Quella, S.K., Loprinzi, C.L., Barton, D.L., Knost, J.A., Sloan, J.A., LaVasseur, B.I., … Novotny, P.J. (2000). Evaluation of soy phytoestrogens for the treatment of hot flashes in breast cancer survivors: A North Central Cancer Treatment Group trial. Journal of Clinical Oncology, 18, 1068–1074. 

The study evaluated soy phytoestrogens for the treatment of hot flashes in breast cancer survivors.

After a baseline documentation week, women received four weeks of either soy tablets or placebo. They then crossed-over to the opposite for the last four weeks. The soy product was formulated in 600 mg tablets. Participants took one tablet three times per day (150 mg of isoflavones day), an amount similar to that consumed with three glasses of soy milk.

Participants included 177 women with a history of breast cancer; 149 participants (84%) provided useable efficacy data for the entire nine weeks of the study.

All participants were randomized in a double-blind crossover design to one of two groups (soy or placebo) and crossed-over after four weeks. Participants were stratified according to age, duration of hot flashes, and the average daily hot flash frequency using a dynamic allocation procedure that balances marginal distributions. They were also stratified by current tamoxifen or raloxifene use (yes or no).

The instrument was a daily questionnaire documenting hot flashes frequency, intensity, and perceived side effects.

The soy product did not alleviate hot flashes in breast cancer survivors. No toxicity was observed. These data failed to suggest any patient preference for the soy compound over the placebo preparation.

Optimal daily dose of soy required to recognize a clinical response may be questioned. Data related to estimated intake of 150–200 mg daily in the Asian diet endorsed the choice of 150 mg/day. Experience from conventional HRT suggests that length of time on the soy isoflavones (four weeks) may be too short to elicit a clinical response. Study durations of less than three months have been excluded from overviews of the effects of HRT.