Phytoestrogen and fiber intakes in relation to incident vasomotor symptoms: results from the Study of Women's Health Across the Nation.

OBJECTIVE: Although reduction of vasomotor symptoms (VMS; hot flashes and night sweats) has been reported in postmenopausal women who used isoflavones, a clear dose response has not been shown, has largely not been reported for perimenopausal women, and has largely only been reported for reducing prevalent VMS, not preventing newly developing VMS. We analyzed longitudinal data from the Study of Women's Health Across the Nation for the relation of dietary phytoestrogen and fiber intake to incident VMS in this multiracial/ethnic cohort.

METHODS: The Study of Women's Health Across the Nation included 3,302 premenopausal and early perimenopausal women, 1,651 of whom reported no VMS at baseline and were followed with annual visits for 10 years. Dietary intakes of isoflavones, coumestrol, lignans, and fiber were assessed by a food frequency questionnaire at baseline and in annual visits 5 and 9 and interpolated for intervening years. The number of days experiencing VMS in the past 2 weeks was self-reported annually. Using multinomial logistic regression with generalized estimating equations, we modeled incident VMS in relation to isoflavones, lignans, fiber, coumestrol, or total phytoestrogen intake and covariates.

RESULTS: No consistent monotonic relations were observed between any dietary phytoestrogen or fiber and incident VMS, although adjusted odds ratios for some individual quartiles were statistically significant.

CONCLUSIONS: For certainty of any effect of dietary phytoestrogens or fiber on the prevention of incident VMS, a randomized, placebo-controlled, double-masked trial with sufficient numbers of women in different racial/ethnic, menopausal status, and metabolic groups over years of follow-up is required, but our results suggest that a clinically significant or large effect is improbable.