Vitamin D is not seen anymore only as a phosphocalcic hormone, but also as having an effect on global health (anti-infective, anti-inflammatory, anti-tumour roles and cardiovascular protection).

The link between vitamin D deficiency and osteomalacia lesions is well-known. In paediatrics, systematic vitamin D supplementation of infants and toddlers, associated with milk enrichment, has allowed an almost total disappearance of rickets. Vitamin D repletion was defined as the minimal concentration that enables the prevention of rickets in children and osteomalacia in adults, that is approximately 8 ng/mL (20 nmol/L). However, in 2010, most of the international experts agreed to set minimal threshold of 25 OH vitamin D serum concentration, higher than the one previously admitted, with a limit of 20 ng/mL (50 nmol/L) to define a vitamin D deficiency and a limit of 30 ng/mL (75 nmol/L) to define vitamin D insufficiency in adults. In the paediatric population, the consensus is less obvious and we consider that a serum concentration of minimum 20 ng/mL is necessary.

A study on more than 200 children from Lyon, followed in the paediatric nephrology unit and having a renal function normal or sub-normal, demonstrated an important prevalence of vitamin D deficiency (75%) in adolescents and pre-adolescents.

Concurrently, the appearance of new bone imaging techniques (especially high-resolution peripheral quantitative computed tomography HR-pQCT) improved bone status evaluation in a non-invasive manner.

Given the new pathophysiological data on pleiotropic role of vitamin D (bone, cardiovascular system, adipose tissue) and given the proportion of French children possibly suffering from vitamin D deficiency, it seems urgent to actualize current recommendations regarding systematic supplementation in vitamin D. This transversal study on 200 healthy children and adolescents will allow to have an overview of vitamin D status in French healthy children and adolescents, studying with non-invasive, safe, reliable and innovative tools, the theoretical targets of vitamin D (bones, cardiovascular system and nutritional status); and then to lay the foundations of therapeutic trials aiming to evaluate the mode of vitamin D supplementation for healthy children and adolescents; while having a cohort for HR-pQCT measurements, that will allow us to have French reference range in a 10-17 year-old population, for this innovative, non-invasive and low radiation exposure technique.

To study the correlation between bone assessment by high-resolution peripheral quantitative computed tomography (HR-pQCT) and vitamin D serum concentration.

Inclusion Criteria:

• 10 year-old \