Jane Teas

Seaweed and soy foods are consumed daily in Japan, where breast cancer rates for postmenopausal women are significantly lower than in the West. Likely mechanisms include differences in diet, especially soy consumption, and estrogen metabolism. Fifteen healthy postmenopausal women participated in this double-blind trial of seaweed supplementation with soy challenge. Participants were randomized to 7 wk of either 5 g/d seaweed (Alaria) or placebo (maltodextrin). During wk 7, participants also consumed a daily soy protein isolate (2 mg isoflavones/kg body weight). After a 3-wk washout period, participants were crossed over to the alternate supplement schedule. There was an inverse correlation between seaweed dose (mg/kg body weight) and serum estradiol (E2) (seaweed-placebo = y = 0.28 × dose − 42.8; r = 0.70; P = 0.003), which was linear across the range of weights. Soy supplementation increased urinary daidzein, glycitein, genistein, and O-desmethylangolensin (P = 0.0001) and decreased matairesinol and enterolactone (P < 0.05). Soy and seaweed plus soy (SeaSoy) increased urinary excretion of 2-hydroxyestrogen (2-OHE) (P = 0.0001) and the ratio of 2-OHE:16α-hydroxyestrone (16αOHE₁) (P = 0.01). For the 5 equol excretors, soy increased urinary equol excretion (P = 0.0001); the combination of SeaSoy further increased equol excretion by 58% (P = 0.0001). Equol producers also had a 315% increase in 2:16 ratio (P = 0.001) with SeaSoy. Seaweed favorably alters estrogen and phytoestrogen metabolism and these changes likely include modulation of colonic bacteria.

Incidence of the metabolic syndrome is increasing worldwide, with notable exceptions of some Asian countries where seaweeds are commonly consumed. 13 men (mean age 47.4+/-9.9 yr) and 14 women (average age 45.6+/-12.2 yr) with at least one symptom of the metabolic syndrome were recruited in Quito Ecuador to a randomized double-blinded placebo-controlled trial. Subjects were assigned to either Group 1 (1 m placebo, followed by 1 m 4 g/d seaweed [Undaria pinnatifida]) or Group 2 (1 m of 4 g/d seaweed, followed by 1 m of 6 g/d of seaweed). Blood pressure, weight, waist circumference, inflammation biomarkers, and lipids were measured monthly. Repeated measures analysis of variance with Tukey's multiple comparison tests were used for statistical analysis. In Group 2, systolic blood pressure decreased 10.5 mmHg after a month of 6 g/d seaweed (95% CI: 4.1, 16.8 mmHg; p<0.05), primarily in subjects with high-normal baseline blood pressure. Waist circumference changed only for women participants, with a 2.4 cm decrease in Group 1 after treatment with placebo (95% CI: 1.0, 3.7 cm; p<0.01). In Group 2, women had a mean decrease of 2.1 cm after 4 g/d (95% CI: 0.4, 3.7 cm; p<0.05) and a further 1.8 cm decrease after 1 m 6 g/d seaweed (95 % CI: 0.1, 3.4, p<0.05). No other changes were observed. Consumption of 4 to 6 g/d seaweed, typical for most people in Japan, may be associated with low metabolic syndrome prevalence.

Dietary seaweeds, common in Asia and in Asian restaurants, have become established as part of popular international cuisine. To understand the possibility for iodine-induced thyroid dysfunction better, we collected samples of the most common dietary seaweeds available from commercial sources in the United States, as well as harvester-provided samples from Canada, Tasmania, and Namibia. Altogether, 12 different species of seaweeds were analyzed for iodine content, and found to range from 16 µg/g (±2) in nori (Porphyra tenera) to over 8165 ± 373 µg/g in one sample of processed kelp granules (a salt substitute) made from Laminaria digitata. We explored variation in preharvest conditions in a small study of two Namibian kelps (Laminaria pallida and Ecklonia maxima), and found that iodine content was lowest in sun-bleached blades (514 ± 42 µg/g), and highest amount in freshly cut juvenile blades (6571 ± 715 µg/g). Iodine is water-soluble in cooking and may vaporize in humid storage conditions, making average iodine content of prepared foods difficult to estimate. It is possible some Asian seaweed dishes may exceed the tolerable upper iodine intake level of 1100 µg/d.