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Radioactive Iodine and the Effect of Consuming Seaweed

The potential effect of consuming seaweed to prevent radioactive iodine uptake has been a point of contention. In this article, the author, who is the chairman of the Foundation for Growth Science and an acknowledged expert in the field of endocrinology, metabolism and nuclear medicine, explains radioactive iodine 131 (I-131) uptake and the effect of consuming seaweed based on the results of studies conducted in the 1950s.

Keywords: iodine-131, health, nuclear power stations, children, radioactive substances, radiation, seaweed, thyroid gland, Minoru Irie

At a recent board meeting held by the Foundation for Growth Science the topic of radioactive iodine released during the Great East Japan Earthquake in March last year was discussed, giving me the opportunity to speak about the effects of consuming seaweed. Later on that day, Dr. Noboru Kobayashi contacted me through the editor of Child Research Net (CRN) and asked me to write on the topic of seaweed and its effects on the human body. Dr. Kobayashi, a member of the Board of Directors of the Foundation (of which I am currently Chairman), has provided us with valuable comments and suggestions over the years. Established in 1977, the Foundation has addressed and worked on various issues of human growth, including both physical and mental development.

The human body requires many different substances for its physical development: in terms of hormone secretion, growth hormones and thyroid hormones play an important role. There are two types of thyroid hormone, both containing iodine in their molecules. If you do not eat foods that contain iodine, the thyroid gland cannot produce thyroid hormones, which can stunt physical and mental growth. Along with the theme of growth hormone, our Foundation also deals with the problems of growth impairment due to lack of iodine.

Fortunately, people in Japan include seaweed in their daily diet, thus iodine deficiency has not been a public health problem: however, there are people around the world, especially in developing countries, who suffer from iodine deficiency due to insufficient iodine intake. According to the statistics published by international organizations in 1993, there were 10 million people suffering from a serious developmental disorder due to iodine deficiency; and when all potential patients (found mainly in developing countries) were included, the number reached 1.6 billion. Our Foundation is contributing internationally to solving this issue.

With these facts in mind, let us move on to the main subject. Since the nuclear accident occurred in Fukushima in March last year, the release of radioactive substances into the environment is worrying for many people in the nation, in particular for parents who have young children, especially when they are continually bombarded with non-stop news and speculations. Therefore, as a researcher studying the relationship between the thyroid gland and iodine consumption, I will explain the effect of eating seaweed based on results of our previous work.

It was around 1954, 58 years ago, when we started conducting clinical trials and studies on the thyroid gland. There were very few methods to objectively identify the function of the thyroid gland: besides clinical observation we could only use a measure of basal metabolic rate or an indirect measure of thyroid hormone levels in the blood; therefore, it was difficult to confirm the presence of a thyroid dysfunction. Eventually, a new method was invented to overcome this challenge: a radioactive iodine uptake (RAIU) test. The RAIU test measures radioactive iodine (I-131) uptake in the thyroid gland 24 hours after a small amount of I-131 is given to a patient. The primary function of the thyroid gland, one of the endocrine glands, is to take iodine in any form and convert it into thyroid hormones. This method takes advantage of such characteristics of the thyroid gland in order to evaluate thyroid function.

The iodine we consume in food is non-radioactive iodine (I-127), but we take in radioactive I-131 and deposit it in the thyroid gland just as we do with I-127. This is why the I-131 uptake test is used to examine thyroid function. The normal range for 24 hour I-131 thyroid uptake is said to be 10 to 40%: compared to this range, Graves' disease, one of the most typical types of thyroid hyperfunction, shows a higher rate, while hypothyroidism due to chronic inflammation or atrophic thyroiditis shows a lower rate.

Soon after we started using this method, we noticed that some patients showed a lower rate of I-131 uptake, no matter whether they appeared to have normal thyroid function or thyroid hyperfunction and Graves' disease. This has led us to hypothesizing that these patients had consumed more iodine-rich foods than others. When asked about their daily diet, most of them replied that they had been eating large quantities of kelp and wakame seaweed daily or just a few days before the test.

