Cystic fibrosis (CF) is an autosomal recessive disease caused by alterations in the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene, characterized by multisystemic alterations, mainly in the lung, intestine, sweat, and bile ducts. In addition to pulmonary involvement, the presence of exocrine pancreatic insufficiency also increases the risk of survival, as it is associated with malnutrition and deficiency of fat-soluble vitamins, such as vitamin D.

Vitamin D, in addition to its role in bone health, in the case of CF patients with chronic inflammation, it has been suggested that many of the cytokines that regulate the inflammatory response contain elements that respond to vitamin D, so vitamin D could play an essential role in the regulation of the inflammatory response in CF, which could favor lung function.

However, more than 50% of CF patients present vitamin D insufficiency or deficiency, despite the different schemes suggested for supplementation in different age groups, which suggests that new strategies are needed to normalize vitamin D levels, which will allow us to see its clinical effect on the inflammatory response, by decreasing the number of exacerbations and thus perpetuating or improving lung function, as well as on bone mineral health.

Randomized double-blind clinical trial in patients aged 5 to 18 years attending the CF Comprehensive Care clinic in Mexico City. The intervention group will receive an intensification of vitamin D supplementation, with increments of 4000 IU depending on serum levels, the control group will receive supplementation according to the treatment guidelines of the Cystic Fibrosis Foundation, and the dose will be doubled depending on serum levels. In both groups vitamin D levels will be measured at 3, 6, 9, and 12 months, spirometry and sputum culture will also be performed to evaluate the pulmonary function, densitometry to evaluate the bone mineral density and body composition, together with anthropometric evaluation and quantitative bone ultrasound at each visit, in the biochemical evaluation, calcium/creatinine ratio and alkaline phosphatase will be taken at each visit.

Inclusion Criteria:

• Cystic Fibrosis

Exclusion Criteria:

• Another chronic disease (HIV, cancer, renal failure)
• 25-OH-VitD levels < 10 ng/ml or > 30 ng/ml

For the blinding of the maneuver, supplementation will be done with cholecalciferol (vitD3) in dispersible tablets of the same size and color, with doses of 2000 IU (0.05 mg) for the control group and 4000 IU (0.1 mg) for the intervention group. The capsules should be dissolved in 30 ml of water. The dispensing of the medication according to the intervention group and vitamin D levels will be performed by the investigator assigned to blinding of the principal investigator, the containers of the tablets will be of the same color and size and only the investigators responsible for the dispensing will know the corresponding dose