People who boost their vitamin D levels with supplements reduce their risk of respiratory tract infections, such as the flu, by up to 12%, according to a new systematic review and meta-analysis study of 25 randomized controlled trial (RCT) studies published in The BMJ.

The study reported that this equates to one person being spared a respiratory tract infection (RTI) for every 33 taking vitamin D supplements. The benefit is greater in those receiving daily or weekly vitamin D versus bolus or monthly dosing, with the number needed to treat dropping to 20. The impact in Canada, based on a population of 35 million people, shows vitamin D could spare 1.75 million people having one RTI per year.

Vitamin D researchers have suspected for a long time that RCT studies, which used bolus or monthly dosing and reported minimal benefits, were diminishing the true positive effects of vitamin D, especially in systematic review and meta-analysis. This latest study, “Vitamin D supplementation to prevent acute respiratory tract infections: Systematic review and meta-analysis of individual participant data,” was able to prove that bolus or monthly dosing is ineffective. It found:
•    12% reduction of RTIs based on all studies and all dosing regimens. 
•    19% reduction of RTIs for those receiving daily or weekly vitamin D doses.
•    No statistically significant effect on risk of RTIs for those receiving bolus or monthly dosing.
•    70% reduction in RTIs for those with low vitamin D blood levels below 25 nmol/L and receiving weekly or daily dosing of vitamin D.