By Fredi Kronenberg, PhD, and Adriane Fugh-Berman, MD
Many food plants contain phytoestrogens, compounds that have estrogen-like activity, either directly or when metabolized. Although most mammalian estrogens are steroids, the major classes of phytoestrogens (isoflavones, lignans, and coumestans) are phenolic compounds. Isoflavone precursors are found in soybeans, other beans, clover, and alfalfa.1 Lignan precursors are found in whole grains, seeds, fruits, and vegetables, especially flaxseed (linseed), rye, millet, and legumes.2 Lignan precursors are metabolized by gut bacteria to enterolactone and enterodiol, also termed mammalian lignans. Isoflavone precursors also are affected by gut bacteria, which remove a glycoside unit to create the active, unconjugated isoflavones, genistein, daidzein, and equol.
In the human body, these substances have some of the biological activities of human estrogens. Soybeans, various soy food products, and the constituent isoflavones genistein and daidzein have received the most study in recent years. High dietary soy intake in Japan and other Asian countries has been suggested to be the major reason for the apparently lower prevalence of menopausal symptoms.3,4
Hot Flashes
Clinical studies continue to explore the relationship between phytoestrogen intake, menopausal symptoms, and long-term health concerns of menopausal women. Hot flashes, however, have received the most attention.
Eleven randomized controlled trials have examined soy or isoflavone supplementation for hot flashes. Four found a positive effect, five found a negative effect, and two found mixed effects (see table). Only three of eight studies with treatment phases that lasted longer than six weeks showed significant improvement in hot flashes at the end of the study. The longest study to date (24 weeks) showed no benefit for hot flashes (or other symptoms) at 24 weeks.
Two other trials did not separately assess hot flashes and are not included in the table. An additional rand-omized, placebo-controlled study tested soy supplements (containing 118 mg isoflavones daily) in 94 menopausal women, 80% of whom were experiencing menopausal symptoms, an atypical assortment that included unloved feelings (we’re not sure whether this means feeling unloved or reflects unpopular statements made at family meals), lightheadedness, facial hair, and dry skin. Symptoms were reported to be mostly mild, and the number of women with hot flashes is not reported. Seventy-five women completed the trial; there were no significant differences between groups in any measure.
Comparisons among studies are difficult since each study used a different product, including a high-soy diet, a soy beverage, different types of soy protein, and tablets or capsules containing soy extracts, isolated isoflavones, or a mixture. Amounts of isoflavones and protein also differed. Other differences make comparability among studies difficult. Different menopause symptom indices (particularly, different hot flash scoring systems) were used; studies included different age ranges and menopausal status of subjects (some studies included only postmenopausal women, some only perimenopausal women, and some included both). One would expect more frequent, and more severe, hot flashes in the younger women, and an increase in other symptoms such as vaginal dryness with increasing age.
Vaginal Effects
None of the studies appears to have asked women specifically about comfort during sex; one study found no effect of soy protein containing 80.4 mg isoflavones/d on vaginal dryness.5 Several studies assessed vaginal maturation index; only one6 of four studies6-9 found a positive effect. Another randomized, placebo-controlled, four-week trial of soy (comprising one-third of daily calories, about 165 mg isoflavones/d) in 97 menopausal women (91 completed) found no significant effect on vaginal epithelium.7
Safety
Soy foods have been a staple in Asian cuisine for thousands of years and are presumed safe. Supplementing the diet with beans or bean products should not cause any problems. No such presumption of safety can be made for the isolated, often high-dose, isoflavones currently sold over the counter.
Conclusion
In most studies, only modest effects (primarily on hot flash severity) were seen, and most benefits disappeared after six weeks (for hot flashes, even three months is barely adequate to adequately assess efficacy). As in most studies of pharmaceutical estrogen therapy for menopausal symptoms, there was a large placebo effect: Symptoms decreased in all groups—often by as much as 50-60%. Longer studies must be done to determine whether this placebo effect declines over time. Additional studies are needed to determine whether there are differences among whole foods, soy protein, and isoflav-one extracts in the effect on symptomatology and whether there is a dose-response relationship. It would be valuable to examine the wide range of legumes in addition to soybeans. For example, yellow split peas, black beans, lima beans, kidney beans, and red lentils all contain more genistein than soybeans (although they contain less daidzein).10 All other beans contain far less fat than soybeans.11
Both epidemiological studies and randomized controlled trials suggest that diet plays a role in modulat- ing endocrine activity. The role of diet and biological differences among populations that could explain observed differences in symptomatology remains to be determined.
The conclusions that can be drawn from the data are that soy products have at best mild-to-moderate effects on hot flashes. The effect is not dramatic. Soy foods, beans, and flaxseed are harmless, and may help individual women with menopausal symptoms. The safety of isoflavone concentrates is unknown.
References
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Kronenberg F, Fugh-Berman A. Soy and hot flashes. Altern Ther Women's Health 2002;4(9):68-71
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