Seaweed, soy, and estrogen metabolism in healthy postmenopausal American women.

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Introduction: Seaweed and soy foods are common in Japan and Korea where the incidence and mortality of breast cancer are significantly lower than in the US. Most attention has focused on soy foods and their phytoestrogen content. Seaweeds are known to have an antibiotic effect in vitro and in vivo studies support the idea that dietary seaweed modifies gastrointestinal bacteria populations. We investigated the possibility that dietary seaweed could act as a probiotic when consumed with soy, and enhance the gastrointestinal metabolism of phytoestrogens, especially the increasing the production of equol. Equol production is associated with decreased breast cancer risk. Methods: In a double-blinded placebo-controlled clinical trial, 28 healthy postmenopausal women (average age = 58 years) were recruited to our 17 week study. Ten of the women had been treated for early breast cancer but were disease free at the time of the study. None of the women took antibiotics during the study, and alcoholic beverage consumption was limited to one or fewer drinks per week. The women were randomized to either seaweed or placebo for six weeks, followed by a week when soy supplementation (2 mg isoflavones/kg body weight) was added. A 3-week washout period separated the two arms of the study, after which women were crossed over to the alternate intervention arm. Blood samples for estradiol, estrone, and sex hormone binding globulin were obtained at baseline and each of the 6 clinic endpoints. Women collected 48-hour urine specimens at each of the 7 time points for phytoestrogen determination. Results: Blood levels of estrone and estradiol did not change during the study, but SHBG levels significantly decreased during the seaweed plus soy supplementation period. No urinary phytoestrogen excretion was reported during the placebo or seaweed interventions, but urinary phytoestrogens were detected during both the soy and seaweed plus soy intervention periods. Equol production was only seen in women who had never been treated for breast cancer, and not in any of the breast cancer survivors. For 6 of the 7 disease-free women who produced equol when taking soy, the combination of seaweed plus soy further increased equol levels. Conclusions: The presence of seaweed in the Asian diet may act as a probiotic, enhancing intestinal conversion of phytoestrogens, particularly the production of equol, and could account for some of the breast cancer protective effects of dietary soy.