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Soy Protein: Panacea or Poison?

By Don Matesz

The soy bean is the darling of the health food industry these days, and soy products are most highly and fraudulently touted of all health food products. Reading all the popular press on soy products, one would naturally believe that soy is a panacea and ideal substitute for animal protein in human diets. It is claimed that soy protein is of the same quality as animal protein, yet lower in fat and devoid of cholesterol. Soy promoters make many unsubstantiated claims, especially regarding soy isoflavones, estrogen-like molecules (called xenoestrogens) found the bean. It is claimed that these isoflavones from soy products will prevent breast cancer and osteoporosis and serve as estrogen replacements for post-menopausal women. In fact, according to some prominent soy researchers, the evidence for these claims is very weak (see below).

Promoters of soy commonly point to the low rates of heart disease, breast and prostate cancer, osteoporosis, and symptoms of menopause in Asia and claim that this is due to the "soy-based" Asian diet. But it is a stretch of imagination to call the Asian diet "soy-based". According to the non-profit Soy On Line Service (, the typical Japanese diet contains only 0.08 to 0.13 mg isoflavones per kg of body weight per day. That means only 5.6 to 9.1 mg of isoflavones per day for a 70 kg/154 lb. person. This amount is found in about 8 grams-less than one third of an ounce-of whole soybeans! Compare this to the average 3 to 4 pounds of food eaten daily by the typical individual. One-third of an ounce represents only 0.5 percent of the Japanese diet; one ounce is 1.5 percent of the diet.

This pattern of soy consumption is found throughout Asia, where soy is consumed primarily as a part of very salty fermented seasonings, in the form of shoyu (wheat and soy sauce), tamari (wheat-free soy sauce), or miso paste. Asians are not snacking on soy "nuts", drinking soy milk, using isolated soy protein, and eating tofu burgers or soy hot dogs three times per day. Thus, Asians do not eat a soy-based diet! It includes some soy but it is not a major article of diet as implied by the word "based."

This casts doubt on the claim that soy is the secret to Asian health. Consuming only as little as 9 grams of soy and 9 mg of isoflavones per day, Asians purportedly have much lower rates of heart disease, breast and prostate cancer, osteoporosis, and menopause compared to Americans. Nevertheless, promoters of soy are suggesting that protection from heart disease, etc. requires consumption of 25 grams of soy protein per day, which can provide up to 5 times as much isoflavones as found in the typical Asian diet.

In addition, traditional fermented soy foods are quite different from the processed soy foods promoted in "health food" markets. Soy beans contain various harmful anti-nutrients, including trypsin inhibitors that block protein digestion and phytates that block mineral absorption. Traditional long term fermenting (3 to 48 months) of soy beans to produce soy sauces and miso pastes destroys significant amounts of these anti-nutrients, whereas modern factory processing to produce soy sauces, soy milk, tofu, and mock meats does not remove those harmful elements.

Recently, in response to a petition submitted to the FDA by Protein Technologies International (PTI), a division of DuPont corporation that manufactures Supro® brand soy protein, the FDA has allowed the following health claim to be attached to soy products containing soy protein: "Diets low in saturated fat and cholesterol that include 25 grams of soy protein a day may reduce the risk of heart disease." This claim was allowed supposedly based on some clinical trials which show adding soy protein to a diet low in fat and cholesterol will result in decreases in cholesterol beyond what occurs as a result of a low-fat, low cholesterol diet alone. However, other studies have not shown such a benefit. Therefore, the allowed claim conveys uncertainty, by the phrase "may reduce the risk..." Also note that the possible benefit of soy consumption is promised only in conjunction with a diet low in fat and cholesterol, which itself "may" reduce the risk of heart disease (another uncertainty)-so if your risk goes down when consuming soy as part of a low-fat, low-cholesterol diet, is this due to the soy, or the diet?

Similar uncertainty exists with regard to soy and other putative health benefits. Some studies have suggested that consuming soy reduces hot flashes and other menopausal symptoms and increases bone density among post-menopausal women-while others have not.

Clearly, this is an experiment untried on a large scale. Promoters of soy suggest that it is safe to consume soy in almost any quantity but this has not been proven true. In fact, there is some rather strong evidence that ingesting soy on a daily basis, even in rather modest quantities, and certainly in large quantities, may have serious adverse effects on endocrine functions, immune system functions, and brain cell repair.

