Seasonal variability of vitamin D status in patients with inflammatory bowel disease - A retrospective cohort study

Objectives Vitamin D deficiency predicts unfavorable disease outcomes in inflammatory bowel disease. Endogenous vitamin D synthesis is affected by seasonal factors including sunlight exposure, raising the question whether seasonality determines the risk of vitamin D deficiency and may mask other clinical risk factors. Methods Univariable and multiple regression analyses were performed in a retrospective cohort of 384 patients to determine risk factors for vitamin D deficiency. Since the observed 25-hydroxyvitamin D [25(OH) D] concentrations followed a sinusoidal pattern over the year, all 25 (OH) D concentrations were normalized for the predicted variability of the respective day of analysis based on a sinusoidal regression analysis of 25(OH) D test results obtained in more than 86,000 control serum samples. Results Vitamin D deficiency was highly prevalent in patients with Crohn's disease or ulcerative colitis (63% and 55%, respectively) and associated with winter/spring seasons. After normalization of 25(OH) D concentrations for the day of analysis, vitamin D deficiency was associated with histories of complications related to inflammatory bowel disease, surgery, smoking and ongoing diarrhea while initial disease manifestation during adulthood, ongoing vitamin D supplementation and diagnosis of ulcerative colitis vs. Crohn's disease appeared to be protective. Multiple regression analyses revealed that vitamin D deficiency was associated with disease activity in Crohn's disease and anemia in ulcerative colitis patients. Only few deficient patients achieved sufficient 25(OH) D concentrations over time. However, increasing 25(OH) D concentrations correlated with improved Crohn's disease activity. Conclusions Vitamin D deficiency was highly prevalent in patients with Crohn's disease and ulcerative colitis and dependent on the season of the year. Following normalization for seasonality by sinusoidal regression analysis, vitamin D deficiency was found to be associated with parameters of complicated disease course while increasing 25(OH) D concentrations over time correlated with reduced activity of Crohn's disease.