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 labeled as kombu, from around the world were examined for their iodine content and were found to average 1,542 μg/g when dried [17].

As the Japanese transitioned from a traditional to a Westernized diet, beginning around the 1950's [20], consumption of certain seaweed species declined while others increased. A decrease in kombu consumption (844 to 685 g/year per household) and an increase in wakame consumption (727 to 1234 g/year per household) can be seen between the years of 1963 and 1973 [21]. Consumption of kombu per Japanese household dropped further to 450 g in 2006 (elders ate up to four times more than those under the age of 29) [19]. Since daily seaweed consumption per person in Japan has remained relatively consistent over the last 40 years (4.3 g/day in 1955 and 5.3 g/day in 1995) [22], it is believed that consumption of wakame and nori have made up for the decline in kombu consumption [23, 24]. Both nori and wakame have relatively low iodine contents compared to kombu.

Seaweed consumption frequency differs from person to person in Japan, resulting in a constantly fluctuating iodine intake. Seaweed is served in approximately 21% of Japanese meals [25] with 20-38% of the Japanese male and female population aged 40-79 years consuming seaweed more than five times per week, 29-35% three to four times per week, 25-35% one to two times per week, 6-13% one to two times per month, and 1-2% rarely consuming seaweed [26]. A 2010 food frequency questionnaire on the Japanese Kombu Association website indicates that kelp (assuming kombu) is consumed at a rate of: 27.5% once per week, 25.5% once per month, 18% three or four times per week, and 15.9% once every few months, with only 6.1% of survey respondents stating they consume kelp nearly every day [27].

Seaweed is often cooked to flavor dishes or soup stocks before consumption. When kombu is boiled in water for 15 minutes it can lose up to 99% of its iodine content, while iodine in sargassum, a similar brown seaweed, loses around 40% [28, 29]. Processed kelp is often boiled in dye for half an hour ("ao-kombu" or "kizami-kombu") before hanging to dry [21], a process which can reduce seaweed iodine content before it is consumed. When kelp is used to flavor soup stocks the seaweed is often removed after boiling, resulting in soup stock high in iodine. Twenty samples of supermarket soups with kelp or kelp broth were analyzed by Nishiyama et al. to determine iodine content, revealing a minimum concentration of 660 μg/L (0.66 mg/L) and a maximum concentration of 31,000 μg/L (31 mg/L) [16]. Serving size for soup is typically around 0.25 L, resulting in 165 to 7,750 μg (0.165 to 7.75 mg) of iodine per serving.

Due to variation of iodine content from one seaweed species to the next, along with confusion stemming from wet and dry weight terminology, many inaccurate assumptions have been made regarding the amount of iodine the Japanese actually consume from seaweed. Not all studies, dietary records or surveys specify whether daily or yearly consumption of seaweeds is recorded using wet weight, dry weight or a combination of the two. In some reports seaweed consumption has been estimated at 4-7 g/day dried weight [17, 22, 30, 31], while other reports claim consumption of 12 g/day using both wet and dry weight [32]. Certain seaweeds have a swelling capacity of nearly ten times their dry volume with moisture content typically over 70% when wet and around 7-20% when dried [33, 34]. The difference between wet and dry weight, along with the type of seaweeds being consumed, can result in extreme overestimation (more likely) or underestimation (less likely) of Japanese iodine intake.

Interpreting information to determine Japanese seaweed consumption and resulting iodine intake is a difficult task, and with ever changing diets, a close estimate is all that can be made. Nori and wakame are the most commonly consumed seaweeds in Japan, with nori accounting for 45% and wakame accounting for 30% (75% together) of total seaweed consumption, as stated by the Food and Agriculture Organization of the United Nations [35]. Based on previous estimates and records, dried seaweed consumption of 4-7 g/day [17, 22, 30, 31] results in iodine intakes between 79 and 139 μg/day from nori and wakame when calculated using dried iodine contents of 16 and 42 μg/g respectively [18]. The remainder of iodine intake is derived mainly from kombu consumption, with smaller amounts coming from other seaweeds that have nominal iodine content.

