25-Hydroxyvitamin D is the main form of vitamin D in the body, and low 25-Hydroxyvitamin D is associated with absorption disorders, insufficient intake and reduced natural synthesis. 1. Absorption obstacles: Some digestive diseases such as Crohn’s disease and ulcerative colitis can affect the gastrointestinal tract, which in turn affects digestion and absorption, resulting in the gastrointestinal tract not being able to effectively absorb vitamin D and other nutrients. 2. Insufficient intake: Some people have insufficient intake of vitamin D due to irrational diet structure and partiality. In addition, children and adolescents in the growth and development period of vitamin D demand increases, if not enough intake of vitamin D, can be manifested as a relative lack of intake. 3. Natural synthesis is reduced: part of the vitamin D can be synthesized under ultraviolet radiation, if the sunshine time is not enough, the body can not synthesize enough vitamin D, may appear 25-hydroxyvitamin D low. When the body 25-hydroxyvitamin D is low, the first thing to do is to consult a doctor for a clear diagnosis, follow the doctor’s instructions to standardize the treatment of the primary disease, in addition to the appropriate amount of vitamin D. The use of drugs must follow the doctor’s instructions, do not blindly use their own medication, so as not to cause adverse consequences.