Nearly 20 years ago, the low incidence of hot flashes among Japanese women prompted clinical research into the effects of soyfoods on the alleviation of menopausal symptoms. Earlier this year, the results of the most extensive and sophisticated analysis of the intervention studies to date, which was published in the journal Menopause, showed very clearly that soybean phytoestrogens (isoflavones) cut hot flash frequency and severity by more than half (1). Unfortunately, recent media coverage of a study from the University of California may have unintentionally misled thousands of women into believing that soy may not help alleviate menopausal symptoms.
The objective of this population study was to determine whether soy isoflavones prevent the onset of hot flashes in a multi-ethnic sample of American women (2) This is the first study, to evaluate whether isoflavones prevent hot flashes, from occurring. The research involved over 3,000 premenopausal and early perimenopausal women, about half of whom reported no menopausal symptoms at study entry. The women were followed for approximately 10 years. At study entry and at years 5 and 9, participants were assessed to determine the number of days they reported having hot flashes during the previous two weeks and the amount of isoflavones they consumed.
While these results showed isoflavone intake was unrelated to the onset of hot flashes, the findings were not especially surprising. This is because the isoflavone intake of the women in this study was too low to expect any benefit. Median isoflavone intake was less than one-half milligram per day. To put that very small intake into perspective, there are 25 milligrams in one cup of soymilk. In the analysis published earlier this year, it was found that 50 milligrams of isoflavones, or about two serving of soyfoods, are needed for relief (1).
Another limitation of the more recent population study was that hot flash onset was based on the number of days women experienced hot flashes in a two week period. This type of classification would likely obscure any benefit of isoflavones. Although isoflavones reduce the frequency and severity of symptoms, in most instances they don’t completely eliminate hot flashes (nor does estrogen for that matter). If taking isoflavones prior to the onset of hot flashes caused a woman to develop 5 hot flashes per day instead of 10 per day, the benefit would not have been detected in this study. . Unfortunately, none of the limitations of this study were identified by the media.
1. Menopause 19:776-790, 2012
2. 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.