With the level of growth seen in the soy research field, it remains a challenge for scientists to come to a solid consensus on aspects of soy as they relate to human health. In addition, many different types of studies are performed using a variety of soy products. Conclusions from only one or a few studies on specific soy preparations cannot be simply generalised about the consumption of soy on the whole. As a result, much confusion is generated regarding the health effects of soy. Soya’s safety has been reviewed by a number of major Western committees including the public health community, the UK Committee on Toxicity and opinion leaders such as the US Food and Drug Administration and American Heart Association who agree that soy foods are healthy additions to the diet.(1).
Controversy around soy appears to relate primarily to the difference between observed health outcomes from epidemiological studies of soy food intake and the data obtained using specific isolated and concentrated fractions of soy(2). In contrast to epidemiological studies that look at soy food consumption, research data from animal or human intervention studies use fractions of soy (e.g. soy concentrates or isolates, isolated isoflavones mixtures, pure genistein etc.) By not fully specifying what has been used in epidemiological studies, it is often difficult to understand what researchers mean when they use terms such as “soy” or “soy protein” (3). It is known that these different forms of soy can produce varying results.
Additionally, differences in soy components also arise from the variety of the soybean used, specifically processing techniques, and/or growing and storage conditions (3).
Here’s a closer look at some of the research:
Tofu contains phytoestrogens, one example being isoflavones - plant-derived substances that have a chemical structure similar to that of oestrogen. Due to this unique structure, isoflavones have weak oestrogen-like effects in various tissues, such as reproductive, cardiovascular and skeletal tissues (9,10). There has been concern regarding the oestrogenic effects of soy isoflavones; however it has been estimated that isoflavones are 1/400th to 1/1000th the potency of synthetic oestrogen (11). Phytoestrogens may help to balance oestrogen levels with the ability to either exert oestrogenic activity if body levels of oestrogen are low or anti-oestrogenic activity if body levels are high. However, more research is still required in this area. Like all foods, phytoestrogens are not “magic bullets” and need to be incorporated into a person’s overall diet in a balanced way. In moderation like all foods. How much a person should eat depends on their unique health needs.
In men, there has been concern as to whether soy consumption might lead to infertility by decreasing testosterone levels and semen quality. Kurzer reviewed four published clinical trials investigating the effects of soy isoflavone consumption in men and reported that:
“…there are no adverse effects of soy isoflavone consumption on sperm quality, although there may be small effects of soy consumption on sex hormone-binding globulin and steroid hormones”(8). It is also worthwhile to note that based on a variety of studies, it has been suggested that soy consumption may reduce prostate cancer risk without any significant adverse effects (17-23). Further studies to evaluate the effects of soy consumption on hormone metabolism by the prostate are needed for a better understanding of this effect of soy.
Does soy have a negative impact on thyroid function?
Well-designed human clinical studies have demonstrated that soy has a minimal effect on overall thyroid function in healthy individuals (7,8) Individuals who have a history of thyroiditis and/or consume a diet marginally deficient in iodine may be more susceptible to these effects. If there is a history of thyroiditis problems it is advisable to consume sea vegetables along with the soy in order to ensure there is enough iodine. (Iodine is involved in thyroid hormone synthesis).
Not all soy products are created equal. Find out which soy foods and soy products to choose
1. Henkel J. U.S. Food and Drug Administration. Soy: Health claims for soy protein, questions about other components. Available at http://www.fda.gov/fdac/features/2000/300_soy.html.
2. Messina M, Erdman J Jr, Setchell KD. Introduction to and perspectives from the Fifth International Symposium on the Role of Soy in Preventing and Treating Chronic Disease. J Nutr 2004;134(5):1205S-06S.
3. Erdman JW Jr, Badger TM, Lamoe JW, Setchell KD, Messina M. Not all soy products are created equal:caution needed in interpretation of results. J Nutr 2004;134(5):1229S-33S
7. Duncan AM, Underhill KE, Xu X, LavalleurJ, Phipps WR, Kurzer MS. Modest hormonal effects of soy isoflavones in postmenopausal women. J Clin Endocrinol Metab 1999;84(10):3479-84
8. Persky VW, Turyk ME, Wang L, et al. Effect of soy protein on endogenous hormones in postmenopausal women. Am J Clin Nutri 2002; 75(1):145-53
10. Setchell KD, Cassidy A. Dietary isoflavones: biological effects and relevance to human health. J Nutr 1999;129(3):758S-67S.
11. Hudson T. Perimenopause and menopause alternatives to conventional HRT for symptom management – Women’s Health Update. Townsend Letter for Doctors and Patients; November, 2002.
17. Peterson G, Barnes S. Genistein and biochanin A inhibit the growth of human prostate cancer cells but not epidermal growth factor receptor tyrosine autophosphorylation. Prostate 1993;22(4):335-45.
18. Geller J, Sionit L, Partido C, et al. Genistein inhibits the growth of human-patient BPH and prostate cancer in
histoculture. Prostate 1998;34(2):75-79.
19. Barnes S. Effect of genistein on in vitro and in vivo models of cancer. J Nutr 1995;125(3 Suppl):777S-83S
20. Adlercreutz CH, Goldin BR, Gorbach SL, et al. Soybean phytoestrogen intake and cancer risk. J Nutr 1995;125(3 Suppl):757S-70S.
21. Nagata C, Takatsuka N, Shimizu H, Hayashi H, Akamatsu T, Murase K. Effect of soymilk consumption on serum estrogen Epidemiol Biomarkers Prev 2001;10(3):179-84.
22. Habito RC, Montalto J, Leslie E, Ball MJ. Effects of replacing meat with soyabean in the diet on sex hormone concentrations in healthy adult males. Br J Nutr 2000;84(4):557-63.
23. Mitchell JH, Cawood E, Kinniburgh D, Provan A, Collins AR, Irvine DS. Effect of a phytoestrogen food supplement on reproductive health in normal males. Clin Sci 2001;100(6):613-18.