Teas J, Hurley TG, Hebert JR, Franke AA, Sepkovic DW, Kurzer MS. Dietary seaweed modifies estrogen and phytoestrogen metabolism in healthy postmenopausal women. J Nutr. 2009;139: 939-944.

The overall incidence of breast cancer in resident Japanese-American women (103 cases/100,000 women) is lower than that of the national average in the United States (118 cases/100,000 women). The incidence rate of breast cancer in Japanese women who migrate to the United States increases dramatically from 20 cases/100,000 women to 35 cases/100,000 women after 10 years. These data suggest that lifestyle differences, particularly dietary differences, influence breast cancer susceptibility. The most commonly identified dietary difference is soy (Glycine max) food consumption, which is higher in Japanese women than in American women. However, recent meta-analyses suggest that soy food consumption may lower the risk of breast cancer by only about 20%. Several studies have reported that high serum estradiol concentrations increase the risk of breast cancer and the risk of disease recurrence. Furthermore, studies have shown 2- to 3-fold higher fecal excretion rates of estradiol in postmenopausal Japanese breast cancer survivors who consume a traditional diet than in those who do not, which strongly suggests that dietary effects on colonic bacteria may be a key factor to understanding international differences in breast cancer incidence. Seaweeds are a traditional component of the Japanese diet. Brown seaweed contains a unique, insoluble, indigestible fiber (alginate) that has been shown to directly affect intestinal microflora. The objective of the present study was to evaluate the ability of American brown seaweed (Alaria esculenta), which is closely related to the popular Japanese seaweed wakame (Undaria pinnatifida), to change serum hormone levels and the urinary excretion of estrogen metabolites and phytoestrogens.

Thirty-three healthy, non-smoking, postmenopausal European-American women with no history of cancer (other than breast cancer) residing in Massachusetts were enrolled in this double-blind, randomized, placebo-controlled, crossover study, which was conducted at the University of Massachusetts Medical School. The subjects were randomly assigned to treatment with 5 g/d seaweed powder (seaweed period; 10 gelatin capsules [Beehive Botanicals®; Hayward, Wisconsin]) or 5 g/d maltodextrin (placebo period; 10 capsules with an appearance identical to the seaweed capsules) for 7 weeks. The seaweed was collected by the Maine Seaweed Company from the Sally Islands close to Maine. During week 7, both groups also consumed a soy protein isolate (High Protein Nutritious Food Ingredient Powder; Solae, LLC; St. Louis, Missouri) daily (2 mg isoflavone/kg body weight): seaweed + soy (seasoy period) and placebo + soy (soy period). After a 3-week washout period, the subjects were crossed over to the opposite treatment. Fasting blood samples were collected at baseline and at the end of each treatment period for the measurement of estrone, estradiol, and sex hormone-binding globulin. Urine samples were collected by the women over the 48 hours before each visit for the measurement of phytoestrogens [daidzein, glycitein, genistein, O-desmethylangolensin (O-DMA), enterolactone, enterodiol, matairesinol, secoisolariciresinol, and equol] and estrogen metabolites [2-hydroxyestrogen (2-OHE), 16a-hydroxyestrone (16aOHE₁), and the ratio of these last 2 compounds]. The level of compliance was determined on the basis of urinary iodine concentrations. A P value of <0.05 indicated significance.

Data from a total of 15 women were included in the analysis. The level of compliance was determined to be "very high." Only slight variations in serum hormone levels were observed in response to the supplements. A small but significant decrease in estrone levels was observed during the seasoy period, and a significant decrease in sex hormone-binding globulin was observed during the soy period. An inverse correlation was observed between seaweed dose and serum estradiol levels (P = 0.03). Urinary daidzein, glycitein, genistein, and O-DMA increased significantly (P = 0.0001), and matairesinol and enterolactone decreased significantly (P < 0.05) during the soy period. Urinary 2-OHE and the ratio of 2-OHE to 16aOHE₁ increased significantly (P = 0.0001 and P = 0.01, respectively) during the seasoy period. The 5 subjects who were equol producers experienced a 315% increase in the ratio of 2-OHE to 16aOHE₁ (P = 0.0001). None of the other variables measured changed significantly.

The authors conclude that seaweed consumption "favorably" changed estrogen and phytoestrogen metabolism in healthy postmenopausal women, and "these changes likely include modulation of colonic bacteria." Because this was a pilot study, however, changes in colonic microflora were not directly measured. The authors suggest that future studies measure changes in colonic microflora directly.