Research on Why to Avoid Soy

  Go Easy on the Soy foods

Interview that helps understand why not to eat too much soy. 


Researcher of the Month – David Zava, Ph.D.

David Zava, Ph.D., is a biochemist and an experienced breast cancer researcher who has spent decades looking at breast cancer tissue under the microscope. He helped Aeron Laboratories develop their saliva hormone assay testing procedures, and is now working as a consultant. He is also the recent co-author, with Daniel Herman, of a book, A Woman’s Right to Know: The Breast Cancer Prevention Report due out next fall. Over the past few years, Dr. Zava has contributed much to Dr. Lee’s knowledge and understanding of progesterone, estrogen, and breast cancer, and we’re always intrigued to know what’s on his mind. This month, he wanted to talk about the dangers of eating too much soy, which has been touted recently as a virtual miracle food for menopausal and hormonal problems.

JLML: Dr. Zava, why are you warning women away from too much soy when everything we’re reading and hearing encourages women to eat soy foods as much as possible for menopausal symptoms and to prevent breast cancer?

DZ: People do fine with average soy consumption, eating it here and there, but the way Americans are jumping into it, I think we’re going to have problems. I reviewed the literature on soy very carefully when I was researching my book, and really got down to the nitty gritty, and what I found is that there are an enormous number of toxins (anti-nutrients) in soy. What I mean by this is that there are plant chemicals produced naturally in the soybean that, if not removed first by soaking, slow cooking, and fermentation, can cause serious health problems if you eat too much of them.

JLML: What about using soy foods to treat menopausal symptoms?

DZ: Soy foods can be important as one part of a balanced nutrition and lifestyle approach to menopausal symptoms, but they aren’t a solution by themselves. Even high consumption of soy isolates that are loaded with isoflavones only suppresses menopause symptoms minimally for most women. The recent Bowman-Gray School of Medicine study tested the ability of soy protein isolates to relieve menopausal symptoms. The positive side of the study was that the intensity of hot flashes was reduced; however there was no statistical difference in the number of hot flashes these women were having.

With natural progesterone and/or estrogen replacement therapy, women often get complete relief of menopausal symptoms. Because the natural hormones are safe and effective when used properly, I think the push to use soy is the wrong approach, and is seriously compromising the health and quality of life of millions of women.

JLML: What effects do the anti-nutrients in soy have on the body?

DZ: In studying the literature on soy, I found there are about five types of plant chemicals in the soybean that can be toxic to humans if they are not removed by special processing. Over thousands of years of experimentation, the Asians learned how to remove the toxins from soybeans and reap the remarkably rich nutrients from the bean. The primary toxins in soybeans include allergens, phytates, protease inhibitors, genistein, and goitrogens. The allergens can cause very pronounced allergic reactions in some individuals. This probably accounts for some 10 to 20 percent of the population in the Western world.

The phytates can be a problem because they tightly bind up essential minerals, particularly zinc, preventing them from being absorbed into the body. Some studies have indicated that excessive phytate consumption in children can cause physical stunting because of its zinc-depleting capacity. Phytates are not a problem if adequate animal protein is included in the diet, so I would expect phytates would be more of a problem for vegans. Zinc is needed for over 50 enzyme reactions in the body, including many of those necessary for brain functions.

The third anti-nutrient in soybeans is a phytochemical that inhibits the enzymes that digest protein into the simple building blocks, amino acids. These are called Bowman-Birk or protease inhibitors.

JLML: Aren’t protease inhibitors what AIDS patients are taking now?

DZ: Yes, and we’ve learned the hard way from them that using protease inhibitors inhibits pancreatic enzymes. The last thing in the world you need when you have a wasting disease is something that’s going to prevent you from digesting food. Even a healthy person doesn’t want their digestion compromised.

JLML: And if you are estrogen dominant or taking birth control pills, you already have a problem with high copper and low zinc levels, and this will only make it worse. Dr. Ellen Grant, a hormone researcher from England, believes that this is the imbalance that causes the mood swings and irritability in PMS and in menopausal women.

