As the prevention and treatment of rickets in China continues, the role of vitamin D in children’s bone health has been gradually recognized. Vitamin D is a fat-soluble vitamin, with vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) being the most important, mainly from sunlight and food, which can promote calcium absorption in the body and participate in fetal calcium metabolism through the placenta. Infants and children are at high risk of vitamin D deficiency, which can predispose them to rickets and increase the risk of osteoporosis and fractures in adulthood. If a pregnant woman is deficient in vitamin D, her breastfed infant may develop nutritional rickets due to insufficient reserves of nutrients in breast milk. Older children are less likely to have vitamin D deficiency than infants because of increased activity, increased exposure to sunlight, and increased access to vitamin D in food. The causes of vitamin D deficiency in infants and children are as follows: Inadequate storage during fetal life: The fetus obtains vitamin D from the mother through the placenta and stores it in the body to meet its needs for a period of time after birth, so infants with vitamin D deficiency during maternal pregnancy, premature birth or twin births Insufficient vitamin D in the early postnatal period. Lack of sunlight exposure: Firstly, the ultraviolet rays of sunlight cannot pass through ordinary glass, and infants and children have less outdoor activities, so vitamin D production is seriously insufficient; secondly, high-rise buildings block sunlight exposure, and atmospheric pollution (haze) can absorb part of the ultraviolet rays; finally, due to the reduction of sunlight exposure in winter, it will affect skin synthesis of vitamin D. In recent years, the houses in China are built higher and higher, and PM2,5 is high but not falling, which seriously affects children’s sunlight exposure. The sunlight exposure of children is seriously affected. As a result, vitamin D deficiency is becoming more frequent not only in infants but also in older children. Inadequate intake: Natural foods contain very little vitamin D, such as dairy (including human milk and cow and goat milk), poultry egg yolk, and meat, and cereals, vegetables, and fruits contain almost no vitamin D. Based on these high-risk factors, the new guidelines published by the American Academy of Pediatrics in November 2008 advance and extend the age range for vitamin D supplementation, and the preventive dose has been increased. The new guidelines recommend: 1. 400 IU/d of vitamin D supplementation for newborns starting a few days after birth and continuing through childhood and adolescence; 2. 400 IU/d of vitamin D supplementation for any breastfed infant, with or without formula; 3. No less than 400 IU/d of vitamin D supplementation for women in late pregnancy and breastfeeding, regardless of the season. Reasons for the recommendations 1. Symptoms of vitamin D deficiency can appear in the neonatal period, especially when the mother is deficient in vitamin D. 2. Serum 25-(OH)D3 levels are generally low in exclusively breastfed infants who are not supplemented with vitamin D, especially when the mother is deficient in vitamin D and when the infant is born in winter. 3. The amount of sunlight exposure that allows the infant to reach adequate serum 25-(OH)D3 levels cannot be precisely determined. 4. supplementation with 400 IU/d of vitamin D can result in serum 25-(OH)D3 levels >50 nmol/L in exclusively breastfed infants; 5. infants must consume about 1000 ml of formula per day to consume 400 IU of vitamin D, but it is almost impractical to consume such large amounts of formula; 6. vitamin D is needed continuously during childhood and adolescence before bone growth ceases to promote the deposition of calcium salts in the bones and to facilitate height growth.