INTERPRETATION Results indicate that vitamin D has a protective effect against RTI , and dosing once-daily seems most effective .
BACKGROUND Low levels of 25-OH vitamin D are associated with  respiratory tract infection ( RTI ) .
However , results from r and omized controlled trials are inconclusive .
Therefore , we performed a systematic review and meta- analysis to assess the preventive effect of vitamin D supplementation on RTI .
Vitamin D supplementation did not prevent influenza-like illness as diagnosed retrospectively by questionnaires in subjects participating in randomized clinical trials
Background : Vitamin D deficiency has been associated with a number of diseases , including influenza . Whether or not this reflects a causal relationship is unknown . We therefore wanted to examine if supplementation with vitamin D would affect the incidence and severity of influenza-like disease  . Methods : Question naires on influenza were sent to subjects participating in ongoing placebo-controlled intervention studies with vitamin D supplementation , up until the end of April 2010 . Results : Five hundred and sixty-nine subjects from 10 different clinical trials were included in the study , of whom 289 were r and omized to receive vitamin D ( 1111–6800 IU/day ) and 280 to receive placebo .  Influenza-like disease during the previous fall/winter was reported in 38 subjects in the vitamin D group and 42 in the placebo group ( non-significant ) , of whom 25 and 26 subjects , respectively , fulfilled our clinical criteria for influenza . In these latter subjects , the duration of illness was significantly longer among those in the vitamin D group than among those in the placebo group ( median 7 ( range 2–60 ) days vs median 4 ( range 2–18 ) days ; p = 0.007 ) . However , this difference was not statistically significant if all 38 ( vitamin D ) and 42 ( placebo ) subjects who reported symptoms were included . Conclusion : Our results do not support the hypothesis that high doses of vitamin D supplementation will have a pronounced effect on influenza-like disease in population s not targeted for high influenza risk
INTERPRETATION Results indicate that vitamin D has a protective effect against RTI , and dosing once-daily seems most effective .
BACKGROUND Low levels of 25-OH vitamin D are associated with  respiratory tract infection ( RTI ) .
However , results from r and omized controlled trials are inconclusive .
Therefore , we performed a systematic review and meta- analysis to assess the preventive effect of vitamin D supplementation on RTI .