Individuals with hormone-receptor positive breast cancer and who regularly consume soy products throughout their lives may have a better effect from tamoxifen therapy.
Lifetime Genistein Intake Increases the Response of Mammary Tumors to Tamoxifen in Rats
Authors: Zhang, X. et. al.
Source: Clin Cancer Research doi 10.1158/1078-0432.CCR-16-1735
This study attempted to assess the effects of a traditional Asian diet of regular soy intake upon tamoxifen and breast cancer. Genistein is an isoflavone — a component of soy. Laboratory rats were split into two groups. One group was fed daily supplements of genistein starting at 15 days after birth. The second group was fed a regular diet, without genistein. These groups were representative of a typical Asian diet ("high soy") and a typical Caucasian diet ("no soy"). Every rat was induced to have breast cancer. Following that, some rats in the "no soy" group were converted to a "high soy" diet. When a rat developed a tumor beyond a certain size, daily tamoxifen was added to its diet. Further, some of the rats in the "no soy" group were started on daily genistein only after they developed large tumors.
The rats with breast cancer and a "high soy" diet only after their tumors became large had higher risks of cancer recurrence compared to the rats in the "high soy" diet from childhood and compared to the rats in the "high soy" diet begun in adulthood. There was less suppression of antitumor activity and more immune cell activity in the rats on the "high soy" diet from childhood.
This study suggests that people who consume soy products on a regular basis, beginning early in life, may have better outcomes with their hormone-receptor positive breast cancer and tamoxifen. People who start consuming soy on a regular basis in adulthood may see the same benefits. There may be no benefit in starting soy consumption after a breast cancer diagnosis. Certainly, more studies that include human subjects are necessary. But this investigation adds to the theories regarding soy products and breast health.