Soy has been one of the most polarizing herbs used in natural health care. Soy food is a rich source of isoflavones--a class of phytoestrogens that has both anti-estrogenic and anti-carcinogenic properties. Some believe it has great benefits for hormone balancing as well as reducing cancer and cardiovascular risks while others feel that it is a harmful plant that destroys both the earth and wreaks havoc on the human body. The answer we believe is somewhere in the middle and ultimately depends on what form of soy you are speaking about. The genetically modified (GM) version of the plant is most likely the culprit behind all of the negative exposure and ill effects that soy may cause, whereas organic, non-GM soy can have many positive effects in the body. We utilize soy a great deal in our clinical practice mostly to balance female hormones so we wanted to get to the bottom of the controversy and take a look at the scientific literature available on soy. What we found was slightly surprising in the fact that it was nearly impossible to locate any studies that showed soy to have a negative effect. The only information we came across that lead us to believe soy was harmful came from a Google search leading to various blogs and magazine articles. Searching reputable sources such as Pubmed and the Cochrane library only directed us to countless articles showing positive effects which we have briefly summarized below. As a disclaimer, we understand that clearing rainforests and fields to monocrop the GM version of the plant is harmful both to the planet and humans and we always advocate for the use of organic non GM soy.
Phytoestrogens are plant sources of estrogen-like compounds and are often misunderstood to act solely as excess hormone in the body and therefore said to be harmful in conditions that are worsened by estrogen. This is misleading and does not explain how phytoestrogens work on the receptors in the human body. To quickly clarify, phytoestrogens act as weak estrogens in the body. This means that if your estrogen is too high the phytoestrogen will weakly bind to estrogen receptors and block the excess or harmful estrogen from having an effect and it will be eliminated from the body with proper liver support. On the other hand, if you have low estrogen levels the phytoestrogen will again weakly bind to the receptors and have a slight estrogenic effect to help increase the amount of estrogen in the body. Therefore, phytoestrogens are helpful in both high and low estrogen situations.
Soy and Cancer
The studies that have been published in this area show that soy has protective effects in cancers that are hormone related. We came across many journal articles and we have summarized a few of the meta-analyses below.
Post-diagnosis Soy Food Intake and Breast Cancer Survival: A Meta-analysis of Cohort Studies.
Chi F, Wu R, Zeng YC, Xing R, Liu Y, Xu ZG.
Asian Pac J Cancer Prev. 2013
11 602 patients studied
This meta- analysis showed that soy food intake might be associated with better survival, especially for ER negative, ER+/ PR+, and postmenopausal patients.
Soy intake and risk of endocrine-related gynaecological cancer: a meta-analysis.
Myung SK, Ju W, Choi HJ, Kim SC.
The results of the current study showed protective effects of soy intake on the risk for endocrine-related gynecological cancers.
Association of soy and fiber consumption with the risk of endometrial cancer.
Goodman MT, Wilkens LR, Hankin JH, Lyu LC, Wu AH, Kolonel LN.
Am J Epidemiol. 1997
High consumption of soy products and other legumes was associated with a decreased risk of endometrial cancer.
Similar reductions in risk were found for increased consumption of other sources of phytoestrogens such as whole grains, vegetables, fruits, and seaweeds.
Legume, soy, tofu, and isoflavone intake and endometrial cancer risk in postmenopausal women in the multiethnic cohort study.
Ollberding NJ, Lim U, Wilkens LR, Setiawan VW, Shvetsov YB, Henderson BE, Kolonel LN, Goodman MT.
J Natl Cancer Inst. 2012
A reduced risk of endometrial cancer was associated with total isoflavone intake (highest vs lowest quintile, ≥7.82 vs <1.59 mg per 1000 kcal/d, RR = 0.66, 95% CI = 0.47 to 0.91), daidzein intake (highest vs lowest quintile, ≥3.54 vs <0.70 mg per 1000 kcal/d, RR = 0.64, 95% CI = 0.46 to 0.90), and genistein intake (highest vs lowest quintile, ≥3.40 vs <0.69 mg per 1000 kcal/d, RR = 0.66, 95% CI = 0.47 to 0.91).
This study suggests that greater consumption of isoflavone-containing foods is associated with a reduced risk of endometrial cancer in this population of nonhysterectomized postmenopausal women.
