In recent years, scientific evidence has questioned the health benefits of soy, and more importantly, has brought to light possible adverse effects. Tell me it's not soy!!

Remember when soy or soya was the wonder food of the 90s? Even government health agencies such as the U.S. FDA went so far as to allow soy food labeling with health claims on the association between soy protein and the reduced risk of coronary heart disease. ¹

For vegetarians, soy and soy products is the perfect substitute for meat and dairy products. And for those who are lactose intolerant, especially little babies, soy milk is a god-sent milk substitute that was both protein-rich and healthy.

In addition to its alleged health benefits, soy and soy products are cheap and easily available. In the form of tofu or bean curd, soy is purported to be the perfect protein source without the bad health effects of meat, plus low in fat and calories. ²

Aside from being more than 77% water, soy is rich in vitamins and minerals, low in fat, carbs and sugar and contains:

• Omega-3-fatty acids, also known as "the heart-friendly fats."
• Isoflavones, antioxidants which are thought to have some some anti-cancer properties.
• Cholesterol-lowering compounds

Sounds like it's the perfect health food, but is it?

• In 2004, the Weston A. Price Foundation submitted a petition to the U.S. FDA to withdraw the privilege of soy products to carry a heart disease health claim. The petition submitted a body of data "which establishes a lack of consensus among experts, qualified by scientific training and experience, about claims that soy protein prevents heart disease or even lowers cholesterol". ³
• According to a review by Italian researchers, "available data does not appear to unequivocally support beneficial effects of soy isoflavones, and warn against their wide use, in the absence of satisfactory clinical findings." ⁴
• The main isoflavones in soy, genistein and daidzein, have antioxidant properties but also are phytoestrogens or endocrine-disrupting substances which can create havoc with hormonal balance. There are contradicting studies as to whether these phytoestrogens prevent or promote the development of breast cancer. ^5-7
• A recent clinical trial reported that soy protein and isoflavones are not effective in improving bone mineral density in postmenopausal women. This contradicts the claims that soy supplements may prevent osteoporosis. Researchers at the Osteoporosis Research Center in Omaha, reported that even calcium-fortified soy milk is not comparable to cow milk as a calcium source. Calcium from the soy milk is absorbed only at 75% the efficiency of absorption from dairy milk. ^8,9
• Soy protein is said to slow down aging by maintaining bone mineral density and cognitive function. Again, the current state of evidence is contradictory. Most research studies did not find any significant positive effects of soy consumption on aging.
  In fact, researchers at the National Institute on Aging suggest that high soy consumption in midlife is linked to cognitive decline and atrophy of the brain later in life. In 2004, Dutch researchers investigated using soy isoflavones as an alternative to estrogen therapy in postmenopausal women. The reported results "do not support the hypothesis that the use of soy protein supplement containing isoflavones improves cognitive function, bone mineral density, or plasma lipids in healthy postmenopausal women when started at the age of 60 years or later." A 2007 study reported that supplementation with soy isoflavones do not significantly affect cognitive functioning in postmenopausal women. ^10-12
• Not to add insult to injury, it is important to point out that most soy products sold in the United States and Canada are GMO which has its own set of issues. ¹³

So, is there a dark side to soy?

The Weston A. Price Foundation summarizes the potential dangers of soy: ¹⁴

• Soy is rich in phytic acid that hinders the uptake of calcium, magnesium, copper, iron and zinc. High phytate diets have caused growth problems in children.
• Soy contains trypsin inhibitors which interfere with protein digestion and may cause pancreatic disorders.
• The phytoestrogens in soy disrupt hormonal balance and may cause infertility and breast cancer.
• Phytoestrogens also interfere with the functioning of the thyroid gland. In infants, consumption of soy formula has been linked to autoimmune thyroid disease.
• Soy contains vitamin B12 analogs that are not easily absorbable and may actually increase the body's requirement for vitamin B12 and vitamin D.
• Processing of soy protein results in the formation of lysinoalanine which is toxic and nitrosamines which are highly carcinogenic.
• Free glutamic acid or MSG, a potent neurotoxin, is formed during soy food processing and additional amounts are added to many soy foods.
• Some soy foods were found to contain high levels of aluminum which is toxic to the nervous system and the kidneys.

Stronger evidence of the hormone disrupting action of phytoestrogens in soy has been observed in men. There have been anecdotal reports of adult men developing abnormal breasts (gynecomastia) as described in an article in Men's Health. ¹⁵

Some of the peer-reviewed studies on this topic are summarized:

• In 2005, Canadian researchers reported an inverse relationship between soy intake and testosterone levels in men. This may have the effect of reducing the risk for prostate cancer but reducing male fertility as well.
• In animals, a 2008 Chinese study observed that exposure of juvenile rats to the phytoestrogen daidzein resulted in erectile dysfunction that persisted in adulthood. The same group of researchers observed abnormal histological changes in the penile tissues of daidzein-exposed rats. Thus, the phytoestrogen daidzein seems to induce changes in the structure and function of the reproductive organs of rats.
• Harvard researchers reported in 2008 that the isoflavones in soy may lead to decreased fertility in human males and produced data which suggest that higher intake of soy foods and soy isoflavones is associated with lower sperm concentration. ^16-19

And as early as 1991, Japanese researchers reported that excessive soy consumption can suppress thyroid function in elderly patients and cause goiter. ²⁰

If soy has some negative health effects on adults, how about children exposed to soy at an early age?

