The Effect of Soy Isoflavones on Menopausal Vasomotor Flushing

Abstract

The use of over the counter soy products and soy-rich diets has become popular among women in an attempt to control vasomotor flushing associated with menopause as an alternative to estrogen-based hormonal therapies. Soy isoflavones are structurally similar to 17β-estradiol and possess selective estrogen receptor modulator (SERM)-like activity. It has been suggested that hot flushes can be reduced in women whose diets incorporate soy products, but trials testing this hypothesis have been inconsistent. To determine the effect of long-term use of isoflavone soy protein supplementation on frequency and intensity of hot flushes. The Women's Isoflavone Soy Health (WISH) trial was a randomized placebo-controlled trial designed to determine the impact of isoflavone soy protein (ISP) supplementation on health outcomes (including atherosclerosis, osteoporosis, cognition and breast density) in a healthy population of postmenopausal women. 350 healthy postmenopausal women were randomized 1;1 to 2.5 yrs of ISP supplementation or placebo. Participants completed daily flushing diaries prior to randomization and throughout intervention; numbers of mild, moderate and severe flushes were recorded. This is a secondary analysis of the WISH trial, focusing on 110 women who reported at least seven weekly hot flushes of any level at baseline. Flushing intensity level was assigned a score of 1, 2, or 3 corresponding to mild, moderate, or severe flushing, respectively. The frequency of each level was multiplied by the corresponding intensity score and summed to create a composite flushing score. Data were analyzed using linear mixed effects models, with the repeatedly measured flushing composite score as the dependent outcome variable. An unstructured variance-covariance matrix was used, assuming a common covariance structure for placebo and ISP groups, to determine if the average linear change in composite flushing score over the trial differed between treatment groups. Flushing scores did not differ by treatment group at baseline or at any post-randomization period. The treatment-by-time interaction was not statistically significant (p=0.40), indicating no difference in the longitudinal change in the composite flushing score between treatment groups. The use of dietary isoflavone soy protein supplementation has no significant impact on either frequency or intensity of hot flushes.