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 .
A randomized controlled trial of vitamin D3 supplementation for the prevention of symptomatic upper respiratory tract infections.
Vitamin D has been shown to be an important immune system regulator . Vitamin D insufficiency during winter may cause increased susceptibility to upper respiratory tract infections ( URIs ) . To determine whether vitamin D supplementation during the winter season prevents or decreases URI symptoms , 162 adults were r and omized to receive 50 microg vitamin D3 ( 2000 IU ) daily or matching placebo for 12 weeks . A bi-weekly question naire was used to record the incidence and severity of URI symptoms . There was no difference in the incidence of URIs between the vitamin D and placebo groups ( 48 URIs vs. 50 URIs , respectively , P=0.57 ) . There was no difference in the duration or severity of URI symptoms between the vitamin D and placebo groups [ 5.4+/-4.8 days vs. 5.3+/-3.1 days , respectively , P=0.86 ( 95 % CI for the difference in duration -1.8 to 2.1 ) ] . The mean 25-hydroxyvitamin D level at baseline was similar in both groups ( 64.3+/-25.4 nmol/l in the vitamin D group ; 63.0+/-25.8 nmol/l in the placebo group ; n.s . ) . After 12 weeks , 25-hydroxyvitamin D levels increased significantly to 88.5+/-23.2 nmol/l in the vitamin D group , whereas there was no change in vitamin D levels in the placebo group . There was no benefit of vitamin D3 supplementation in decreasing the incidence or severity of symptomatic URIs during winter . Further studies are needed to determine the role of vitamin D in infection
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 .