Sunshine is an important determinant of vitamin D status even among high-dose supplement users: secondary analysis of a randomised controlled trial in Crohn's disease patients

Sunshine is considered to be the most important source of vitamin D. Due to an increased risk of skin cancer, sun avoidance is advised, but this directly contributes to the high prevalence of vitamin D deficiency. The simple solution is to advise vitamin D supplementation. The aim of this study was to examine the absolute and relative contribution of sunshine and supplementation to vitamin status. This study was a secondary analysis of an RCT of 92 Crohn's disease patients in remission (49% female, median age = 44). Participants were randomized to 2000 IU day−1 of vitamin D3 or placebo for 1 year, with 25‐hydroxyvitamin D (25(OH)D) being measured at baseline and every 4 months. Based on participant's place of residence, daily ambient UVB dose at wavelengths that can induce vitamin D synthesis (D‐UVB) was obtained. Cumulative and weighted ambient D‐UVB (cw‐D‐UVB) exposure prior to each blood draw was calculated for each participant. Linear regression analysis and multilevel modeling were used to examine the association between UVB exposure, supplementation and 25(OH)D concentration. There was considerable annual variation in D‐UVB, cw‐D‐UVB and 25(OH)D. Both supplementation and cw‐D‐UVB were found to be strongly associated with 25(OH)D: in multilevel model, an increase of approximately 6 nmol L−1 for every 100 kJ m−2 in cw‐D‐UVB was found, among those receiving placebo and supplementation (P < 0.0001). Treatment was associated with increase of 23 nmol L−1 (P < 0.0001). Sunshine is an important determinant of 25(OH)D concentration, even in those who are taking high‐dose vitamin D supplements and reside at a higher mid‐latitude location.