Here are the details:

Protein Quality
Soy salesmen claim that soy protein is equal to animal protein. Their claims are aided by the FDA's endorsement of the Protein Digestibility Corrected Amino Acid Score (PDCAAS), which uses soy protein as the standard. However, the PDCAAS disregards digestibility, rating proteins only by their amino acid score; and further, the FDA simply defined soy protein as the standard, even though animal proteins have superior digestibility and amino acid profiles. Outside the FDA, nutritional biochemists still rate proteins according to biological value, and the standard for biological value is human milk protein (an animal protein), which is given a score of 100. The biological value of soy protein is only 73, and all animal proteins rate superior to soy protein on the scale of biological value, with egg and whey protein at the top of the heap, equal to human milk.

Reproductive Function
Soy isoflavones are estrogen-like molecules. Environmental toxicologists refer to such things as xenoestrogens. Many health care professionals are extremely concerned that human health is adversely affected by increasing intake of xenoestrogens, including soy isoflavones, because they stimulate various undesirable growth processes in girls and women, and may interfere with normal hormone dependent development of boys and functional capacities of men.
In 1997, researchers reported that "The daily exposure of infants to isoflavones in soy infant formulas is 6- to 11 fold higher on a body weight basis than the dose that has hormonal effects in adults consuming soy foods. Circulating concentrations of isoflavones in the seven infants fed soy-based formula were 13000-22000 times higher than plasma oestradiol concentrations in early life, and may be sufficient to exert biological effects, whereas the contribution of isoflavones from breast-milk and cow-milk is negligible." [Lancet 1997 Jul 5, 350: 9070, 23-7]

In 1998 researchers from New Zealand reported that the rate of isoflavone intake in infants fed soy-based formulas, cereals, dinners, and biscuits "is much greater than that shown in adult humans to alter reproductive hormones." They advised: "Since the available evidence suggests that infants can digest and absorb dietary phytoestrogens in active forms and since neonates are generally more susceptible than adults to perturbations of the sex steroid milieu, we suggest that it would be highly desirable to study the effects of soy isoflavones on steroid-dependent developmental processes in human babies. [Proc Soc Exp Biol Med 1998 Mar 217: 3, 247-53]

Previous to these recent studies, in 1982, pediatric endocrinologists in Puerto Rico reported an increase in the incidence of premature breast development in girls under eight years of age. Of 130 cases studied, 85 involved breast development in girls under 18 months of age. Of those 85 cases, 22 were found associated with use of soy formula, rich in isoflavones. [Am J Dis Child 1986 Dec 140: 12 1263-7]

The evidence for cancer preventive properties of soy foods and isoflavones is rather weak.1 Some epidemiological studies have shown a relation between soy consumption and protection from breast and endometrial cancer, but they have not been able to separate the effect of soy from that of other significant factors of Asian diets, such as low fat and high fiber, fish, whole grain, vegetable, fruit and non-soy legume intake-especially since soy is such a small part of Asian diets. Meanwhile, some clinical evidence suggests that high doses of isolated soy protein and isoflavones (not parts of traditional Asian diets) actually may be cancer promoters.

In 1996, one group of researchers fed 38 grams of soy protein isolate containing 38 mg of isoflavones to both pre- and post- menopausal women for five months. It is of interest to note that these researchers reported "No changes were found in plasma prolactin, sex hormone binding globulin, cholesterol, high density lipoprotein cholesterol and triglyceride concentrations."[emphasis added] Perhaps the women weren't also on a low fat, low cholesterol diet? Or maybe they were on such a diet and that doesn't reliably reduce cholesterol or tryglycerides either?

They did find, however, that there was a greater production of breast fluid in premenopausal women during the months of soy consumption, and "plasma estradiol [estrogen] concentrations were elevated erratically throughout a 'composite' menstrual cycle during the months of soy consumption." Further, they reported "Of potential concern was the cytological detection of epithelial hyperplasia in 7 of 24 women (29.2%) during the months they were consuming soy protein isolate" and they concluded "this pilot study indicates that prolonged consumption of soy protein isolate has a stimulatory effect on the premenopausal female breast, characterized by increased secretion of breast fluid, the appearance of hyperplastic [abnormal] epithelial cells, and elevated levels of plasma estradiol." [Cancer Epidemiol Biomarkers Prev 1996 Oct 5: 10, 785-94]

In 1997, another group of researchers studied the effect of soy isoflavone genistein on breast cancer cells and reported that "dietary estrogens at low concentrations do not act as antiestrogens but act like DDT and estradiol to stimulate human breast cancer cells to enter the cell cycle" [Environ Health Perspect 1997 Apr 105 Suppl 3, 633-6].

