Dangers of Dietary Isoflavones At Levels Above Those Found In Traditional Diets

soy-iconCargill has received “self-determined” GRAS status for its AdvantaSoyTMClearTM isoflavone supplement to be used as an additive for beverages, nutrition bars, yoghurt, meal replacements and confections. See our summary of Studies Showing Adverse Effects of Isoflavones for ample proof of the dangers of adding phytoestrogens (isoflavones) to common foods. Deleterious effects include endocrine disruption, thyroid suppression, immune system suppresion, suppression of sperm production, DNA breakage and increased incidence of leukemia, breast cancer, colon cancer, infertility, growth problems and subtle changes in sexually dimorphic behaviors.

Dangers of Isoflavones

It has been argued that high levels of soy isoflavones such as genistein, daidzein and genistin in Asian diets protect the inhabitants of Japan and China from certain degenerative diseases, especially breast and prostate cancer. Actually, consumption of soy in traditional Asian diets is low. A 1975 report lists soyfoods as minor sources of protein in Japan and China.1 Major sources of protein listed were meat including organ meats, poultry, fish and eggs. Average isoflavone consumption in Asian diets ranges from 3-28 mg/day, as shown in the table below.

Studies indicate that isoflavone consumption at levels slightly exceeding those found in tradition diets results in thyroid suppression and endocrine disruption. The AdvantaSoyTMClearTM supplement would add 30-50 mg of isoflavones to a 100-gram serving of various common western foods, levels that exceed the amounts found in traditional diets and that are in the range of levels shown to cause problems, especially for sensitive individuals. Note that this level is also greater than the amount provided by 25 mg soy protein isolate, the amount determined by the FDA to warrant a health claim. It is not only possible but likely that many individuals will consume two or more servings of foods to which the Cargill isoflavones have been added, especially as these foods will be promoted with much advertising touting their health benefits. Two or more servings of such foods would provide 60-100 mg isoflavones per day, an amount that provides the estrogen equivalent of the contraceptive pill2 and one that clearly poses dangers after only a brief period of daily intake.


China (1990 survey)3 3 mg/day
Japan (1996 survey)4 10 mg/day
Japan (1998 survey)5 25 mg/day
Japan (2000 survey)6 28 mg/day
In Japanese subjects receiving adequate iodine, causing thyroid suppression after 3months7 35 mg/day
In American women, causing hormonal changes after 1 month8 45 mg/day
In American women, causing changes presaging breast cancer after 14 days9 45 mg/day
FDA recommended amount10 24 mg/day
AdvantaSoyTMClearTM 30-50 mg/ 100 g serving

As evidence on the toxicity of soy isoflavones accumulates, warnings have begun to appear in the popular press. An article appearing in the Washington Post Health Section was entitled: “You have to be soy careful: tofu and similar foods may be beneficial, but some experts fear that too much could be unsafe.”11 Writing for the New York Times, health columnist Marian Burros published the following comment on isoflavone supplements, which provide 50-100 mg isoflavones per capsule: “Against the backdrop of widespread praise. . . there is growing suspicion that soy–despite its undisputed benefits–may pose some health hazards. . . . Not one of the 18 scientists interviewed for this column was willing to say that taking isoflavones was risk free.”12

The addition of isoflavones to common foods poses a clear danger to the public and should not be allowed.


  1. Nutrition during Pregnancy and Lactation. California Department of Health, 1975.
  2. Bulletin de L’Office Federal de la Sant√© Publique, No 28, July 20, 1992
  3. This exhaustive study of Chinese diets found that legume consumption ranged from 0 to 58 grams per day, with an average of 13 grams. According to the researchers, about two-thirds of this was supplied by soy beans, giving average consumption of about 9 grams of soy products per day. Chen J, Campbell TC, Li J, Peto R. Diet, Lifestyle and Mortality in China. A study of the characteristics of 65 counties. Monograph, joint publication of Oxford University Press, Cornell University Press, China People’s Medical Publishing House, 1990. Isoflavone content is estimated be about 3 mg per day based on an average amount of 30 mg total isoflavones per 100 grams of tofu. USDA-Iowa State University Database on the Isoflavone Content of Foods 1999.
  4. Fukutake M, Takahashi M, Ishida K, Kawamura H, Sugimura T, Wakabayashi K. Quantification of genistein and genistin in soybeans and soybean products. Food Chem Toxicol 1996;34:457-461.
  5. Nagata C, Takatsuka N, Kurisu Y, Shimizu H. Decreased serum total cholesterol concentration is associated with high intake of soy products in Japanese men and women. J Nutr 1998 Feb;128(2):209-13.
  6. Nakamura Y, Tsuji S, Tonogai Y. Determination of the levels of isoflavonoids in soybeans and soy-derived foods and estimation of isoflavonoids in the Japanese daily intake. J AOAC Int 2000;83:635-650.
  7. Y Ishizuki, et al, “The effects on the thyroid gland of soybeans administered experimentally in healthy subjects,” Nippon Naibunpi Gakkai Zasshi 1991, 767: 622-629. Thirty grams of roasted pickled soybeans per day were administered to the test subjects. Isoflavone content for roasted pickled soybeans is not available but raw Japanese soybeans contain 119 mg total isoflavones per 100 grams, giving a rough total of 35 mg isoflavones per day.
  8. Cassidy A, Bingham S, Setchell KD. Biological effects of a diet of soy protein rich in isoflavones on the menstrual cycle of premenopausal women. Am J Clin Nutr 1994;60(3):333-340.
  9. McMichael-Phillips DF and others. Effects of soy-protein supplementation on epithelial proliferation in the histologically normal human breast. Am J Clin Nutr 1998 Dec;68(6 Suppl):1431S-1435S.
  10. The FDA recommended amount for adults is 25 grams of soy protein isolate per day. This provides about 24 mg isoflavones per day based on a total isoflavone content of 97 mg for 100 grams of soy protein isolate. USDA-Iowa State University Database on the Isoflavone Content of Foods 1999.
  11. Washington Post Health Section, January 30, 2001.
  12. Burros M. Doubts Cloud Rosy News on Soy. New York Times, January 26, 2000.

Sally Fallon Morell is the founding president of the Weston A. Price Foundation and founder of A Campaign for Real Milk. She is the author of the best-selling cookbook, Nourishing Traditions (with Mary G. Enig, PhD) and the Nourishing Traditions Book of Baby & Child Care (with Thomas S. Cowan, MD). She is also the author of Nourishing Broth (with Kaayla T. Daniel, PhD, CCN).

One Response to Dangers of Dietary Isoflavones At Levels Above Those Found In Traditional Diets

  1. sara gorevan says:

    Are soy isoflavones and flax isoflavones the same?

Leave a reply

© 2015 The Weston A. Price Foundation for Wise Traditions in Food, Farming, and the Healing Arts.