DZ: That’s right. But again, if you eat animal protein with legumes the phytates are not a problem. Genistein is the next on the list of anti-nutrients. When I first began to study soy’s anticancer properties I thought genistein was the answer. My first publication reflected my sentiments. As I “read on” in the scientific literature, I became less and less convinced that genistein was beneficial. Its structure is similar to that of estradiol, and it fits nicely into the estrogen receptor and turns the receptor on much like estradiol. However, genistein has many other actions in the human body. It inhibits a number of different enzymes, some of which are responsible for synthesizing estrogens, including those that convert androgens to estrone (aromatase) and estrone to estradiol (17-HSD). It also is a potent inhibitor of tyrosine kinases, enzymes that shuttle high-energy phosphate molecules in the cells for the purpose of driving cellular processes like cell proliferation. Cancer cells tend to over-express tyrosine kinases, so genistein has been shown to be quite useful in blocking cancer cell proliferation. However, this can be a double-edged sword because normal cells also need some tyrosine kinase activity. This includes hair follicles, memory neurons in the brain, and so forth.

A third thing very high levels of genistein does is block glucose transport into cells by inhibiting an enzyme called GLUT-1. This is one of the major glucose transporters that sits on the outside of brain cells, red blood cells, and in many other places, and shuttles glucose into the cells. Because the brain is very dependent on glucose for its energy source, I would be concerned that too much genistein over a prolonged period of time eventually would be toxic to the brain.

I don’t think soy in the form consumed by Asians over thousands of years is any problem as regard to brain function. In fact, it may be beneficial. What concerns me, however, is that we just don’t have any information on what long-term impact excessive amounts of genistein in processed soy foods, powders, or in pill form will have on brain function, since very high genistein inhibits at least three of the metabolic pathways needed to maintain normal brain function.

The fifth anti-nutrient in soybeans is called a goitrogen. This is a chemical that latches on to iodine, preventing it from absorbing into the body from the gastrointestinal tract. Iodine is needed to make thyroid hormone. Low thyroid function has been associated with poor brain development. Anyone who has been deficient in thyroid hormone understands quite well what impact this can have on normal brain function, especially at a time in life as we grow older and “fuzzy thinking” creeps into our vocabulary.

JLML: How should we weigh the protective effects of genistein against breast cancer that we read and hear so much about in the popular media?

DZ: Surprisingly there’s actually very little evidence in the literature either from epidemiological studies or experimental animal studies that prove soy protects against breast cancer. That’s not to say it doesn’t have other beneficial properties, such as heart-protective benefits, but only that it doesn’t appear to protect the breast from cancer. Many people have made the assumption that genistein, which is present in very high levels in soybeans, acts like tamoxifen and protects against breast cancer. What I have found in my research is that genistein acts like a pro-estrogen, not an anti-estrogen like tamoxifen. The suggestion by the popular press that genistein at the concentrations found in foods inhibits the growth of breast cancer cells is just plain wrong and not based on scientific fact. Several abstracts submitted to the Second Conference on Soy held in Brussels, Belgium, reported that soy protein isolates increased the proliferation of normal breast cells in intact humans. This is entirely consistent with what I found working with human breast cancer cells in test tubes using levels of genistein commonly found in the blood of individuals consuming a lot of soy foods. Others have found similar results, that is genisein is a pro-estrogen in human breast cancer cells. So given the lack of evidence that soy foods protect against breast cancer, I find it amazing that there is so much emphasis on it for breast cancer prevention.

JLML: So this is a food that is a medicine in moderation, and a potential poison in excess. What guidelines would you give both women and men for eating soy foods?

DZ: If you are not allergic to soy foods, eat them primarily as fermented soy products, which includes miso and tempeh, because many of the anti-nutrients have been reduced in this form. If you eat soy in the form of tofu, soymilk, or powders for example, eat it with a balance of mineral-rich sea vegetables, such as kombu and nori, and animal protein, preferably fish, to counter the higher content of anti-nutrients such as phytates and digestive enzyme inhibitors.

JLML: Thank you, Dr. Zava, this is important information for women to have.

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