The growth of some types of breast tumors is stimulated by the estrogen. However, the evidence that estrogen therapy increases breast cancer risk is unimpressive. In fact, in the Women’s Health Initiative trial, which involved over 10,000 women, estrogen therapy actually statistically significantly reduced risk of developing invasive breast cancer.
Several lines of evidence suggest it is the combination of estrogen plus progestin that increases risk, not estrogen alone. This distinction is important because while soy contains the phytoestrogens isoflavones, soy doesn’t contain compounds with progestin activity. However, new observational data from Finland suggest that neither estrogen nor combination therapy increases breast cancer risk.
In total, 489,105 women using hormone therapy between 1994 and 2009, traced from a nationwide reimbursement register, were followed from hormone therapy initiation (3.3 million cumulative exposure years) to breast cancer death (n = 1,578 women). The breast cancer mortality risk was reduced in all hormone therapy users with exposure for at most 5 years (standardized mortality ratio 0.56; 95% CI, 0.52-0.60), more than 5 to 10 years (0.46; 0.41-0.51), or more than 10 years (0.62; 0.56-0.68). The death risk reductions in estrogen therapy users tended to be larger in all age groups compared with estrogen plus progestin users
Mikkola, T.S., Savolainen-Peltonen, H., Tuomikoski, P., Hoti, F., Vattulainen, P., Gissler, M., and Ylikorkala, O. Reduced risk of breast cancer mortality in women using postmenopausal hormone therapy: a Finnish nationwide comparative study. Menopause. 2016, 23, 1199-1203.