Sunlight is an important determinant of vitamin D serum concentrations in cystic fibrosis

Background/Objectives: The increase of bone disease in adult cystic fibrosis (CF) patients is partly attributed to inadequate serum concentrations of 25-OH cholecalciferol (25 (OH) D) blamed on fat malabsorption. Based on physiological, clinical and biochemical observations this pathogenesis is debatable. The objective was to ascertain the relative importance of different 25 (OH) D sources. Subjects/Methods: Over 4 consecutive years, 474 annual 25 (OH) D serum concentrations from 141 CF patients of all ages were compared with values of healthy peers and weighed against annual ultraviolet B (UVB) exposure. Results: Ranked per month, 25 (OH) D concentrations depicted a curve strikingly parallel to the amount of UVB exposure in the preceding months. A significant difference exists between 25 (OH) D concentrations in the 'Months with high UVB exposure' (May-October) and the 'Months with low UVB exposure' (November-April) but not with healthy controls in the same period. Conclusions: 25 (OH) D concentrations clearly respond to the amount of sunshine in preceding months. They are not clearly influenced by daily oral supplements of 800 IU of cholecalciferol. Sun exposure should be encouraged, and the recommended dosage of oral supplements increased