The unpredictability of seasonal variations in serum vitamin D levels in children with asthma and/or rhinitis

Background: Some studies have showed that seasonality is an important determinant of vitamin D (vitD) status. Objective: We evaluated whether there are differences in individual trends of serum vitD level over one year in asthmatic and rhinitic children. Materials and methods: Ninety-two asthmatic and rhinitic paediatric patients were followed up for one year and their serum vitD level was detected at three-month intervals, once in each season. Results: We observed higher vitD levels at the end of summer and lower at the end of winter. However, the individual seasonal trend was very variable and unpredictable. If it is true that in a given season the majority of patients followed one direction (increase or decrease of serum vitD levels), nevertheless a substantial percentage behaved differently and unpredictably. For example, at the end of spring, 70% of patients showed an increase in serum vitD levels, but 30% showed a decrease. In addition, five individuals had a value ≥50 ng/ml in September and showed serum vitD levels ≥30 ng/ml throughout the year; 16 patients presented vitD value ≥40 ng/ml in September and always had ≥20 ng/ml in the other months. Conclusions: The wide and unpredictable variability of the individual trend of serum vitD levels should be taken into account before deciding whether or not a drug supplementation is appropriate.