Assessment of Japanese iodine intake based on seaweed consumption in Japan: A literature-based analysis

Japanese iodine intake exceeds that of most other countries, primarily due to substantial seaweed consumption. Iodine is an essential element required for thyroid hormone synthesis, believed to impart some of its antioxidant and antiproliferative activity in the prevention of cardiovascular disease and cancer [1-8]. Seaweeds have the unique ability to concentrate iodine from the ocean, with certain types of brown seaweed accumulating over 30,000 times the iodine concentration of seawater [9]. The amount of iodine the Japanese consume daily from seaweeds has previously been estimated as high as 13.5 to 45 mg/day by sources that use ambiguous data to approximate intake [10,11], an amount 4.5 to 15 times greater than the safe upper limit of 3 mg/day set by the Ministry of Health, Labor and Welfare in Japan [12]. While high iodine intake from seaweed consumption is believed to have numerous health benefits, it has been reported to negatively affect individuals with underlying thyroid disorders [13-16]. To prevent excessive consumption it is imperative for people seeking health benefits from a high iodine diet to be knowledgeable of the amount of iodine the Japanese consume daily. In this paper we use a combination of dietary records, food surveys, urine iodine analysis, and seaweed iodine content to provide a reliable estimate of Japanese iodine intake, primarily from seaweeds.In Japan, over 20 species of red, green, and brown algae (seaweed) are included in meals [17]. Iodine content varies depending on species, harvest location and preparation, and is typically highest in fresh cut blades and lowest in sun bleached blades [18]. The three most popular seaweed products in Japan are nori (Porphyra), wakame (Undaria) and kombu (Laminaria). Dried iodine contents range from 16 μg/g in nori to over 8,000 μg/g in kelp flakes; Japanese kombu and wakame contain an estimated 2353 μg/g and 42 μg/g respectively [18,19]. Ten different species of Laminaria, a type of kelp commonly l