7 nmol/L at the end of winter. Patients without any additional vitamin D intake through oral supplementation or sun exposure had lower

mean serum 25OHD levels of 48.4 nmol/L at the end of summer and 42.7 nmol/L at the end of winter (Fig. 1). Fig. 1 Mean serum 25OHD levels (nanomoles per litre) at the end of summer and winter. Patients were classified as ‘vitamin D intake only by ultraviolet Eltanexor (UV) light’ if they did not use oral vitamin D supplementation and met one or two of the following criteria: regular solarium visits and sun holiday in the last 6 months. Patients who used oral supplementation without being exposed to ultraviolet light (no solarium visits or sun holidays) were classified as ‘vitamin D intake only by oral AZD7762 clinical trial supplementation’. If patients used both oral supplementation and additional UV light, they were classified as ‘combined vitamin D intake by UV light and oral supplementation’ In general, a decreased risk of vitamin D deficiency was seen in patients who used daily oral vitamin D supplementation during summer (p  =  0.029) and winter (p  <  0.001). Higher dosages of supplementation did not lower the risk of developing vitamin D deficiency, although a non-significant negative trend was seen

between the daily dosage of vitamin D supplementation and the risk of being vitamin D deficient (p  =  0.09). Discussion This prospective cohort study demonstrates that vitamin D deficiency, with a prevalence of 39% at the end of summer, is a common problem in IBD patients. Furthermore, strong seasonal variation of vitamin D levels was observed, with a decline of mean serum 25OHD levels from 55.1 nmol/L at the end of summer to 48.4 nmol/L at the end of winter, leading to an overall vitamin D deficiency prevalence of 57% in the sun-deprived months. To our knowledge, this is the largest study up till now which investigates the seasonality of vitamin D levels in a cohort of adult IBD outpatients. Our results are in line with the few data currently available concerning

vitamin D deficiency in IBD patients. McCarthy et al. described in 44 CD patients prevalence rates of vitamin D deficiency of 18% (cut-off point, <50 nmol/L) late-summer and 50% late-winter [14]. Kuwabara et al. reported vitamin D deficiency prevalence rates of even 76% in 70 IBD patients at the end of Masitinib (AB1010) summer (cut-off point, <50 nmol/L) [10]. Generally, we can conclude that our study, which is characterized by a large and representative IBD outpatient cohort, confirms the high prevalence of vitamin D deficiency which was presumed in preliminary studies. Prevalence rates of vitamin D deficiency in the general population are better documented compared to the relatively small subgroup of IBD patients; unfortunately, the usefulness of these prevalence data for comparison with our diseased group is limited. In the Netherlands, representative population-based studies are lacking.