In the September 1998 issue of Cancer Research, William G Helferich, Ph.D., from the University of Illinois, reported that human estrogen-dependent breast cancer cells injected into mice multiplied if the mice were fed genistein. Helferich says "We've seen a lot of good research that genistein is a cancer preventer, but it is dangerous to people who already have cancer....Caution is warranted."2

An epidemiological study done in China found that high soy intake did not provide protection from breast cancer ["Diet and breast cancer in Shanghai and Yianjin, China," Br J Cancer 1995 71: 1353-8]. At a recent conference on soy foods and health, soy advocate and author of The Simple Soybean and Your Health Mike Messina Ph.D. stated, "It's simply not possible as yet to draw any conclusions about soy consumption and cancer prevention, but further research is certainly warranted" [Honolulu Star Bulletin 11/19/99,]. This is completely contrary, of course, to the image of soy presented by advertisers who would like women to believe that ingesting soy will prevent breast cancer.

Other research has shown that some premenopausal women ingesting 60 grams of soy protein daily (45 mg isoflavones) have leutenizing hormone levels 33% of normal and follicle stimulating hormone levels only 53% of normal levels [Am J Clin Nutr 1994 Sep 60:3, 333-40]. These levels are low enough to inhibit ovulation in some individuals. In 1994 a team of researchers reported that in vitro "Genistein, and inhibitor of tyrosine kinases, including c-kit, blocked oocyte growth and disrupted follicle morphology." In translation, genistein blocked egg growth and caused abnormal changes in the shape of the follicle [Dev Biol 1994 Jan 161:1, 194-205].

There is good reason to suspect soy estrogens as a cause for male infertility. Over the past 30 to 50 years, as human exposure to xenoestrogens (including soy isoflavones, in so many processed foods containing soy protein) has increased, there has also been an increase in incidence of developmental disorders of male reproductive organs and a decrease in sperm counts. Researchers have found that soy isoflavones genistein and daidzein inhibit the activity of an enzyme required for proper metabolism of steroids critical to hormonal functions-in particular, this enzyme is required for the production of testosterone. [Biochem Biophys Res Commun 1995 Oct 24 215:3, 1137-44.]

Soy isoflavones have been shown to interfere with normal reproductive functions on the cellular level. In 1996, researchers showed that genistein is a strong inducer of DNA strand breaks, thus clastogenic and mutagenic. [Food Chem Toxicol 1996, 35: 605-13] In 1998, researchers reported that "coumestrol and genistein are clastogenic in cultured mammalian cells and lead to gene mutations."[Z Lebensm Unters Forsch A 1998, 206: 367-73] Below we will report how this may affect the immune and nervous systems.

Soy and Thyroid Function

A significant body of research has suggested that the estrogen-like soy isoflavones in 25 grams of soy protein can seriously disrupt other endocrine functions. In the late 1950s and early 1960s there were reports that infants fed soy formula developed goiter (enlarged thyroid). A study done in Japan in 1991 found that diffuse goiter and hypothyroidism appeared in half of the subjects after consuming 30 gm per day of pickled roasted soybeans for three months [Ishisuki Y et Al., "The effects on the thyroid gland of soybeans administered experimentally in healthy subjects," Nippon Nibunpi Gawk Zasshi (1991) 67:622-629].
In 1997 it was shown that the supposedly healthful soy estrogens/isoflavones suppress thyroid activity.3 As thyroid inhibitors, soy isoflavones are considered to be somewhat more potent than common anti-thyroid drugs, working to inhibit action of an enzyme that converts the amino acid tyrosine to thyroid hormone. Daniel R. Doerge, Ph.D., one of the researchers from the FDA National Center for Toxicological Research involved in the study that isolated and studied the anti-thyroid mechanism of the soy isoflavones, says: "I don't think you can get into trouble if you eat a few soyfoods within the bounds of a balanced diet...But I see substantial risks from taking soy supplements or eating huge amounts of soy foods for their putative disease-preventive value. There is definitely potential for interaction with the thyroid."4

Soy and Immune Function
Several studies have shown that soy isoflavones suppress the immune system. In fact, genistein has been studied as a potential drug to give to transplant recipients, who are always given immune suppressant drugs to prevent their bodies from rejecting the foreign transplant tissues. One team of researchers studied the immunosuppressive potential of genistein and reported, "Our data suggest that genistein is a powerful immunosuppressive agent, with no toxic effects on T cells, and has the potential for use in the prophylaxis and treatment of allograft rejection" [Transplantation 1991 Feb 51:2, 448-50]. In other words, genistein has a immune suppressant effect comparable to that of immune suppressant drugs given to transplant patients.

