Conclusions Vitamin D supplementation  was safe and it protected against acute respiratory tract infection overall .
Patients who were very vitamin D deficient and those not receiving bolus doses experienced the most benefit .
Objectives To assess the overall effect of vitamin D supplementation on  risk of acute respiratory tract infection  , and to identify factors modifying this effect .
Environmental, personal, and genetic determinants of response to vitamin D supplementation in older adults.
CONTEXT AND OBJECTIVE Suboptimal vitamin D status can be corrected by vitamin D supplementation , but individual responses to supplementation vary . We aim ed to examine genetic and nongenetic determinants of change in serum 25-hydroxyvitamin D ( 25(OH)D ) after supplementation . DESIGN AND PARTICIPANTS We used data from a pilot r and omized controlled trial in which 644 adults aged 60 to 84 years were r and omly assigned to monthly doses of placebo , 30 000 IU , or 60 000 IU vitamin D3 for 12 months . Baseline characteristics were obtained from a self-administered question naire . Eighty-eight single-nucleotide polymorphisms ( SNPs ) in 41 c and i date genes were genotyped using Sequenom MassArray technology . Serum 25(OH)D levels before and after the intervention were measured using the Diasorin Liaison platform immunoassay . We used linear regression models to examine associations between genetic and nongenetic factors and change in serum 25(OH)D levels . RESULTS Supplement dose and baseline 25(OH)D level explained 24 % of the variability in response to supplementation . Body mass index , self-reported health status , and ambient UV radiation made a small additional contribution . SNPs in CYP2R1 , IRF4 , MC1R , CYP27B1 , VDR , TYRP1 , MCM6 , and HERC2 were associated with change in 25(OH)D level , although only CYP2R1 was significant after adjustment for multiple testing . Models including SNPs explained a similar proportion of variability in response to supplementation as models that included personal and environmental factors . CONCLUSION Stepwise regression analyses suggest that genetic variability may be associated with response to supplementation , perhaps suggesting that some people might need higher doses to reach optimal 25(OH)D levels or that there is variability in the physiologically normal level of
Conclusions Vitamin D supplementation  was safe and it protected against acute respiratory tract infection overall .
Patients who were very vitamin D deficient and those not receiving bolus doses experienced the most benefit .
Objectives To assess the overall effect of vitamin D supplementation on  risk of acute respiratory tract infection  , and to identify factors modifying this effect .