Six researchers including myself worked for the University of Tokyo Hospital at that time conducting a study to measure the rates of I-131 thyroid uptake in the patients who were given kelp for 7 - 14 days. Thin-sliced konbu seaweed was considered ideal for the test as it could be digested quickly. Thus, in the first stage of the study, around 7 to 16 grams of thin-sliced konbu seaweed (served as one or two bowls of soup) were given per day to the patients. The result was that the rates of I-131 uptake decreased to the range of 0.6 to 4.1% in two weeks after consuming kelp daily from the range of 15 to 20% measured at the beginning of the study. In addition, in 6 to 20 days after stopping the consumption of kelp, the rates returned to the initial values.

Next, we gave 10 grams of kelp to four patients whose thyroid function was normal: of them, one patient was given kelp over 4 days, and three patients for one day. The result shows that the rate of I-131 uptake in the patient consumed kelp over 4 days decreased to 3.1% from 18%, while the rates of those who consumed kelp for one day decreased to 2.0 - 4.2% from 14 - 39%. The rates of all the patients returned to the initial values in 3 to 6 days after the kelp consumption was terminated. We also measured the rate of I-131 uptake in four patients with Graves' disease who were given 10 grams of kelp over 4 to 6 days. The uptake rate in the four patients also decreased significantly.

Furthermore, we measured the uptake rates in five patients with diseases other than thyroid dysfunction who, for various reasons, have by choice been consuming seaweed over the years. Their rates were consistently lower than 10%. This research was published in a Japanese academic journal 54 years ago in 1958 (note 1). Our research provides insight into the importance of considering seaweed consumption such as kelp for the RAIU test as they may affect the test results. Since then, it has become a common practice in Japan to reduce the consumption of iodine-containing foods as much as possible for at least one week before the RAIU test.

The amount of iodine contained in seaweed varies depending on their type. Dried laver seaweed was also used for the above study; however, the rate of I-131 uptake in seven patients whose thyroid function was normal remained almost the same before and after consuming 1.2 - 3.2 grams of dried laver seaweed over 2 to 14 days. This is probably due to a low content of iodine in dried laver seaweed as well as a small amount of consumption per meal. In addition, it was discovered that not only seaweed but also many processed foodstuffs using seaweed contain iodine: our research group led by Dr. Yozen Fuse has recently published an article about such processed foods, reporting that soup stock, noodle sauce and instant food products that are popular among Japanese consumers contain a lot of iodine (note 2).

Now we move on to the topic on the release of radioactivity from Fukushima. The radioactive leaks due to the Great East Japan Earthquake occurring in March of this year are currently the focus of great attention and a matter of debate in Japan including Fukushima and surrounding areas, as well as around the world. If a large amount of radioactive iodine is absorbed by the thyroid gland, there could be a possibility of thyroid cancer, especially for young people, such as the case of the Chernobyl accident. However, as our study indicates, sufficient intake of iodine-containing foods has the potential to prevent radioactive iodine uptake in the thyroid gland. It may be reasonable to think that it takes time to realize such an effect since this occurs in the process of digestion and absorption of food; nevertheless, our study provides evidence that a decrease in radioactive iodine uptake was observed in the patients who consumed seaweed for even only one day.

On March 14, 2011, days after the devastating earthquake, the National Institute of Radiological Sciences of Japan announced the guidelines to reduce health risks when a large amount of radioactive iodine is taken into the body. It stated that a physician may prescribe potassium iodine, but over-the-counter medicine containing iodine such as an iodine liquid for cuts and sores should not be used as they are not intended for internal application; and that seaweed products are not effective as the amount of iodine in seaweed is not always constant and take a long time to be absorbed into the body. However, it should be noted that these guidelines were prepared for emergencies such as sudden exposure to massive radioactive substances. In the case of chronic exposure to small amounts of raioactive substances, I believe that daily intake of a certain amount of seaweed should rather be encouraged.

