Seawater contains the most abundant and stable iodine. The iodine concentration in seawater is 50 micrograms/L - 60 micrograms/L, which is known as the "iodine library". Due to evaporation, part of the iodine in the seawater enters the air, so about 400,000 tons of iodine enter the atmosphere every year. This iodine then falls to the land in the form of rain (snow) water. This is the cycle of iodine in nature. The iodine content of volcanic rock soil is as high as 9.0 mg/kg. The iodine content of soil in areas with a large slope angle, inland areas, and soil formed after ice water erosion is low. The soil of coastal areas and islands has high iodine content. Organisms obtain iodine from nature, and the concentration of iodine after concentration is higher than that in the natural environment. Therefore, after concentration, plants have higher iodine content than soil; animals have higher iodine content than plants; and seafood has higher iodine content than land products. Marine organisms are high in iodine. The foods with the highest iodine content are seafood, such as kelp, seaweed, fresh hairtail, dried clams, dried clams, scallops, mussels, sea cucumbers, jellyfish, lobster, etc.; kelp has the highest iodine content, reaching more than 240 mg/kg in dried kelp ; followed by sea shellfish and fresh fish (about 800 micrograms/kg). However, the iodine content in salt is very small. The more refined the salt, the less iodine it contains. The iodine content in sea salt is about 20 micrograms/kg. If each person consumes 10 grams of salt per day, they can only obtain 2 micrograms of iodine. , far from meeting the needs for preventing iodine deficiency diseases. On land, eggs and milk have the highest iodine content (40 μg/kg to 90 μg/kg), followed by meat. Freshwater fish has lower iodine content than meat. Plants have the lowest iodine content, especially Fruits and vegetables. 80%-90% of the iodine in the human body comes from food, 10%-20% is obtained through drinking water, and 5% of the iodine comes from the air. Therefore, iodine in food is the main source of iodine in the human body. Iodide in food can only be absorbed after being reduced to iodine ions, and iodine combined with amino acids can be absorbed directly. Calcium, fluoride, and magnesium in the gastrointestinal tract hinder the absorption of iodine, especially under conditions of iodine deficiency. When the human body is deficient in protein and calories, it will hinder gastrointestinal iodine absorption. The respiratory tract and skin can also absorb small amounts of iodine. The physiological role of iodine in the human body is achieved through the synthesis of thyroid hormone. Thyroid hormone is one of the most important hormones in the body. Its main physiological roles are: (1) Maintaining the body's energy metabolism (decomposing substances to provide the energy required for life activities) energy) and thermogenesis (maintenance of body temperature). The reduction in thyroid hormone synthesis caused by iodine deficiency can lead to impairment of basic life activities and reduced physical fitness. This effect is lifelong. (2) Promote physical development. Thyroid hormone regulates bone development, sexual development, muscle development, and height and weight of children during growth and development. Lack of thyroid hormone can lead to symptoms and signs of developmental delay such as delayed physical development, delayed sexual development, short body, and muscle weakness. (3) Brain development. The brain development of fetuses or infants must depend on thyroid hormone during a certain period. Its deficiency will lead to varying degrees of brain development delays and varying degrees of intellectual disability after birth. This disorder is basically irreversible, and supplementing iodine after the critical period will not help. However, iodine supplementation can also ensure normal physical development, correct hypothyroidism, restore physical fitness, and indirectly improve intellectual activity.