Whether or not to eat soy is a puzzle for the everyday consumer. It can be even trickier if you’re a woman with uterine fibroids. And yet, the latest research shows we’re not that much closer to finding a clear-cut answer.
Soy is rich in phytoestrogens or plant estrogens called isoflavones, which are similar in structure to human estrogen but not as potent. Because they’re weaker, it’s believed these plant estrogens could act as anti-estrogens and serve as a potential protection against several hormonal diseases like breast cancer.1
Uterine fibroids, although noncancerous, are tumors thought to respond to estrogen and progesterone levels. For this reason, researchers question whether soy intake will protect or worsen the condition.
Published in Nutrition & Cancer, a review of current research looked at relevant studies investigating any potential links between soy intake and uterine fibroid risk.1 A Chinese case-control study involving 73 women with fibroids and 210 without examined the women’s dietary habits, physical activity, and stress through validated self-administered questionnaires.1,2 Results showed that soy food intake wasn’t linked to uterine fibroids in either premenopausal or postmenopausal women. A similar Japanese cross-sectional study that looked at 285 premenopausal Japanese women also found no association.1,3 The same was found true for the Black Women’s Health Study made up of 22,120 premenopausal US women.1,4 These studies show that soy intake doesn’t seem to be correlated to fibroid risk in adult women, but could soy intake early in life affect fibroid development and growth later on? The Study of Environment, Lifestyle & Fibroids (SELF), an ongoing cohort study of 1,696 African American women ages 23-34 years, surveyed 1,553 women (89%) concerning soy intake during infancy.5
Out of the 1,553 SELF participants, 22% had fibroids and 13% reported being fed soy formula. Results showed that soy formula-fed women had a slightly lower prevalence of fibroids (20%) in comparison to women who were not given soy formula (23%)—these were values considered too similar to be significant. There also wasn’t any association between soy formula intake and fibroid number.5
What did stand out to researchers was that among women with fibroids, those who were soy formula-fed as infants had significantly larger fibroids in comparison to those not fed soy formula. On average, soy formula feeding was linked with a 32% increase in fibroid diameter of the largest fibroid and a 127% increase in total fibroid volume. The researchers concluded that this study supports the theory concerning the effects of early life phytoestrogen exposure on the uterus.5 Even though no link was found between soy formula and fibroid prevalence and number, animal studies have shown that when exposed to a soy isoflavone called genistein early in life, it can affect the developing uterus and promote both of these occurrences.6
If the results to these studies don’t seem very cohesive, researchers examining possible fibroid links within the Black Women’s Health Study found data that was even more conflicting.7 After following 23,505 premenopausal women, 12-year follow-ups reported 7,268 had fibroids. But the researchers also stated “there was little evidence of an association between [fibroids] and…exposure to soy formula in infancy” and that “these findings do not support the hypothesis that intrauterine and early life factors are strongly related to [fibroid] risk.”7
As the latest research has shown, the jury is still out on whether or not soy intake is connected to fibroids. As we reviewed, it may not affect adult women but may influence a developing uterus in ways that could later lead to fibroid growth.
- Parazzini, F., Di Martino, M., Candiani, M., et al. (2015). Dietary components and uterine leiomyomas: a review of published data. Nutr Cancer, Mar; 67(4): 569-579.
- He, Y., Zeng, Q., Dong, S., et al. (2013). Associations between uterine fibroids and lifestyles including diet, physical activity and stress: a case-control study in China. Asia Pac J Clin Nutr, 22(1): 109-117.
- Negata, C., Nakamura, K., Oba, S., et al. (2009). Association of intakes of fat, dietary fibre, soya isoflavones and alcohol with uterine fibroids in Japanese women. Br J Nutr, May; 101(10): 1427-1431.
- Wise, L., Radin, R., Palmer, J., et al. (2010). A prospective study of dairy intake and risk of uterine leiomyomata. Am J Epidemiol, Dec; 171(2): 221-232.
- Upson, K., Harmon, Q., & Baird, D. (2016). Soy-based infant formula feeding and ultrasound-detected uterine fibroids among young African-American women with no prior clinical diagnosis of fibroids. Environ Health Perspect, Nov; 124(6): 769-775.
- Greathouse, L., Bredfeldt, T., Everitt, J. (2012). Environmental estrogens differentially engage the histone methyltransferase EZH2 to increase risk of uterine tumorigenesis. Mol Cancer Res, Apr; 10(4): 10.1158/1541-7786.MCR-11-0605.
- Wise, L., Radin, R., Palmer, J., et al. (2012). Association of intrauterine and early life factors with uterine leiomyomata in black women. Ann Epidemiol, Dec; 22(12): 847-854.77