Childhood obesity is one of the most serious public health challenges of the 21st century, with an increasing prevalence over time in developed countries. Overweight and obese children and adolescents are likely to remain so into adulthood and to develop chronic diseases at a young age, such as diabetes and cardiovascular disease.

Obese patients, whether adults or children, are likely to have low serum vitamin D levels due to sequestration and/or volumetric dilution of this fat-soluble vitamin in adipose tissue. Studies have established a link between vitamin D deficiency or insufficiency and chronic diseases such as hypertension, type 2 diabetes and other metabolic problems.

Determining physiological 25(OH)D levels to ensure optimal phosphocalcic metabolism and bone mineralisation requires the use of functional markers: parathyroid hormone (PTH) levels, assessment of the intestinal calcium absorption fraction, assessment of bone mineral density and bone mineral content using absorptiometry.

Vitamin D deficiency leads to malabsorption of calcium and phosphate in the digestive tract, with concentrations, especially of calcium, tending to fall in plasma, resulting in hypersecretion of PTH, which mobilises bone calcium to maintain subnormal blood calcium levels.

Each unit increase in BMI is associated with lower serum vitamin D concentrations: given these low concentrations in this population associated with the risk of developing pathologies, it is important to ensure adequate vitamin D supplementation.

The latest paediatric recommendations recommend, for children aged between 1 and 18 with vitamin D deficiency, a supplement of 2,000 IU/day for at least 6 weeks or a bolus of 50,000 IU once a week for at least 6 weeks.

There are different dosage regimens for the replacement of vitamin D deficiency depending on the country: there is a lack of data on the appropriate dosage and administration regimens for vitamin D supplementation in cases of deficiency, particularly in obese children and adolescents. A prospective, randomised clinical trial will make it possible to define the vitamin D supplementation regimen best suited to increasing serum vitamin D levels in these children and adolescents suffering from obesity.

Inclusion Criteria:

• Aged between 5 to 18 year-old
• Being obese (BMI >97th percentile, > IOTF 30, for age and gender using the WHO references)
• Patients (parents) having given their informed consent
• Patient having insurance from the national health system

Exclusion Criteria:

Children will be excluded from the study if:

• Symptomatic vitamin D deficiency (tetany, muscular hypotonia, hypocalcaemic seizure)
• Vitamin D supplementation in the 3 months preceding the inclusion visit (V1)
• Signs of rickets at the X-ray (osteopenia and cortical thinning of the long bones, stress fractures, and metaphyseal widening and fraying)
• Chronic disease such as granulomatous conditions, Williams syndrome, or hypothyroidism predisposing to hypocalcaemia or in case of hypercalcaemia (calcium > 2.65 mmol/L), liver/kidney disease, malabsorption diseases;
• Hypercalciuria (urinary Calcium/Creatinine > 0.7 mmol/mmol), calcium nephrolithiasis, hypervitaminosis D (25-(OH)D > 250 nmol/L); nephrocalcinosis;
• Ongoing treatment with anticonvulsants/barbiturates or steroids which increase the catabolism of 25(OH)D;
• Ongoing treatment with thiazides diuretics which reduce urinary excretion of calcium;
• Contraindications to the class of drugs under study, e.g. known hypersensitivity or allergy to class of drugs or the investigational product;
• Pregnancy, breastfeeding;
• Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia, etc. of the participant;
• Simultaneous enrolment to another study which could influence the results of the current study;
• Patient under legal protection or deprived of liberty.