show that phytoestrogens may actually be protective against hormone-related cancers by blocking more potent estrogenic substances from occupying estrogen receptors. Though not all studies agree (they never do!), the preponderance of epidemiological evidence shows that Southeast Asian women, who typically consume high amounts of soy (10-50 g/day), have a four to six-fold decreased risk of breast cancer compared to American women who typically consume negligible amounts of this legume (1-3 g/day).(4-5) The difference in these cancer rates is believed due to the phytoestrogens in soy.

Aromatase Inhibitors (estrogen-blocking drugs) Vs. Phytoestrogens

Although the verdict is still out on this issue, I wouldn’t recommend taking concentrated soy or other phytoestrogen substances on a daily basis if I had a hormone-sensitive cancer NOR would I make a big deal out of avoiding all phytoestrogen containing foods.

Aromatase inhibitors (estrogen blockers) such as Arimidex work (we think) by blocking the body’s formation of estrogen. Phytoestrogens appear to work, at least in part, by actually blocking the estrogen receptors. The end result is similar: decrease the ability of strong estrogens to bind to estrogen receptors, either by blocking their production (the drugs) or blocking their receptor (phytoestrogens). Some studies have shown that use of phytoestrogens has a similar effect as the drugs (1-5) but without the long list of negative side effects.

Arimidex side effects:

Possible Side Effects of ARIMIDEX.

  • Based on information from a study in patients with early breast cancer, women with a history of blockages in heart arteries (ischemic heart disease) who take ARIMIDEX may have a slight increase in this type of heart disease compared to similar patients who take tamoxifen.
  • ARIMIDEX can cause bone softening/weakening (osteoporosis) increasing the chance of fractures. In a clinical study in early breast cancer, there were more fractures (including fractures of the spine, hip, and wrist) with ARIMIDEX (10%) than with tamoxifen (7%).
  • In a clinical study in early breast cancer, some patients taking ARIMIDEX had an increase in cholesterol.
  • Skin reactions, allergic reactions, and changes in blood tests of liver function have also been reported.
  • In the early breast cancer clinical trial, the most common side effects seen with ARIMIDEX include hot flashes, joint symptoms (including arthritis and arthralgia), weakness, mood changes, pain, back pain, sore throat, nausea and vomiting, rash, depression, high blood pressure, osteoporosis, fractures, swelling of arms/legs, insomnia, and headache.

“Other than that, Mrs. Lincoln, how was the play?”

  … from the manufacturer’s website:

Several studies have found that the isolated soy phytoestrogen genistein, but not other phytoestrogens, countered the effect of aromatase inhibitors. (6-8)

On the other hand, though I would not take concentrated genistein with Arimidex, neither would I be concerned about eating small amounts of phytoestrogens that occur naturally in many foods. Again, there are studies which show phytoestrogens to be PROTECTIVE in hormone-sensitive cancers. I wouldn’t take concentrated forms of soy products or soy powder with estrogen-blocking drugs but neither would I avoid normal dietary amounts of phytoestrogen-containing foods. Since soy has the largest concentration of genisteins, I would not eat this every day; on the other hand, I wouldn’t skip my favorite tofu and veggie stir-fry when eating at The China Wok, either!

Finally, the “soy” that occurs in Cal-Mag amino is the isolated amino acids from same, used as protein chelators of the minerals to increased absorption. The phytoestrogen component of whey has been removed; only the amino acids (protein fractions) are used in Wellness Club Nutritionals.

Far more important than fussing about small amounts of phytoestrogens in food, a ketogenic diet is the single most important “treatment” that a person can use to both prevent and treat cancer. A brief description of the benefits of this diet, as written for doctors, can be found in this previous HealthBeat News article.

One final note. Asking your conventional oncologist about the advisability or non-advisability of a natural or nutritional substance is usually like asking your acupuncturist about brain surgery: it is outside their scope of practice, meaning they don’t have the information to be able to give you a good answer.

What does a conventional doctor do when they don’t know? Do they say “I don’t know”? Rarely. Instead, the thought is “If I don’t know the answer, then don’t do it.” Unfortunately, this mindset isn’t just “erring on the side of caution” and has in fact often steered people away from helpful treatments.

I wouldn’t look to a conventional oncologist for sound advice on diet (most say it doesn’t matter, eat anything you want and just “keep up your weight”), supplemental nutrition or herbs for cancer unless they have done some serious extra-curricular studies on the subject. Most are not even aware of the numerous references and benefits of a ketogenic diet on cancer.

I find no justification for avoiding phytoestrogens as found in food in instances of breast or prostate cancer, but I DO recommend avoiding concentrated genisteins and soy products with a history of breast cancer, especially when taking estrogen-blocking drugs.

1.) Power KA, Thompson LU. Can the combination of flaxseed and its lignans with soy and its isoflavones reduce the growth stimulatory effect of soy and its isoflavones on established breast cancer?  Mol Nutr Food Res. 2007 Jul;51(7):845-56.
2.) Bergman Jungeström M, Thompson LU, Dabrosin C. Flaxseed and its lignans inhibit estradiol-induced growth, angiogenesis, and secretion of vascular endothelial growth factor in human breast cancer xenografts in vivo. Clin Cancer Res. 2007 Feb 1;13(3):1061-7.
3.) Touillaud MS, Thiébaut AC, Fournier A, Niravong M, Boutron-Ruault MC, Clavel-Chapelon F. Dietary lignan intake and postmenopausal breast cancer risk by estrogen and progesterone receptor status.  J Natl Cancer Inst. 2007 Mar 21;99(6):475-86.
4.) Messina MJ, Persky V, Setchell KD, Barnes S. Soy intake and cancer risk: a review of the in vitro and in vivo data. Nutr Cancer 1994;21:11331.
5.) Birt DF, Hendrich S, Wang W. Dietary agents in cancer prevention: flavonoids and isoflavonoids. Pharmacol Ther. 2001;90:157-161.
6.) Ju YH, Doerge DR, Woodling KA, Hartman JA, Kwak J, Helferich WG. Dietary genistein negates the inhibitory effect of letrozole on the growth of aromatase-expressing estrogen-dependent human breast cancer cells (MCF-7Ca) in vivo. Carcinogenesis. 2008 Nov;29(11):2162-8. Epub 2008 Jul 16.
7.) Edmunds KM, Holloway AC, Crankshaw DJ, Agarwal SK, Foster WG. The effects of dietary phytoestrogens on aromatase activity in human endometrial stromal cells. Reprod Nutr Dev. 2005 Nov-Dec;45(6):709-20.
8.) de Lemos ML. Effects of soy phytoestrogens genistein and daidzein on breast cancer growth. Ann Pharmacother. 2001 Sep;35(9):1118-21.

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