A robust systematic review and meta-analysis has revealed regular vitamin D supplementation is strongly protective against acute respiratory infections (ARIs), including colds and flu.
Observational studies have shown consistent links between serum vitamin D levels and susceptibility to ARIs. However, previous meta-analyses, reviewing whether vitamin D supplementation can reduce the risk of ARIs, have shown significant heterogeneity of effect between the results. These studies were too different in design, method or intervention to provide an accurate outcome.
In a recent meta-analysis, researchers collated and systematically reviewed all randomised, double-blind, placebo-controlled trials of vitamin D supplementation for ARIs up until 31 December 2015. There were no language or study size restrictions. Twenty-five high quality studies met the final eligibility requirements.
The difference with this research is that the reviewers directly addressed heterogeneity by using individual participant data (IPD). This allowed for variations in participant characteristics and dosing regimens, with independent subgroup outcomes, and identified which of these affected the results.
The overall results, using the IPD, showed that vitamin D supplementation statistically and significantly reduced the risk of ARIs, with no safety issues.
Subgroup analysis revealed the risk of ARIs was more than halved in those with baseline 25-hydroxyvitamin D levels below 25nmol/L (moderate deficiency), and while there was a risk reduction with levels above 25nmol/L, this was not statistically significant.
However, when combining dosage and serum levels, they found that daily or weekly supplementation of vitamin D, compared to a large dose given periodically, was strongly protective, even in those with vitamin D baseline levels above 25nmol/L.
The overall conclusion was that the protective effect of vitamin D supplementation is strongest in those with low vitamin D levels, and when supplementation is taken on a daily or weekly basis.
Vitamin D may protect against viruses and bacteria by inducing antimicrobial peptides and is important for other innate immune system functions.
Acute respiratory tract infections are a major cause of global morbidity and mortality, placing a substantial health and economic burden on society, correlated with an estimated 2.65 million deaths globally in 2013. In a 2014 study, researchers extrapolated that there are around 68.9 million cases of colds and flu in Australia annually.
These findings support the introduction of further public health care measures to improve vitamin D status, especially in areas where deficiency is common.
- Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: Systematic review and meta-analysis of individual participant data. BMJ 2017;356:i6583. [Full text]
- Chen Y, Kirk MD. Incidence of acute respiratory infections in Australia. Epidemiol Infect 2014;142(7):1355-1361. [Abstract]