Low vitamin D is the following situations: first, insufficient intake, such people will have a partial diet, picky eating, etc., so through the normal diet can not take in enough vitamin D. People who lose weight because of the intake of grains and cereals or the amount of fruits and vegetables with high vitamin and trace element content is insufficient, and thus will have low vitamin D. Secondly, people in special situations need more, such as pregnant women, people with high fever, workers in high temperature conditions, sick patients, patients with long-term alcoholism, increased metabolic rate of catabolism, so the demand for vitamin D is also increased, and this is not timely supplementation, which will cause secondary hypovitaminosis D. Third, patients with gastrointestinal disorders, such as long-term diarrhea, and even patients with liver disease, may also suffer from secondary hypovitaminosis D.