Menopausal Hot Flashes Not Prevented by Eating More Soy

Jenni Laidman

November 05, 2012

Eating soy each day will probably not prevent hot flashes and night sweats in women heading into menopause, according to results from a study tracking the dietary habits of 1651 women.

The research, published online October 31 and in print March 2013 in Menopause, followed up women in premenopause and early perimenopause who had not experienced hot flashes or night sweats when the study began. Over a 10-year period, the researchers tracked the dietary habits of this subset of women, looking for a relationship between the consumption of phytoestrogens and fiber to incident vasomotor (VMS) symptoms of menopause.

Ellen B. Gold, PhD, professor and chair of the Department of Public Health at the University of California, Davis, and colleagues measured consumption of isoflavones, coumestrol, lignans, and fiber using a food frequency questionnaire at baseline and again in years 5 and 9. The researchers compared those dietary records with the women's recollection of their menopausal symptoms during the 2 weeks before their study visits. The research data are part of the Study of Women's Health Across the Nation (SWAN), which includes 3302 premenopausal and early perimenopausal women.

The study differed from earlier research in several ways: It looked at whether soy intake prevented the onset of hot flashes in women who had not had the symptoms previously, it included a range of racial and ethnic groups, it included both premenopausal and perimenopausal women, and it focused on dietary soy rather than soy supplements. It was also much larger than previous trials that have evaluated the relationship of soy and menopause.

Although the investigators state that the true effect of dietary phytoestrogens or fiber on VMS would require a randomized, placebo-controlled, double-blinded trial, they conclude, "[O]ur results suggest that a clinically significant or large effect is improbable."

Although most relationships between the consumption of various phytoestrogens failed to reach statistical significance, there were a few exceptions that the authors say probably arose by chance. For instance, the lowest quartile of lagged isoflavones intake in white and black women had a significantly reduced odds ratio (OR) for 1 to 5 days of VMS (OR, 0.76; 95% confidence interval [CI], 0.61 - 0.95; P = .016.) In addition, the P value for quartile trend was significant (P = .018) for 1 to 5 days of VMS. Also, although the VMS frequency in Asian women showed no relationship to coumestrol intake, the lowest quartile of lagged coumestrol intake showed an increased adjusted OR compared with the highest quartile when looking at 6 days or more of VMS in the 2 weeks before (OR, 2.59; 95% CI, 1.19 - 5.63), with a trend P value of .052.

Those anomalies, however, did not affect the authors' conclusions.

"Although some adjusted ORs were statistically significant, patterns were largely not monotonic for the amount of intake of each phytoestrogen or fiber in relation to the frequency of VMS. Thus, the few significant individual point estimates may well have been because of chance, given the many statistical comparisons that were made," the authors write.

No "Magic Bullet"

"[W]e were hopeful that certain dietary intakes would provide good alternatives to hormone therapy," Dr. Gold said in a university press release. "Unfortunately, based on our study, soy-related foods did not turn out to be the 'magic bullet.' "

In 2011, the North American Menopause Society issued a position paper calling treatment with isoflavones "reasonable" in postmenopausal women, based on the positive results in 11 of 14 small studies on soy supplementation involving between 39 and 177 women each. Although those studies found a reduction in hot flashes and night sweats among postmenopausal women, studies that included women in perimenopause did not show the effect.

The society's paper also called for larger studies, including those that look at dietary fiber.

Women in the new study ranged in age from 42 to 52 years, had a menstrual period within the last 3 months at baseline, had an intact uterus and at least 1 ovary, and were not using hormone-replacement therapy.

The study of this population also failed to find support of the idea that higher soy consumption in women of Asian ancestry led to reduced VMS symptoms.

"The 'Eastern' dietary pattern, which is high in phytoestrogens, has been one of the proposed explanations for the ethnic differences in hot flash occurrence," study coauthor Gail Greendale, MD, a geriatric medicine specialist with the University of California, Los Angeles, Health System, said in the news release. "Our findings do not support the theory that higher phytonutrient intakes are associated with lower hot flash rates."

The study of Women's Health Across the Nation, including the Study of Women's Health Across the Nation Phytoestrogen Ancillary Study, received grant support from the National Institutes of Health, Department of Health and Human Services, through the National Institute on Aging, National Institute of Nursing Research, and National Institutes of Health Office of Research on Women's Health. The authors have disclosed no relevant financial relationships.

Menopause. Published online October 29, 2012. Abstract