Seasonal flu is here and it has been reported that the flu vaccine has not been that helpful in preventing infection. I started to think about else can I do beside the usual to reduce risk for myself, my family and my patients. It is known that vitamin D deficiency influences the immune system and can impact on our body’s response to various respiratory infections like the flu. There is a theory that the seasonal change in our body’s vitamin D levels influences contributes to the epidemic pattern that we see of the flu each year. I think that the jury is still out on this, but it can not be ignored that we are experiencing a particularly bad flu season that was preceded by some significantly cold weather driving us all indoors. This article looks at some of the recent available evidence of how vitamin D supplementation could impact our susceptibility to viruses like the flu.

Vitamin D is involved in our immune system’s response to infections.  It supports the induction of what is called the innate immune system.  This is the part of our immune system that responds quickly to perceived threats such as harmful microbes.  It increases “antimicrobial peptides” aptly called “defensin” that induces a process called autophagy.  I liken this to unleashing little pac men that eat up the microbes (not exactly, but you get the point).  In a 2017 meta-analysis of 10,933 study participants researchers among 25 individual studies examined the impact of vitamin D supplementation and the incidence of acute respiratory tract infection.  Vitamin D supplementation reduced the risk of respiratory tract infections.  They calculated the NNT (Number Needed to Treat- the number of people we need to supplement to prevent one infection) to be 33.  If the vitamin D was given daily or weekly the NNT was equal to only 20!  The efficacy of the vitamin D was better if given daily in lower doses rather than large whopper doses.  As expected, if patients were deficient at baseline, a greater effect was seen (NNT= 4).  Having tested a lot of people in New England for vitamin D deficiency, I can safely say this is almost everybody!  This is good quality evidence that vitamin D supplementation may make sense in helping prevent respiratory infections.

In studies that specifically look at the influenza virus, there are several studies that indicate some protection against influenza type A (which is consistent with the current circulating strain).  This effect was seen in a randomized controlled trial in school children.  The incidence of physician diagnosed influenza type A was reduced by half in the “treated” group.

Best Vitamin D Supplements

We may not be able to prevent the flu with the vaccine this year but perhaps a drop or two of vitamin D3 would be helpful daily.  There are certainly no reports of harm in any of the studies I looked at.  For adults, unless I have tested their levels, I typically recommend 2000 IU D3 daily during the winter months.  Vitamin D is a fat soluble vitamin so more is not necessarily better.  It can be tested for at the lab to better personalize dosing but this amount is typically safe.  For school age children, the dose utilized in the study referenced was 1200 IU D3 daily, to keep it simple I give children up to 1000 IU daily in a liquid drop.  Please consult with your own physician before starting supplements to make sure they are appropriate for you and your family.

References:

1. Vitamin D Supplementation to prevent acute respiratory tract infections: systematic review and meta analysis of individual participant data  BMJ 2017: 356 Martineau et al

2. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Am J Clinical Nutrition2010 May;91(5):1255-60. doi: 10.3945/ajcn.2009.29094. Epub 2010 Mar 10. Urashima et al.