The main physiological function of vitamin (vitamin) D, commonly known as cod liver oil, is to regulate calcium and phosphorus metabolism in the body and maintain plasma calcium and phosphorus levels, thereby maintaining normal growth and development of individual teeth and bones. Therefore, vitamin D is especially important for children during the growth period. Yang Fan, Department of Pediatric Health Care, Second Hospital of West China, Sichuan University The main physiological functions of vitamin D include: ① Promoting the absorption of calcium and phosphorus by the epithelium of the small intestinal mucosa. ②Promote the reabsorption of calcium and phosphorus by the proximal tubule of the kidney. ③There are two opposite effects on bone. On the one hand, when blood calcium decreases, 1, 25-(OH)2D acts synergistically with parathyroid hormone (PTH) to lyse bone salts through osteoclasts, absorb calcium and phosphorus from bone, and maintain normal plasma calcium and phosphorus concentrations; on the other hand, 1, 25-(OH)2D promotes the calcification of epiphyseal plate cartilage and bone-like tissue, and maintains calcium and phosphorus levels. On the other hand, 1, 25-(OH)2D promotes calcification of epiphyseal cartilage and osteoid tissues, maintains the saturation of calcium and phosphorus in plasma and facilitates the deposition of bone salts. Therefore, if vitamin D is deficient, the most obviously affected are the bones out of the rapid growth period, resulting in rickets or osteochondrosis. CD4+ T cells are the direct targets of 1, 25-(OH)2D. CD4+ T cells can be functionally divided into two subsets: Thl and Th2. Th1 and Th2 are mutually suppressive T cells. 1,25-(OH)2D regulates the Th1/Th2 immune shift, and when the nutritional status of the body’s 1,25-(OH)2D is altered, the body is in a Th1/Th2 immune drift. 1,25-(OH)2D is able to regulate the phenotype and function of antigen-presenting cells, especially dendritic cells, through various mechanisms, in addition to direct action on T cells. Both in vivo and in vitro experiments have demonstrated that 1,25-(OH)2 vitamin D3 and its analogues induce the acquisition of tolerance in dendritic cells. Based on these immunological properties, vitamin D is used in the prevention of infections, autoimmune diseases (multiple sclerosis, rheumatoid arthritis, systemic lupus erythematosus, etc.) and certain tumors and type II diabetes. Results of prospective observational studies have shown that vitamin D supplementation during infancy and early childhood , may reduce the incidence of type I diabetes. Sources and Intake of Vitamin D Natural foods contain limited amounts of vitamin D. Relatively high levels are found in certain fatty fish and fish oil, liver and aquatic mammalian fat. Another major source of vitamin D is that synthesized by the skin after exposure to ultraviolet light. An adult with light skin color can produce (10,000-20,000) IU of vitamin D in 24 h from exposure of the whole body to UV light for (10-15) min in summer; for a person with dark skin color, 5-10 times the sun exposure time is required to produce a comparable amount of vitamin D. Obtaining vitamin D through sunlight is influenced by many factors, including time spent outdoors, skin color shade, body weight, latitude of living environment, season, cloud thickness, air pollution level, and skin exposure area. Therefore, it is very difficult to objectively evaluate the amount of vitamin D an individual, especially an infant or a child, obtains through exposure to UV light. Considering the potential risk of skin cancer, the American Academy of Pediatrics (AAP) currently recommends that children up to 6 months of age should avoid direct UV exposure, encourage outdoor activities, and recommend the use of sunscreen and protective clothing for outdoor activities. For infancy, childhood and adolescence, vitamin D supplementation is necessary. For breastfed infants, vitamin D supplementation should be started a few days after birth at a dose of 400 IU/d. For artificially fed infants, since commercially available formula is fortified with vitamin D, the amount of fortification is generally (40-100) IU/100 kal (1 kal = 4.18 J). If the infant consumes 1 L/d of milk, the daily vitamin D requirement can be met. If the intake does not reach the above milk level, then vitamin D supplementation is required to meet the requirement. It is important to note that vitamin D deficiency is not specific to infancy and early childhood and can occur at any time throughout life. Recent epidemiological data show that vitamin D deficiency in older children and adolescents has been reported worldwide. This group has a reduced intake of vitamin D fortified milk and does not meet the recommended intake requirements, so the AAP recommends that older and adolescent children take a single preparation containing 400 IU of vitamin D or a multivitamin combination daily. In summary, vitamin D not only plays an important role in maintaining normal growth and bone health in individuals, but also regulates the body’s immunity and protects against certain chronic diseases. Therefore, attention should be paid to the dietary vitamin D intake and supplementation of individuals to maintain a good vitamin D nutritional status.