Vitamin D is an essential nutrient that regulates the activity of calcium and bone hormones throughout life. A study has suggested that higher vitamin D levels can protect the body from multiple diseases . Additional research over the last 20 years has shown that vitamin D plays an important role in a variety of physiological processes. Long-term vitamin D deficiency not only causes abnormal bone development and impedes the absorption of calcium and phosphorus, but also increases the risk of chronic diseases such as infection , cancer [2, 3], autoimmune diseases , asthma  and infectious diseases , as well as cardiovascular and vascular diseases [7, 8]. Vitamin D deficiency is common worldwide and recent studies have suggested that vitamin D may be involved in the development of the nervous system because its deficiency may lead to the occurrence of depression . Infants and young children (0–4 years old) are at a critical stage of physical growth and development. Vitamin D deficiency may lead to metabolic syndrome in childhood and adverse effects into adulthood . Therefore, screening for vitamin D levels in children, especially infants aged 0–4 years, is particularly important.
Receptors for vitamin D are found in almost all organs of the body, with various studies showing them present in the brain, heart, skin, reproductive organs, small intestine, colon, and even in activated T and B lymphocytes and B-islet cells . More than 90% of the body's own vitamin D is produced by the skin, while dietary supplements may contribute 5–10% of total vitamin D . Under ultraviolet radiation (UV-B), 7-dehydrogenated cholesterol, the precursor of vitamin D, is transformed into vitamin precursor D3 through the skin and isomerized into vitamin D3, which is transported to the liver through the blood, where it is synthesized into 25(OH)D and finally converted to the active form 1, 25 (OH)2D in the kidney catalyzed by 1α1-hydroxylase . Vitamin D enters cells through receptors on the cell surface and binds to retinol X receptors and the vitamin D receptor complex in the nucleus, where it regulates target genes to produce calcium-binding proteins, which bind calcium ions transported out of intestinal epithelial cells and released into the blood. If the blood calcium concentration is high, calcium is deposited in bone for storage or excreted out of the body through the kidney in urine. If the blood calcium concentration is low, calcium is supplemented by bone calcium . Lack of vitamin D leads to abnormal calcium metabolism and eventual changes in bone, resulting in deformities. Diet alone cannot meet daily vitamin D needs because vitamin D levels in natural foods are limited. Children who spend less time outdoors are more likely to have lower serum vitamin D levels.
Although there is no standard on the optimal level of serum 25-hydroxyvitamin D, the general consensus has generally emphasized that 25-hydroxyvitamin D levels above 50 nmol/L are normal [15, 16]. Serum 25(OH)D levels are classified into five categories by the American Academy of Pediatrics : severely deficient, < 12.5 nmol/L; deficient, > 12.5 and < 25 nmol/L; insufficient, > 25 and < 50 nmol/L; sufficient, > 50 nmol/L and < 250 nmol/ L; and toxic, > 250 nmol/L. However, recommendations for vitamin D do not take into account the optimal level of serum 25(OH)D required for general health in infants and young children, whose threshold level is even more controversial than in adults because less data are available. A retrospective study showed that the most advantageous serum concentration of 25 (OH)D for bone mineral density, lower extremity function, dental health and reduced risk of fractures from falls in infants and young children should be approximately 90 to 100 nmol/L . The results of this study also suggested an infant serum vitamin D concentration threshold of 75 nmol/L to avoid the sequelae caused by vitamin D deficiency [18, 19].