"Overall, the study said one person would be spared infection for every 33 taking vitamin D supplements. That is more effective than flu vaccination, which needs to treat 40 to prevent one case, although flu is far more serious than the common cold."
That was some of the media interpretation of the paper - "systematic review and meta-analysis of individual participant data" - published by Adrian Martineau and colleagues  looking at the collected data on vitamin D supplementation "on risk of acute respiratory tract infection." Including data on approximately 11,000 'randomised' participants reported in 25 studies, authors assessed whether the quite messy data on vitamin d supplementation potentially decreasing the risk of acute respiratory tract infection showed any semi-definitive trends.
Results: "Vitamin D supplementation resulted in a statistically significant reduction in the proportion of participants experiencing at least one acute respiratory tract infection." Further: "Use of vitamin D did not influence risk of serious adverse events of any cause... or death due to any cause. Instances of potential adverse reactions to vitamin D were rare." And finally: "Subgroup analysis revealed that daily or weekly vitamin D supplementation without additional bolus doses protected against acute respiratory tract infection, whereas regimens containing large bolus doses did not."
I note that in the BBC news report on the Martineau paper we are told: "Public Health England (PHE) says the infections data is not conclusive, although it does recommend supplements." This slightly counter-intuitive position follows more general advice from the powers-that-be that perhaps we should all be taking a little more vitamin D (see here) given what is emerging when it comes to the varied functions of the sunshine vitamin/hormone. But bear in mind that supplementation comes with potential risks too (see here) particularly when people forget to treat their vitamins and minerals as what they are: biologically active pharmaceutics. Neither is everyone completely sold on the idea that vitamin D 'could stop colds or flu' as an accompanying editorial to the Martineau paper makes clear : "The results are heterogeneous and not sufficiently applicable to the general population."
What such research does advance however, is that vitamin D is a potentially important nutrient (more so for some groups) and one that we should be [cautiously] dedicating a lot more investigation to for all-manner of possible reasons (see here and see here) outside of just bone health and the English disease. And within that scheme of research, don't forget a few things: (a) there's more to vitamin D metabolism than just 'getting enough' and (b) even today, science is still finding out new things about the chemistry of vitamin D . In short, the scheme of science around vitamin D needs to be broad...
 Martineau AR. et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ 2017; 356: i6583.
 Bolland MJ. & Avenell A. Do vitamin D supplements help prevent respiratory tract infections? BMJ 2017; 356: j456.
 Pauwels S. et al. 1β,25-Dihydroxyvitamin D3: A new vitamin D metabolite in human serum. J Steroid Biochem Mol Biol. 2017 Feb 10. pii: S0960-0760(17)30040-7.
Adrian R Martineau, David A Jolliffe, Richard L Hooper, Lauren Greenberg, John F Aloia, Peter Bergman, Gal Dubnov-Raz, Susanna Esposito, & et al (2017). Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data BMJ : 10.1136/bmj.i6583
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