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 .
Vitamin D supplementation and upper respiratory tract infections in adolescent swimmers: a randomized controlled trial.
Observational studies identified associations between vitamin D insufficiency ( serum 25(OH)D < 30ng·ml-1 ) and risk of upper respiratory infection ( URI ) .  Swimmers are highly prone to URIs , which might hinder their performance . The aim of this study was to examine if vitamin D3 supplementation reduces URI burden in vitamin D-insufficient swimmers . Fifty-five competitive adolescent swimmers with vitamin D insufficiency were r and omized to receive vitamin D3 ( 2,000IU·d-1 ) or placebo for 12 winter weeks . A URI symptom question naire was completed weekly . Serum 25(OH)D concentrations were measured by radio-immunoassay before and after supplementation . We used linear regression to examine the relation between the change in 25(OH)D concentrations during the trial , and the duration and severity of URIs . There were no between-group differences in the frequency , severity , or duration of URIs . Exploratory analyses revealed that in the placebo group only , the change in 25(OH)D concentrations during the trial was highly associated with the duration of URIs ( r = -0.90,p < .001 ) , and moderately associated with the severity of URIs ( r = -0.65,p = .043 ) . The between-group differences for duration  were highly significant . Vitamin D3 supplementation in adolescent swimmers with vitamin D insufficiency did not reduce URI burden . However , larger decreases in serum 25(OH)D concentrations were associated with significantly longer and more severe URI episodes
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 .