Sunlight exposure behaviour and vitamin D status in photosensitive patients: longitudinal comparative study with healthy individuals at U.K. latitude

Summary Background Low vitamin D status is prevalent in wintertime in populations at northerly latitudes. Photosensitive patients are advised to practise sun avoidance, but their sunlight exposure levels, photoprotective measures and resulting vitamin D status are unknown. Objectives To examine seasonal vitamin D status in photosensitive patients relative to healthy individuals and to assess quantitatively behavioural and demographic contributors. Methods This was a longitudinal prospective cohort study (53·5°N) examining year‐round 25‐hydroxyvitamin D [25( OH )D] levels, sun‐exposure behaviour and oral vitamin D intake in photosensitive patients diagnosed at a photoinvestigation unit ( n  =   53), compared with concurrently assessed healthy adults ( n  =   109). Results Photosensitive patients achieved seasonal 25(OH)D variation, but insufficient (< 20 ng mL −1 ; 50 nmol L −1 ) and even deficient (< 10 ng mL −1 ; 25 nmol L −1 ) levels occurred at the summer peak in 47% and 9% of patients, respectively, rising to 73% and 32% at the winter trough. Adjusting for demographic factors, the mean values were lower than for healthy volunteers by 18% [95% confidence interval (CI) 4–29] in summer ( P  = 0·02) and 25% (95% CI 7–39) in winter ( P  =   0·01). Behavioural factors explained 25(OH)D differences between cohorts. Patients demonstrated lower weekend ultraviolet B doses ( P  <   0·001), smaller skin surface area exposure ( P  = 0·004) and greater sunscreen use ( P  <   0·001), while average oral vitamin D intake was low in both groups (photosensitive: 2·94 μg per day). Supplementation and summer surface area exposure predicted summer peak and winter trough 25(OH)D levels. A 1 μg per day increment in supplementary vitamin D raised summer and winter 25(OH)D by 5% (95% CI 3–7) and 9% (95% CI 5–12), respectively (both P  <   0·001). Conclusions Photosensitive patients are, through their photoprotective measures, at high risk of year‐round low vitamin D status. Guidance on oral measures should target this patient group and their physicians.