Soy: Impact on Health

By Cat, Apr 2007

Update links in text to my old site, after moving them.

Soy is considered a health food and excellent source of protein. However un-fermented soy contains several toxic anti-nutrients (see Soy’s Antinutrients for more), which can lead to serious disease, as discussed in this article. A better choice would be fermented soy products (see Soy: Friend or Foe? for more on fermented soy)

The following essay the second segment condensed “Newest Research On Why You Should Avoid Soy:  Cinderella’s Dark Side” (1), by Sally Fallon & Mary G. Enig, Ph.D. (1). While there is a fair amount of political content in the original article, I only focus on the health content.  Those who are interested in the political content can read the entire article. 

Another article on the problems of Soy can be found in the July/August 2007 issue of Utne Reader: “The Dark Side of Soy” (2), by Mary Vance Terrain. 

Soy & Thyroid Function

The thyroid is a small endocrine (hormone) gland in the neck, just below the Adam’s apple. It produces iodine-containing hormones that control the body’s metabolic rate–the rate at which the body produces energy and heat.  [Refer to my essay on Thyroid Function on my old site for more on this subject.]

Scientists Daniel Sheehan and Daniel Doerge, from the National Center for Toxicological Research, have demonstrated that genistein*, a phytoestrogen in soy foods, causes irreversible damage to enzymes that synthesize thyroid hormones in rats.  Dr Doerge writes, “The association between soybean consumption and goiter in animals and humans has a long history.  Current evidence for the beneficial effects of soy requires a full understanding of potential adverse effects as well.”

Dr Claude Hughes reports that rats born to mothers fed genistein* had decreased birth weights compared to controls, and onset of puberty occurred earlier in male offspring.  He asserts that his research on rats “will be at least somewhat predictive of what occurs in humans.”

* See Soy’s Anti-Nutrients for more on genistein.

Soy and Heart Disease

Much has been made of the claim that soy consumption lowers cholesterol and prevents heart disease.  The study used to support this claim was the 1995 meta-analysis by Dr. James Anderson, sponsored by Protein Technologies International, and published in the New England Journal of Medicine.

However, this study is problematic on several grounds.  First and foremost is the method used.  A meta-analysis is a review and summary of results of many clinical studies on the same subject.  The use of meta-analysis to draw general conclusions has been sharply criticized by members of the scientific community because they make faulty assumptions and indulge in creative accounting.  [In other words, they compare apples with oranges and conclude turnips.]  And there is the added temptation of excluding studies that do not support the desired conclusion.  Such was the case with the 1995 meta-analysis of soy:  8 of 37 studies were excluded for this reason.

Another problem with the meta-analysis is that only those persons whose total cholesterol exceed 250 mg/dl could obtain the stated benefits. The majority of our American population fall below this mark and would thus not benefit from increased soy intake.

But perhaps the most significant problem with this study is the primary assumption that lowering cholesterol lowers risk of heart disease.  Studies using diet or drugs to lower cholesterol have consistently resulted in a greater number of deaths in the treatment groups than in control groups.  Why should lowering cholesterol by increased soy production be any different?  (These are deaths from stroke, cancer, intestinal disorders, accident and suicide).

Soy & Brain Aging

At a recent soy symposium in Washington DC, Dr Lon White reported on a study of Japanese Americans living in Hawaii, that showed a significant statistical relationship between two or more servings of tofu a week and “accelerated brain aging.”

Study participants who consumed tofu in mid-life had lower cognitive function in late life, and a greater incidence of Alzheimer’s disease and dementia.  Those who ate a lot of tofu appeared 5 years older than actual age, by the time they were 75 or 80.  These changes were traced to the negative effects of soy’s isoflavones (phytoestrogens) .

Soy and Cancer

(See also “Soy & Breast Cancer, Menopause, Osteoporosis” below)

Promoters of soy cite low incidence of breast, uterus and prostate cancer among Japanese as a reason to consume more soy, because, they claim, the Japanese eat 30 times as much soy as North Americans.  What these promoters fail to mention is that the Japanese (and Asians in general) have much higher rates of other types of cancer, particularly of the esophagus, stomach, pancreas, and liver.  And Asians throughout the world also have much higher rates of thyroid cancer.  The logic that links low rates of reproductive cancers to soy consumption requires attribution of high rates of thyroid and digestive cancers to the same foods.

