Rolf Jorde, Stina Therese Sollid, Johan Svartberg, Ragnar Martin Joakimsen and Guri Grimnes
Objective: There has been a huge increase in numbers of serum 25-hydroxyvitamin D (25OHD) measurements in spite of uncertainty concerning the benefits of vitamin D supplementation, there is a sufficient serum 25OHD level and to interpret serum 25OHD measurements. Methods: 255 subjects who participated in a 5-years intervention study with vitamin D for the prevention of type 2 diabetes and who were allocated to placebo were included. Serum 25OHD levels were measured annually, and questionnaires on sunny vacations and vitamin D supplementation filled in. Results: The serum 25OHD levels were ~ 20 nmol/L higher in the summer than the winter months; those taking vitamin D supplements had ~ 5 nmol/L higher 25OHD levels than those not taking supplements; a recent sunny vacation increased the serum 25OHD levels 8-16 nmol/L; and there was a high degree of tracking of serum 25OHD with correlation coefficient (r) between baseline and the following annual measurements between 0.67 and 0.75. Conclusions: If considering a serum 25OHD level of 50 nmol/L as sufficient and that this level should be attained year-round, and taking season, vitamin D supplementation status and recent sunny vacation into account, a single measurement will in most situations be reliable and sufficient for making clinical decisions without need for numerous repeat measurements.
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