Effect of vitamin D supplementation and ultraviolet B exposure on serum 25-hydroxyvitamin D concentrations in healthy volunteers: a randomized, crossover clinical trial


  • Funding sources The work was supported by the Research Foundation of the Norwegian Radium Hospital and South-Eastern Norway Regional Health Authority.
  • Conflicts of interest None declared.



Solar ultraviolet (UV) radiation during the summer and vitamin D supplementation are two major sources of vitamin D for humans at northern latitudes. However, little is known about the relative efficiency of these two vitamin D sources.


The main goal was to compare the efficiency of high-dose oral vitamin D3 supplementation (2000 IU per day for 30 days) with a simulated summer UV exposure [10 sunbed sessions to a total dose of 23·8 standard erythema doses (SED)] to improve vitamin D status.


Healthy volunteers were randomized into two groups: group 1 received vitamin D supplementation followed by 10 whole-body sunbed exposures; group 2 started with 10 sunbed exposures followed by vitamin D supplementation.


The oral supplementation with vitamin D3 resulted in a mean (SEM) serum 25-hydroxyvitamin D [25(OH)D] increase of 25·3 (5·4) nmol L−1. A similar increase, 19·8 (5·4) nmol L−1, was observed after simulated summer UV exposure. At the end of the study, serum 25(OH)D concentrations were similar in both groups.


Twice-weekly whole-body sunbed exposure to a dose of 4·8 SED is equal to 2000 IU daily of oral vitamin D supplementation for 30 days and enough to achieve and maintain serum 25(OH)D concentrations > 75 nmol L−1 in ~55% of cases. Based on our calculations, this dose corresponds to a cumulative weekly whole-body exposure of 3·4 SED (~ 40 min around midday during the summer at the latitude of Oslo).