2017 年 28 巻 1 号 p. 28-34
Although high dietary iodine intake due to consumption of edible kelp (kombu) is observed, there are few health problems associated with iodine in Japan. Since several kinds of isoflavones contained in soybeans act as goitrogens, the consumption of soybean may modify the adverse effects of excess iodine derived from kombu. In the present study, to clarify the effects of eating soybean meal on thyroid function and tissue distribution of iodine, we measured iodine content of serum and several organs in rats fed casein diet or soybean meal diet with or without kombu powder. Four types of experimental diet (casein diet, casein diet with 5% kombu, soybean meal diet, soybean meal diet with 5% kombu) were prepared using dried kombu powder (Saccharina diabolica; iodine content, 7.18 mg/g). Twenty-four male 4-week-old Wistar rats were divided into four groups and fed these four diets for four weeks each. No significant differences were observed in animal growth, thyroid weights, serum total thyroxine, triiodothyronine and thyroid stimulating hormone concentrations among the four groups. Irrespective of dietary protein (casein or soybean), the iodine content in thyroid gland of all four groups was significantly increased to a similar level by the administration of kombu. Iodine concentrations in serum, liver and kidney were also increased by the kombu, but the increases of iodine in these tissues were significantly lower in rats fed soybean than in those fed casein indicating that soybean meal reduced the content of iodine in serum, liver and kidney. These results suggest that the consumption of soybean products could prevent the manifestation of adverse effects due to an excess iodine intake from kombu.