It is a stretch to claim the Japanese eat 30 times as much soy as North Americans.  Japanese and other Asians consume on average 9 grams (less than 2 teaspoons) of soy per day (per 1990s studies), primarily as a fermented condiment, compared with American vegetarians who consume at least a cup of soy per day, primarily as SPI or TVP as a meat substitute.  These Americans eat at least 24 times the amount of soy as the average Asian!

So, what conclusions can be drawn about soy and cancer?

A 1994 meta-analysis (compiling and comparing data from different studies with different dietary criteria) by Mark Messina, published in Nutrition and Cancer, noted that in 26 animal studies, 65% reported protective effects from soy.  In this analysis, he neglected to include at least one study in which soy-feeding caused pancreatic cancer (the 1985 study by Rackis).

In the human studies he listed, the results were mixed.  A few showed some protective effect, but most showed no correlation at all between dietary soy and cancer rates.  He concluded “the data in this review cannot be used as a basis for claiming that soy intake decreases cancer risk.”  Yet in his subsequent book (The Simple Soybean and Your Health, by Mark Messina), he recommends 1 cup, or 230 grams, of soy products per day to prevent cancer.

Soy & Breast Cancer, Menopause, Osteoporosis

Thousands of women are now consuming soy, hoping to protect themselves against breast cancer, heart disease, and the effects of menopause. This hope is based on the health claim that the isoflavones (phytoestrogens) found in soy provide this protection, despite many published reports about the toxicity of these particular phyto-estrogens.

  • A 1996 study reports that women consuming soy protein isolate had an increased incidence of epithelial hyperplasia, a condition that presages malignancies.
  • A 1997 study concluded that because dietary genistein (a soy isoflavone) stimulates breast cells to enter the cell cycle, women should not consume soy products to prevent breast cancer.
  • Administration of a soy-based feed to tropical birds (parrots) in the early 1990s caused striking and deadly effects.  Male birds developed colored plumage months earlier than normal, which was considered an asset.  But in the ensuing years, problems were noted:  decreased fertility; precocious maturation; deformed, stunted and stillborn babies; premature deaths, especially among females.  The birds suffered beak and bone deformities, goiter, immune system disorders, and pathological aggressive behavior.  Autopsy revealed digestive organs in a state of disintegration.  And all of these problems could be traced back to genistein, an isoflavone present in soy.
  • A 1991 Japanese study reported that consumption of 30 grams (2 Tablespoons) of soybeans per day for one month resulted in a significant increase in thyroid-stimulating hormone.  Some subjects developed goiter and hypothyroidism; many complained of constipation, fatigue, and lethargy, even though their iodine intake was adequate.  In 1997, the FDA’s national Center for Toxicological Research traced the problem to the same genistein and other soy isoflavones.
  • Most studies on effect of isoflavones on postmenopausal symptoms show that control subjects report reduction in discomfort (hot flashes, etc.) in degrees equal to subjects given soy.  In vitro studies suggest that isoflavones inhibit synthesis of estradiol and other steroid hormones.
  • The claim that soy prevents osteoporosis is absurd, given that soy foods block calcium, and cause vitamin D deficiencies (calcium and vitamin D are vital for the building of bone tissue).  It is far more likely that the reports of lower rates of osteoporosis in Asians than Westerners is due to other dietary factors than soy consumption.  The Asian diet provides plenty of vitamin D from shrimp, lard and seafood, and plenty of calcium from bone broths.  The reason for the increase in osteoporosis among westerners is because we have substituted soy oil for butter, our traditional source of vitamin D and other fat-soluble activators needed for calcium absorption.


  1. and following pages:  Page 2, Page 3  Please refer to  Page 3 of this article for list of their sources.
  2. The Dark Side of Soy by Mary Vance Terrain in Utne Reader, July/August 2007:

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