Phytoestrogen intake and endometrial cancer risk.
Horn-Ross PL, John EM, Canchola AJ, Stewart SL, Lee MM.
J Natl Cancer Inst. 2003
Isoflavone (OR = 0.59, 95% CI = 0.37 to 0.93 for the highest versus lowest quartile of exposure) and lignan (OR = 0.68, 95% CI = 0.44 to 1.1) consumptions were inversely related to the risk of endometrial cancer. These associations were slightly stronger in postmenopausal women.
Obese postmenopausal women consuming relatively low amounts of phytoestrogens had the highest risk of endometrial cancer
Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women's Health Study.
Lee SA, Shu XO, Li H, Yang G, Cai H, Wen W, Ji BT, Gao J, Gao YT, Zheng W.
Am J Clin Nutr. 2009
Adult soy food consumption, measured either by soy protein or isoflavone intake, was inversely associated with the risk of premenopausal breast cancer, and the association was highly statistically significant (P for trend < 0.001).
High intake of soy foods during adolescence was also associated with a reduced risk of premenopausal breast cancer (RR: 0.57; 95% CI: 0.34, 0.97). Women who consumed a high amount of soy foods consistently during adolescence and adulthood had a substantially reduced risk of breast cancer.
This large, population-based, prospective cohort study provides strong evidence of a protective effect of soy food intake against premenopausal breast cancer.
All of the studies show lower rates and/or recurrence of cancer in populations that consume the highest amounts of soy. These are not all of the studies done showing positive effects but we had to omit the others due to time and limited space of this blog. We were unable to locate any studies that showed negative effects of soy with cancers. From this we must conclude that benefits of soy for cancer vastly outweigh the risks.
Soy and Cholesterol
Soy has been shown numerous times to lower cholesterol levels and reduce the risk of cardiovascular disease. The results of a meta-analysis done in The American Journal of Clinical Nutrition in 2005 by Siyan Zhan and Suzanne C Ho titled A Meta-analysis of the effects of soy protein containing isoflavones on the lipid profile show that Soy protein containing isoflavones significantly reduced serum total cholesterol, LDL (bad) cholesterol, and triacylglycerol and significantly increased HDL (good) cholesterol. Again, studies showing negative effects of soy on cholesterol and cardiovascular health could not be found.
Soy and Men
Soy has been thought to increase a man’s estrogen levels and lead to female characteristics. Although a few case studies of very high dose soy consumption may have resulted in feminization, scientific evidence in humans shows that low to moderate doses of soy in the diet have beneficial effects for men as well and do not result in female features. We have summarized some of the studies below.
Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis.
Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ.
Fertil Steril. 2010
No significant effects of soy protein or isoflavone intake on T, SHBG, free T, or FAI were detected regardless of statistical model.
The results of this meta-analysis suggest that neither soy foods nor isoflavone supplements alter measures of bioavailable T concentrations in men.
Hormonal effects of soy in premenopausal women and men.
J Nutr. 2002
Thus, studies in women have mostly been consistent with beneficial effects, although the magnitude of the effects is quite small and of uncertain significance. Only three intervention studies reported hormonal effects of soy isoflavones in men. These recent studies in men consuming soy foods or supplements containing 40--70 mg/d of soy isoflavones showed few effects on plasma hormones or semen quality. This data does not support concerns about effects on reproductive hormones and semen quality.
Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence.
Fertil Steril. 2010
The intervention data indicate that isoflavones do not exert feminizing effects on men at intake levels equal to and even considerably higher than are typical for Asian males.
Soy and Thyroid
The one area where soy has been shown to have a negative effect is on the thyroid gland, specifically blocking the uptake of the synthetic thyroid hormone Synthroid. If you do not have a thyroid condition and are not taking Synthroid then soy should not negatively affect your thyroid.
After extensively searching through the scientific literature on soy we have come to the conclusion that the benefits outweigh the harms of soy and we will continue to use organic, non-GM soy as a treatment in our clinic. Any belief that soy is harmful may be based solely on theory, personal opinion, or case studies. The studies that did show little or no effect of soy were using only isoflavone isolates and not the whole bean. The research performed on soy showing positive effects is overwhelming, but as with any treatment the form and dose are critical. We always recommend consulting with your health care professional before starting any soy or supplementation routine.