Patients diagnosed with ‘indeterminate colitis’ were excluded from this study. All included patients were screened for vitamin D deficiency at the end of summer 2009 (September–November) and winter 2009–2010 (January–March) at the gastroenterology outpatient department of a large teaching hospital in the centre

of the Netherlands. Written informed consent was obtained from all participants. The study protocol was see more approved by the local Medical Ethics Committee of the Meander Medical Centre. Data collection A standardized questionnaire was used to analyse information on self-reported demographic data i.e. age, sex, ethnicity, health behaviour, physical activity, this website current smoking and alcohol usage. Physical activity was assessed using the SQUASH (Short QUestionnaire click here to ASess Health) questionnaire according to the national physical activity scale [12]. Excessive alcohol usage was defined as >21 alcoholic units per week for men and >14 alcoholic units per week for women. Disease activity of IBD was assessed by the Manitoba IBD index

[13]. This index is based on patient self-reports enclosing IBD-related symptoms in the last 6 months. Other patient characteristics were retrieved from documented medical records in order to obtain data of fractures in the past and corticosteroid usage. Body mass index was measured by calculating weight old in kilograms divided by the square height in meters. For their vitamin D assessment, patients had to undergo serum 25OHD measurement at the end of summer and winter and complete two questionnaires. In these questionnaires, patients were asked to report their daily oral vitamin

D supplementation (including daily dosages and type of supplementation i.e. prescription medication and/or over the counter supplements), medication compliance, preferred exposure to sunlight or shade when outdoors and average number of days per week with >2 midday hours exposure to sunlight during summer. Furthermore, sun holidays in the last 6 months, frequency of solarium visits, calcium intake (dairy products /day) and intake of fatty fish (servings/month), i.e. mackerel, herring and salmon, were assessed. Laboratory measurements Original serum samples were drawn in EDTA, respectively, heparin-containing collection tubes, centrifuged and stored at −30°C. Biochemical and haematological laboratory markers (e.g. haemoglobin (Hb), haematocrit (Ht), red blood cell distribution width (RDW), erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), calcium, phosphate, alkaline phosphatase, albumin, creatinine and thyroid stimulating hormone) were measured at the end of summer (September–November 2009).