25-hydroxyvitamin D is the main form of vitamin D in the human body. The concentration of 25-hydroxyvitamin D in the blood reflects the nutritional status of vitamin D in the body, and its low value reflects vitamin D deficiency in the body. Insufficient intake of vitamin D in daily life, impaired absorption of vitamin D in the gastrointestinal tract or reduced natural synthesis can easily cause low 25-hydroxyvitamin D: 1. Insufficient intake: is the main cause of low 25-hydroxyvitamin D in the body, due to factors such as unreasonable dietary structure and partiality, resulting in insufficient intake of vitamin D and in a state of malnutrition. In addition, children in the growth period have an increased demand for vitamin D, which can be manifested as a relative lack of intake; 2, absorption disorders: for example, chronic diseases of the gastrointestinal tract such as Crohn’s disease involve the gastrointestinal tract, affecting the digestive and absorption functions, and digestive dysfunction can not effectively absorb vitamin D and other nutrients, which can also cause malnutrition, resulting in low 25-hydroxyvitamin D. Generally, it is accompanied by abdominal pain, diarrhea If the sunshine time is not enough, the body cannot absorb and synthesize enough vitamin D, which may lead to low 25-hydroxyvitamin D.