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 D3 supplementation in patients with frequent respiratory tract infections: a randomised and double-blind intervention study
Background Low serum levels of 25-hydroxyvitamin D3 are associated with an increased risk of respiratory tract infections ( RTIs ) . Clinical trials with vitamin D3 against various infections have been carried out but data are so far not conclusive . Thus , there is a need for additional r and omised controlled trials of effects of vitamin D3 on infections . Objective To investigate if supplementation with vitamin D3 could reduce infectious symptoms and antibiotic consumption among patients with antibody deficiency or frequent RTIs . Design A double-blind r and omised controlled trial . Setting Karolinska University Hospital , Huddinge . Participants 140 patients with antibody deficiency ( selective IgA subclass deficiency , IgG subclass deficiency , common variable immune disorder ) and patients with increased susceptibility to RTIs ( > 4 bacterial RTIs/year ) but without immunological diagnosis . Intervention Vitamin D3 ( 4000 IU ) or placebo was given daily for 1 year . Primary and secondary outcome measures The primary endpoint was an infectious score based on five parameters : symptoms from respiratory tract , ears and sinuses , malaise and antibiotic consumption . Secondary endpoints were serum levels of 25-hydroxyvitamin D3 , microbiological findings and levels of antimicrobial peptides ( LL-37 , HNP1–3 ) in nasal fluid . Results The overall infectious score was significantly reduced for patients allocated to the vitamin D group ( 202 points ) compared with the placebo group ( 249 points ; adjusted relative score 0.771 , 95 % CI 0.604 to 0.985 , p=0.04 ) . Limitations A single study centre , small sample size and a selected group of patients . The sample size calculation was performed using p=0.02 as the significance level whereas the primary and secondary endpoints were analysed using the conventional p=0.05 as the significance level . Conclusions Supplementation with vitamin D3 may reduce disease burden in patients with frequent RTIs
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 .