Soy-supplemented diets have been shown to provide relief of menopausal symptoms, and population-based studies have linked soy-rich diets to a low incidence of cardiovascular disease and estrogen-related cancers. Soy contains several phytoestrogens, some of which become biologically active only after metabolism by organisms in the gut. Studies of dietary supplementation with individual phytoestrogens have not elucidated which one of the main subgroups (lignans, isoflavones and coumestrans) provides the strongest clinical effect in menopausal women. Albertazzi and colleagues examined the relationship between blood and urine levels of the principal phytoestrogens and the effects of soy supplementation on the frequency of hot flushes in post-menopausal women.

The study included 104 women who reported having at least eight moderate to severe hot flushes daily. The women were randomly assigned to receive dietary soy supplementation (60 g of soy powder) or placebo for 12 weeks. The women kept a record of their daily symptoms and were assessed by clinicians every four weeks. Blood and urine levels of four phyoestrogens—enterolactone, genistin, daidzin and equol—were obtained. Fifty-one women received soy supplementation and 53 women received placebo. Forty women in the soy group and 39 in the placebo group completed the study.

After 12 weeks of soy supplementation, serum levels of equol, genistin and daidzin were significantly higher than baseline levels. The mean serum equol level rose by 1,008 percent, the mean genistin level by 756 percent and the mean daidzin level by 593 percent. The mean serum enterolactone level increased by 57 percent. Serum levels of each of the four phytoestrogens correlated well with urine levels. Although the mean serum values for each compound increased significantly, serum levels were noted to vary widely in response to soy supplementation. No changes occurred in the placebo group.

The authors believe that individual variation in bioavailability may partly explain the lack of correlation between serum levels of the four phytoestrogens and relief of menopausal vasomotor symptoms. They also speculate that soy may contain as yet unidentified compounds that account for biologic activity. A previous study by the authors revealed that the frequency of hot flushes decreased by approximately 45 percent in women who added soy to their diets. No such benefit has been observed with preparations of individual phytoestrogens.

editor's note: Women seeking “natural” therapies for menopausal symptoms can find a plethora of information and pseudoscience on which to base their decisions. Particularly with perimenopausal women, questions about the use of alternative therapies should be extended to include dietary changes. The patient might not recognize such changes as clinically important. Patients should be advised that comprehensive sources of soy appear to be more effective than supplements of individual phyotoestrogens. The message appears to be “eat tofu,” not “take phytoestrogen tablets.”—a.d.w.