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Purpose: Sunshine is considered to be the most important source of vitamin D. Due to an increased risk of skin 30 cancer sun avoidance is advised, but this directly contributes to the high prevalence of vitamin D deficiency. The 31 simple solution is to advise vitamin D supplementation. The aim of this study is to examine the absolute and 32 relative contribution of sunshine and supplementation to vitamin status. 33 Methods: This study was a secondary analysis of an RCT of 92 Crohn’s disease patients in remission (49% 34 female, median age= 44). Participants were randomised to 2000 IU/day of vitamin D3 or placebo for 1 year, with 35 25-hydroxyvitamin D (25 (OH) D) being measured at baseline and every four months. Based on participant’s place 36 of residence, daily ambient UVB dose at wavelengths that can induce vitamin D synthesis (D-UVB) was obtained. 37 Cumulative and weighted ambient UVB (cw-D-UVB) exposure prior to each blood draw was calculated for each 38 participant. Linear regression analysis and multilevel modelling were used to examine the association between 39 UVB exposure, supplementation and 25 (OH) D concentration. 40 Results: There was considerable annual variation in D-UVB, cw-D-UVB and 25 (OH) D. Both supplementation 41 and Cw-D-UVB was found to be strongly associated with 25 (OH) D: in multilevel model, an increase of 42 approximately 6 nmol/L for every 100 kJ/m2 in cw-D-UVB was found, among those receiving placebo and 43 supplementation (p< 0.0001). Treatment was associated with increase of 23 nmol/L (p< 0.0001). 44 Conclusion: Sunshine is an important determinant of 25 (OH …