**2.1 Participants**

The present work is a cross-sectional study conducted in a sample of 602 individuals from ages 3.1 to 15.4 years after the completion of a clinical examination and blood testing in the period January 2014 to December 2014. Tests were carried out in the Pediatric Endocrinology Unit of our hospital. Tanner's criteria were evaluated in every individual and were used in the assignment of individuals in two different groups: school group (Tanner stage I) and adolescent group (Tanner stages II–V). The characteristics of the place of residence were recorded as urban or rural (< or > 10,000 inhabitants, respectively).

All individuals were healthy Caucasian children living in Navarra, Spain. They were selected from the external consultations of the different pediatric subspecialties and were not affected by any chronic pathology with potential interference in growth, body composition, food ingestion, or physical activity. Any participant under treatment with concrete medications (antiepileptic drugs or glucocorticoids), vitamin D, or calcium supplements was put aside from our sample.