Other researchers have found that genistein works as an immune suppressant by causing chromatin fragmentation [Biochem Biophys Res Commun 1993 Jul 30 194:2, 944-50]. In 1998 researchers reported that "Exposure of mammalian cells to genistein results in DNA damage that is similar to that induced by the topo-II inhibitor and chromosomal mutagen, m-amsa." These researchers found that "genistein is a chromosomal mutagen" that causes mutations and cell death of human lymph cells [Mutat Res 1998 Aug 31 405:1, 41-56].

Soy and Brain Function
According to the Honolulu Star-Bulletin (11/9/99) newspaper, ingestion of soy products has recently been strongly linked to development of dementia. The National Institute of Aging sponsored a study of 3, 634 Japanese-American men in Hawaii, which found that those who ate the most tofu had the most advanced dementia and on autopsy the brains of men who ate more than two servings per week of tofu weighed less than the brains of men who consumed fewer than two servings of tofu per week. The rate of impairment was also found correlated with soy intake. Those who ate no tofu showed mental abilities of men five years younger, while those who ate the most tofu tested as if they were five years older. Of 27 dietary items checked, including meat, only soy was found consistently correlated with increased incidence and severity of dementia [].

This finding is supported by experimental evidence which has shown that soy isoflavones decrease DNA repair and synthesis in the brains of rats and mice [Yakisich JS, et Al, "Early effects of protein kinase modulator on DNA synthesis in rat cerebral cortex," Exp Neurol 1999 Sep; 159 (1): 164-76; Schmitz C, Axmacher B, Sunker U, Korr H, "Age-related changes of DNA repair and mitochondrial DNA synthesis in the mouse brain," Acta Neuropathol (Berl) 1999; 97(1): 71-81] .

According to the Star-Bulletin, this study linking tofu to dementia was presented at a conference on soy and health sponsored by soy foods producers such as DuPont and Archer Daniel Midlands. Commenting on the conference, the lead investigator in the Hawaii study, Dr. Lon White, stated "The majority of scientists said the data they were talking about for beneficial effects [of soy foods] on health is very weak" and doesn't really support health claims for soy foods.
The Money Game

Thus, right now it appears that there is sufficient contradictory evidence to conclude that we don't really know yet whether ingesting soy in large amounts as food or as isoflavone supplements will be beneficial or harmful. Obviously it is best not to consume soy or its isoflavones in the large amounts recommended by soy product manufacturers and "approved" by the FDA. Until more is known, it would be best not to exceed the levels of soy food consumption found in the typical Asian diet, about 8 to 10 grams (one-third ounce) daily-and certainly one should avoid using isolated soy protein or high dose soy isoflavone supplements.
So why are soybeans so highly touted in the popular media? Here is my hypothesis: Up until about 10 years ago, most of the soy grown in the U.S. was fed to cattle or used for industrial purposes, such as making margarine, paints, plastics, drugs, cleaners, emulsifiers, and so on. However, there is a limit to the profit to be made in this use of soy. You can make much more money if you can get people to eat the soy-and especially if you can get people to believe that it is the panacea for many ills, since people willingly pay dearly for panaceas.

The soy pushers are some of the largest, wealthiest corporations in the U.S. For example, Monsanto (the same corporation behind rBGH mentioned above) markets its "Round Up Ready" genetically engineered soybean along with its Round Up pesticide-the soybean is engineered, not to be more nutritious or delicious, but to be capable of withstanding larger doses of Round Up! Another example is Archer Daniels Midland (a.k.a. ADM), which advertises itself as "supermarket for the world," and is heavily invested in producing soy products. DuPont Chemical is also involved in the soy market, through its subsidiary Protein Technologies International, a soy protein powder maker. Green Giant is selling soybean mock meats.

All of these big boys are interested in getting you to eat soy, drink soy, and take soy pills. They don't really care about your health, only about their profits. So you should take all soy advertisements and popular articles and books with a big grain of salt.

*** ***

Don Matesz, M.A., C.N., C.R.T. is an associate member of the International Association of Resistance Trainers and graduate of the American Academy of Nutrition. Don resides in Toledo, Ohio, and is available for fitness consultation by phone and e-mail, and personal training in Toledo. Call (419) 476-2967 for rates and details. You can also reach Don by E-mailing:

1 Fitzpatrick M, "Soy Isoflavones: Panacea or Poison?", submitted to the FDA in an effort to block GRAS status for soy isoflavones, published in Health and Healing Wisdom, Volume 22, No 3, p. 3. Also available at
2 Osborne SE, "Does Soy Have a Dark Side?", Natural Health, March 1999, p. 158.
3 Divi RL et al., "Anti-thyroid isoflavones from the soybean," Biochem Pharmacol (1997)54:1087-1096.
4 Osborne SE, op cit, p. 113.


This article edited and reprinted by permission of  I.A.R.T.

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