Kombu has the highest iodine content of all seaweeds in the Japanese diet. In 2006 consumption of kombu/household/year was 450 g [19], and with an average of 2.55 members per household in Japan in 2005 [36], 0.48 g kombu/person/day was consumed. When calculated, 0.48 g of kombu with an iodine content of 2,353 μg/g [18] equates to 1,129 μg/day of iodine. Assuming negligible iodine intake from the other seaweeds consumed, daily iodine intake from nori, wakame, and kelp can be estimated at 1,208 to 1,268 μg/day (1.2 to 1.3 mg/day). It is reasonable to assume that iodine intake per day based on seaweed consumption frequency and iodine content averages around 1,000-2,000 μg/day (1-2 mg/day).

High iodine intake from seaweed consumption can cause unexpected health problems in a subset of individuals with pre-existing thyroid disorders. Although it is reported that excessive iodine does not cause thyroid antibody positivity, high intake can cause or worsen symptoms for people with previous thyroid autoimmunity or other underlying thyroid issues [47]. Transient hypothyroidism and iodine-induced goiter is common in Japan and can be reversed in most cases by restricting seaweed intake [16, 29, 48–52]. In Asian cultures, seaweed is commonly cooked with foods containing goitrogens such as broccoli, cabbage, bok choi and soy [18]. The phytochemicals in these foods can competitively inhibit iodine uptake by the thyroid gland (i.e., isothiocyanates from cruciferous vegetables) [53–55], or inhibit incorporation of iodine into thyroid hormone (i.e., soy isoflavones) [56, 57].

Certain species of seaweed can concentrate bromine, a halide similar to iodine with no known physiological function, at very high levels [58, 59]. If seaweeds with elevated levels of bromine and low levels of iodine are consumed when the body is in an iodine deficient state, inhibition of thyroid hormone synthesis--due to bromine's attachment to tyrosine residues on thyroglobulin in place of iodine--is plausible [60].

We estimate that the average Japanese iodine intake, largely from seaweed consumption--based on dietary records, food surveys, urine iodine analysis and seaweed iodine content--is 1,000-3,000 μg/day (1-3 mg/day). This estimate compares to a recent report claiming that the average iodine intake of the Japanese from kelp is around 1,200 μg/day (1.2 mg/day) [19]. Iodine intake can vary from day-to-day depending on diet, and it is unlikely for a single persons iodine intake to remain constant for an extended period of time. With the multitude of edible seaweeds (each with different iodine content) consumed in the Japanese diet, it is not appropriate to use a single type of seaweed to determine iodine intake, though many estimates do. Although seaweed provides a majority of the Japanese iodine intake, other food sources (containing far less iodine)--such as fish and shellfish--can increase the total amount of iodine consumed daily.

Japanese iodine intake from seaweed is linked to health benefits not seen in cultures with dissimilar diets. Knowing how much iodine the Japanese consume daily is beneficial for people who wish to consume equivalent amounts of iodine or seaweed supplements while avoiding excessive amounts that may adversely affect health.

TZ received his Bachelor's degree in Biology from Oregon State University in 2009. He is a Research Associate at ZRT Laboratory in Beaverton, Oregon, where he recently developed a test to measure iodine and creatinine levels in dried urine. His current research focuses on iodine deficiency, Japanese iodine intake, halide competition, thyroid disorders and iodine kinetics in the human body.

DZ received his doctorate in Biochemistry from the University of Tennessee in 1974. He is Laboratory Director and President of ZRT Laboratory, which he founded in 1998. Dr. Zava has developed innovative, simple and cost-effective methods to monitor hormone and other analytes associated with health and disease. His current research focus includes endocrinology, breast cancer, and--most recently--the importance of iodine to optimum health. He is co-author of a landmark book, What Your Doctor May Not Tell You About Breast Cancer: How Hormone Balance Can Help Save Your Life.