What does it mean to target specific serum 25-hydroxyvitamin D concentrations in children and adolescents?

Vitamin D in European children-statement from the European Academy of Paediatrics (EAP)

Vitamin D is synthesized in human skin upon sun exposure and is also a nutrient. It regulates calcium and phosphate metabolism and is essential for the maintenance of bone health. Vitamin D supplementation during infancy, in order to prevent rickets, is universally accepted. Many human cell types carry vitamin D receptor, this being a drive for conducting studies on the possible association between vitamin D status and other diseases. Studies have affirmed that a considerable number of healthy European children may be vitamin D deficient, especially in high-risk groups (darker pigmented skin, living in areas with reduced sun exposure and other disorders). However, the definition of deficiency is unclear due to inter assay differences and due to a lack of consensus as to what is an "adequate" 25(OH)D level. Therefore, there is no justification for routine screening for vitamin D deficiency in healthy children. An evaluation of vitamin D status is justified in children belonging to high-risk groups. All infants up to 1 year of age should receive an oral supplementation of 400 IU/day of vitamin D. Beyond this age, seasonal variation of sunlight should be taken into account when considering a national policy of supplementation or fortification.