Approximately 25% of infant formula sold in the U.S. is made from soy. In 1998, toxicologist Daniel Sheehan of the National Center for Toxicological Research of the U.S. FDA wrote that "among human exposures, infant soy formula exposure appears to provide the highest of all phytoestrogen doses, and this occurs during development, often the most sensitive life-stage for induction of toxicity. Large, carefully controlled studies in this exposed infant population are a high priority." ²¹

In 1999, advocacy groups led by the Weston A. Price Foundation called for the removal of all soy-based formula from the marketplace. They based their call on reports that a high number of boys consuming soy are experiencing delayed physical and sexual development whereas girls are going into premature puberty. They attribute these changes to high intake of phytoestrogens. According to Mary Enig, a nutritionist and then president of the Maryland Nutritionists Association, "the amount of phytoestrogens that are in a day's worth of soy infant formula equals five birth control pills." ^22,23

Ten years later, the issue of soy milk still hasn't been clarified or resolved. In a very recent report researchers from the National Institute of Environmental Health Sciences detected high amounts of the isoflavones genistein and daidzein in the urine, saliva, and blood of babies fed with soy-based formula. Compared to non-soy feeding children, concentrations of these compounds were about 500 times higher in soy-fed children. The results, however, could not determine whether the said phytoestrogens are biologically active in babies or are simply excreted into the urine. The authors concluded that further studies "focusing on physical and developmental findings reflecting the effects of estrogen exposure" are needed. ²⁴

The Bottom Line

Soy: friend or foe? Unfortunately, this is a question that is not easily answered.  However, there is a general belief among experts that high intake of soy, especially as a sole source of protein is not advisable, unless there is no other alternative, such as lactose-intolerant babies.

Published January 11, 2010, updated July 12, 2012

References

1. Food Labeling: Health Claims; Soy Protein and Coronary Heart Disease; Final Rule, U.S. FDA Federal Register 64 FR 57699 October 26, 1999
2. Nutrient Database for Standard Reference. Nutrient Data Laboratory, USDA
3. Health Experts Urge FDA To Deny Soy Protein Cancer Prevention Health Claim , Weston A. Price Foundation, July 5, 2004
4. Sirtori CR, Risks and benefits of soy phytoestrogens in cardiovascular diseases, cancer, climacteric symptoms and osteoporosis, Drug Saf. 2001;24(9):665-82
5. Cherniske S, Soy Story Part One: Phytoestrogens, Univerascience.com
6. Wu AH et al, Epidemiology of soy exposures and breast cancer risk, Br J Cancer. 2008 Jan 15;98(1):9-14. Epub 2008 Jan 8
7. Messina Met al, Addressing the soy and breast cancer relationship: review, commentary, and workshop proceedings, J Natl Cancer Inst. 2006 Sep 20;98(18):1275-84
8. Kenny AM, Soy proteins and isoflavones affect bone mineral density in older women: a randomized controlled trial, Am J Clin Nutr. 2009 Jul;90(1):234-42. Epub 2009 May 27
9. Heaney RP et al, Bioavailability of the calcium in fortified soy imitation milk, with some observations on method, American Journal of Clinical Nutrition, Vol. 71, No. 5, 1166-1169, May 2000
10. White LR et al, Brain aging and midlife tofu consumption, J Am Coll Nutr. 2000 Apr;19(2):242-55
11. Kreijkamp-Kaspers S et al, Effect of soy protein containing isoflavones on cognitive function, bone mineral density, and plasma lipids in postmenopausal women: a randomized controlled trial, JAMA. 2004 Jul 7;292(1):65-74
12. Fournier LR et al, The effects of soy milk and isoflavone supplements on cognitive performance in healthy, postmenopausal women, J Nutr Health Aging 2007 Mar-Apr;11(2):155-64
13. Gutierrez D, Did you know? Nearly all corn and soy products purchased at grocery stores are genetically modified and may place your health at risk, NaturalNews.com, January 30, 2012
14. Soy Alert, Weston A. Price Foundation, 2006
15. Thornton J, Is This The Most Dangerous Food For Men? Men's Health, May 19, 2009
16. Dillingham BL et al, Soy protein isolates of varying isoflavone content exert minor effects on serum reproductive hormones in healthy young men, J. Nutr. 135 (3): 584–91
17. Pan L et al, Exposure of juvenile rats to the phytoestrogen daidzein impairs erectile function in a dose-related manner in adulthood, J Androl. 2008 Jan-Feb;29(1):55-62. Epub 2007 Aug 1
18. Huang Y et al, Long-term effects of phytoestrogen daidzein on penile cavernosal structures in adult rats, Urology. 2008 Jul;72(1):220-4. Epub 2008 Mar 21
19. Chavarro JE, Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic., Hum Reprod. 2008 Nov;23(11):2584-90. Epub 2008 Jul 23
20. Ishizuki Y et al, The effects on the thyroid gland of soybeans administered experimentally in healthy subjects, Article in Japanese, Nippon Naibunpi Gakkai Zasshi. 1991 May 20;67(5):622-9
21. Sheehan DM, Herbal medicines, phytoestrogens and toxicity: risk:benefit considerations, Proc Soc Exp Biol Med. 1998 Mar;217(3):379-85
22. Sheehan DM, Tragedy and Hype: Third International Soy Symposium, The Weston A. Price Foundation, March 26, 2000
23. Fallon S et al, Phytoestrogens – Panacea or Poison? Tragedy and Hype: The Third International Soy Symposium – Part II, 2000
24. Cao Y et al, Isoflavones in urine, saliva, and blood of infants: data from a pilot study on the estrogenic activity of soy formula, J Expo Sci Environ Epidemiol. 2009 Feb;19(2):223-34. Epub 2008 Jul 30