Recently published results from the Study of Women's Health Across the Nation (SWAN) received quite a bit of media attention. However, the severe limitations of this study prohibit conclusions from being made. The SWAN is a multi-site longitudinal, epidemiologic study designed to examine the health of women during their middle years. The SWAN cohort was used to determine whether isoflavone intake prevented the onset of hot flashes. The study included 3,302 premenopausal and early perimenopausal women, 1,651 of whom reported no vasomotor symptoms at baseline and were followed with annual visits for 10 years. The results indicated that isoflavone intake was unrelated to hot flash occurrence. However, median daily isoflavone intake was less than 500 micrograms. Clinical trials suggest approximately 50 milligrams per day are needed to alleviate existing hot flashes. It is unreasonable to expect such low intakes to prevent the onset of hot flashes, even if lower amounts of isoflavones are need to prevent the onset of hot flashes than to alleviate existing hot flashes. Not even the Asian women in this study consumed an amount of isoflavones close to the levels shown to be efficacious in clinical studies. Furthermore, efficacy was determined by assessing the number of days women experienced vasomotor symptoms in the past 2 weeks. Consequently, if isoflavone intake caused a woman to have 4 hot flashes per day instead of 8, which is the type of improvement noted in the clinical trials, this benefit would not have been recorded as such in the analysis of the data.
Gold EB, Leung K, Crawford SL, et al. Phytoestrogen and fiber intakes in relation to incident vasomotor symptoms: results from the Study of Women's Health Across the Nation. Menopause 2012.