Dietary Seaweed and Breast Cancer: A Randomized Trial

Brown seaweeds are popular foods in Japan, where the incidence of breast cancer is about 1/6 the rate of that reported for American women. Seaweed is an excellent source of fiber, contains iodine, carotenoids, and both mammalian lignans and isoflavones. Seaweed may help to prevent breast cancer by several different mechanisms involving these and other constituents. In a preliminary study of toxicity and efficacy using 5 g/day of seaweed, we found seaweed was well tolerated and was associated with some biological changes in the variables measured. In this study, we will examine the effects of escalating doses I of seaweed supplementation in a group of healthy postmenopausal women with and without breast cancer. In our cross-over design, women will be randomized to either seaweed or placebo first. We will then give doses of seaweed or placebo (3 g/day for 3 weeks, then 6 g/day for 3 weeks), followed by 1 week of 6 g/day seaweed/placebo plus soy (2 mg isoflavone per kg bodyweight). Our primary outcome variables are changes in circulating estrogen levels, thyroid hormones, and urinary excretion of phytoestrogens. Adherence to our program will be measured by monitoring urinary excretion of iodine.