The Japanese people have the world's highest per capita seaweed consumption, with more than adequate or excessive intake of iodine. In contrast, the people who lived in the surrounding areas of Chernobyl at that time survived on a far small dietary intake of iodine, some had developed slight iodine deficiency. Iodine deficiency, as I have mentioned above, reduces the production of thyroid hormones, hence causes thyroid dysfunction. Therefore, many people who had been exposed to I-131 released during the Chernobyl accident later suffered from permanent damage to their health probably due to a higher iodine uptake in the thyroid gland. It is only recently that the low iodine intake has been revealed by a research group working with our Foundation. Most Japanese people eat a lot of iodine-containing foods daily and have lower radioactive iodine (I-131) uptake; therefore, as our data suggests, with a positive intake of seaweed such as kelp and wakame seaweed, we may further reduce I-131 accumulation in the thyroid gland.

The recommended dietary intake of iodine for adults is around 150µg per day, a little less for children, and around 200µg for pregnant women. The Japanese people are said to consume 0.5 - 3.0mg (500 - 3,000µg) of iodine daily or intermittently depending on the living environment and age. The amount of iodine contained in seaweed varies depending on their type and growing area, but is roughly estimated at 1.3 - 1.5mg/g (approximately 1,300 - 1,500µg/g) for kelp, slightly more for thin-sliced konbu seaweed, and around half of that of kelp for wakame seaweed. For example, a 10x10 centimeters square of kelp contains around 5mg of iodine, therefore, the daily consumption of 2 to 3 grams of kelp in some way can be sufficient to suppress the uptake of radioactive iodine.

In addition, a recent study using a new method for measuring iodine in urine developed by a research group in which we participated has reported that there was only a slight difference in seaweed intake between school-aged children living in the Hokkaido area and those in Tokyo (previously the Hokkaido residents were thought to have a massive intake of seaweed daily). This indicates that people nowadays have similar dietary habits no matter where they live in Japan. It should be noted, however, that in contrast to the above-mentioned studies, long term excessive intake of iodine can lead to hypothyroidism. If you have general malaise, dry skin, and become increasingly sensitive to the cold, consulting a doctor for thyroid function tests is initially recommended.

Lastly, I emphasize that adequate intake of seaweed can prevent chronic uptake of radioactive iodine in the thyroid gland, and thereby preventing radiation diseases in the broadest sense.

Note 1: Shiro Iino, Kunio Matsuda, Jun Ishii, Minoru Irie, and Kazuo Shizume, (1958), "Influence of the Ingestion of the Sea-weeds on the Thyroidal Uptake of I-131" FOLIA ENDOCRINOLOGICA JAPONICA, vol. 34(1), pp. 58-62.
Note 2: Fuse Yozen, Toshinori Ohashi, Yoshimasa Shishiba, and Minoru Irie, (2010), "Study on Iodine Content of Processed Food on the Market to Estimate Iodine intake in Japanese" JOURNAL OF JAPANESE SOCIETY OF CLINICAL NUTRITION, vol. 32(1), pp. 26-51.


Minoru Irie

Graduated from Faculty of Medicine, University of Tokyo in 1952, and entered its Third Department of Internal Medicine in 1953. Obtained a Doctor of Medicine degree from the University in 1958. Awarded a Fulbright Fellowship in 1959 to undertake studies in Boston for three years. Started to work as an assistant at the Third Department of Internal Medicine, University of Tokyo in 1962, lecturer (concurrent post) at the Second Department of Internal Medicine, Gunma University in 1967, part-time lecturer at the First Department of Internal Medicine, Toho University in 1969, professor of the said Department, Toho University in 1971, professor of the Department of Internal Medicine, Toho University Sakura Hospital, director of the said hospital, and director of Toho University Sakura School of Nursing in 1991, and professor emeritus of Toho University in 1994. Assumed the position of 16th-term and 17th-term member of the Science Council of Japan in 1994 and 1997 respectively, and the chairman, Board of Directors of the Foundation for Growth Science in 1999. Specialized areas: general internal medicine, endocrinology, metabolism, and nuclear medicine. He assumed the positions of president for five academic societies relating to the topic of this article as well as for sixteen other academic societies, member of the board of directors for the Japan Radioisotope Association, and honorary member for eight academic societies, the US Endocrine Society and the Growth Hormone Research Society.
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