A Brief Survey

Soy Background Cover


A simple three-letter word may stir up a lot of thoughts, emotions, discussions, and opinions from many people. Others may be quite indifferent about it. Should it be included in a healthy diet? Does it generally promote health, or deter from it? Can it affect people differently, depending on their age, sex, or other attributes?

Much of the controversy surrounding soy is due to it containing phytoestrogens, also called isoflavones. “Phyto” meaning “plant,” and estrogen (of course) meaning estrogen, these plant molecules mimic the estrogen that circulates throughout our bodies and attaches to our estrogen receptors. But the challenge is in knowing how and if these phytoestrogens affect our health and if it should be a cause for concern.

Starting from the early stages of life, researchers studied how soy formulas impacted blood levels of phytoestrogens, i.e., isoflavones, within the blood of infants (1). Infants who were fed only soy formula were compared with infants who were fed cow-milk-based formula only, as well as infants fed only breast milk. Results of the study showed that infants exposed to soy formula had 6 to 11 times the amount of isoflavones that it would take to have a noticeable hormonal effect in adult patients. Moreover, the levels of isoflavones in the blood of soy-fed infants were 13,000-22,000 times greater than those infants having breast milk or cow’s milk formula (1). If nothing else can be said, one thing we can say for sure is this: these levels of phytoestrogens are way beyond the range of what an infant would be exposed to when being breastfed.

How might this soy-feeding early in life affect the trajectory of a child’s health? A study examined how female infants fed soy had a statistically significant difference in the age they started their first period versus female infants not fed soy (2). While this demonstrates correlation only, girls fed soy formula started their period at an average age of 12.4 years old, while infants not fed soy started their period at an average age of 12.8 years old. Furthermore, girls fed soy during infancy had a 25% increase in the likelihood of starting their periods sooner than cow milk formula or breast milk-fed infants (2). This slight difference in ages may seem negligible to some, but starting menstruation at earlier ages has been associated with a higher likelihood of developing markers linked to cardiovascular disease, including increases in body fat percentage, higher blood pressure and glucose, and higher insulin levels when compared to females who started their period later in adolescence (3).

Likewise, boys consuming higher amounts of soy started puberty about a half year sooner than boys who ate the lowest amounts of soy (4).

Knowing the potentially deleterious effects soy consumption has on infants, children, and adolescents, does soy consumption spell trouble for the adult hormonal profile?

A population-based study from the early 2000s reported that Asian-American women who consumed more soy during adolescence and adulthood had a decreased incidence of breast cancer. However, when they ate high amounts of soy in their teenage years but not in their adult years, the risk of getting breast cancer was higher than eating soy during both time periods (5). Unfortunately, the study did not have a large enough sample of women who consumed little soy during adolescence and increased it during adulthood. If they had this information, we would be able to better understand if consuming soy at certain stages of development is best for protecting against breast cancer. Some have called the phenomenon of phytoestrogens preventing breast cancer the “phytoestrogen paradox” because higher levels of normal estrogen produced by the body are linked to higher breast cancer rates. Though still a theory, some predict that phytoestrogens block the estrogen receptors in the body, preventing endogenous estrogen from having an effect on breast cancer progression.

A similar effect was seen in men in regard to prostate cancer: in general, higher soy consumption correlated to a reduced risk of developing prostate cancer (6). Interestingly, the risk of developing prostate cancer was cut in half for Asian male populations with an increased soy intake, but for Western male populations, the risk reduction was only 1% (6). This information warrants further study into if there are certain genes that affect the processing of soy and if it helps in reducing cancer risk for certain populations.

Phytoestrogens may be used as a therapeutic tool for menopausal symptoms. A study examined the use of isoflavone supplements for the reduction of menopausal symptoms in women. After 12 weeks of taking 100 milligrams of isoflavones per day, hot flashes and other symptoms improved by an average of 16% for these women (7). Some menopausal women undergo what is called hormone replacement therapy (HRT) to help with menopausal symptoms, meaning they take estrogen hormones. However, HRT involves risks including but not limited to an increased risk of developing endometrial and breast cancer and developing blood clots. It is thought that soy isoflavones may be a safer alternative to HRT (7).

These studies leave little room for doubt that phytoestrogens found in soy have a physiological effect. That doesn’t necessarily mean we have to deem it as an evil food never to be touched, but we should recognize its potential effects on the body in all stages of life. In the breast cancer (5) and prostate cancer (6) studies, results suggest that soy may be protective. In the menopause study (7), soy may be therapeutic. But perhaps we should be wary of including soy in the diets of infants and children especially, as it may disrupt their natural timing of puberty. Having soy occasionally may have lesser effects than including it on a daily basis, too. Yet many processed foods contain soy, so it is something to watch for.

As with many things in science, we may not be able to fit these conclusions into a nice little box of “good” or “bad.” We can just understand the data and make more informed decisions. Your stage of life, genetic profile, and whether you are male or female may inform your decisions about how much (if any) soy you choose to consume.


  1. Setchell, K. D., Zimmer-Nechemias, L., Cai, J., & Heubi, J. E. (1997). Exposure of infants to phyto-oestrogens from soy-based infant formula. Lancet (London, England), 350(9070), 23–27.
  2. Adgent, M. A., Daniels, J. L., Rogan, W. J., Adair, L., Edwards, L. J., Westreich, D., Maisonet, M., & Marcus, M. (2012). Early-life soy exposure and age at menarche. Paediatric and perinatal epidemiology, 26(2), 163–175.
  3. Karen E. Remsberg, Ellen W. Demerath, Christine M. Schubert, Wm. Cameron Chumlea, Shumei S. Sun, Roger M. Siervogel, Early Menarche and the Development of Cardiovascular Disease Risk Factors in Adolescent Girls: The Fels Longitudinal Study, The Journal of Clinical Endocrinology & Metabolism, Volume 90, Issue 5, 1 May 2005, Pages 2718–2724.
  4. Segovia-Siapco, G., Pribis, P., Oda, K. et al. Soy isoflavone consumption and age at pubarche in adolescent males. Eur J Nutr 57, 2287–2294 (2018).
  5. Anna H. Wu, Peggy Wan, Jean Hankin, Chiu-Chen Tseng, Mimi C. Yu, Malcolm C. Pike, Adolescent and adult soy intake and risk of breast cancer in Asian-Americans, Carcinogenesis, Volume 23, Issue 9, September 2002, Pages 1491–1496,
  6. Lin Yan, Edward L Spitznagel, Soy consumption and prostate cancer risk in men: a revisit of a meta-analysis, The American Journal of Clinical Nutrition, Volume 89, Issue 4, April 2009, Pages 1155–1163,
  7. Ahsan, M., & Mallick, A. K. (2017). The Effect of Soy Isoflavones on the Menopause Rating Scale Scoring in Perimenopausal and Postmenopausal Women: A Pilot Study. Journal of clinical and diagnostic research : JCDR, 11(9), FC13–FC16.


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