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Vitamin D and the Flu

Jacob Schor, ND, FABNO

March 23, 2010


Up until a few days ago, the idea that vitamin D offered protection against the flu was a theory, a good theory, but still just a theory.  As of March 10, this has changed.  It was on that day that The American Journal of Clinical Nutrition published the results of a randomized trial of vitamin D supplements to see if they would prevent seasonal influenza in school children.  The results were impressive.  School children taking 1200 iu of vitamin D a day had a 42% lower risk of catching the flu than kids taking placebo.


The idea that vitamin D offers protection against the flu has surfaced several times over the decades.  One of the earliest and strongest proponents of the idea was Edgar Hope-Simpson although we must clarify that he didn’t fully understand what he was proposing.  Hope-Simpson was a rather famous English general practitioner, who, back in the 1940s, deduced through meticulous record keeping that shingles results from the same virus that causes chickenpox.  Back in 1981 Hope-Simpson proposed that a ‘seasonal stimulus' associated with solar radiation could explain the seasonal patterns of influenza epidemics. Hope-Simpson's 1992 book, The Transmission of Epidemic Influenza , summarized his findings and made an eloquent plea for scientists to identify this ‘seasonal stimulus'.   He carefully documented that influenza epidemics in temperate latitudes peak in the month following the winter solstice.

"In both hemispheres, influenza rates rise significantly for about 2 months on either side of its peak. Outbreaks are globally ubiquitous and epidemic loci move smoothly to and fro across the surface of the earth almost every year in a sinuous curve that runs parallel with the midsummer curve of vertical solar radiation, but lags about six months behind it … Latitude alone broadly determines the timing of the epidemics in the annual cycle, a relationship that suggests a rather direct effect of some component of solar radiation acting positively or negatively upon the virus, the human host, or their interaction … The nature of the seasonal stimulus remains undiscovered…."


Although Hope-Simpson was certain this seasonal stimulus was associated with changing ultraviolet light exposure, he didn't live long enough to identify it.


In a December 2006 paper,  John Jacob Cannell proposed that vitamin D was Hope Simpson’s mysterious ‘seasonal ‘trigger’.  Cannell works as a staff psychiatrist in a prison hospital in California and has long been in the habit of giving the prisoners in his ward regular high doses of vitamin D. When a flu epidemic laid most of the hospital staff and inmates low, Cannell observed that the inmates in his ward seemed to be protected from illness.  Cannell nominated vitamin D deficiency as the mysterious trigger that would explain the seasonality of flu.


Although the arguments Cannell and others have marshaled over the last few years have convinced many medical practitioners that vitamin D would protect against flu infections, this has nonetheless remained a theory.  Now we have evidence that the theory is true.


In this current 2010 paper, Japanese researchers report how from December 2008 through March 2009, they conducted a randomized, double-blind, placebo-controlled trial comparing vitamin D-3 supplements with placebo in schoolchildren.  They tracked how many of the children caught the flu or had asthma attacks.  A total of 18 of 167 (10.8%) children taking vitamin D caught the flu compared with 31 of 167 (18.6%) children in the placebo group.  The effect of the vitamin D on children who had asthma was more striking.  Of the children in the study who had previously been diagnosed with asthma, asthma attacks almost disappeared with vitamin D use: only 2 children receiving vitamin D-3 had asthma attacks compared with 12 children receiving placebo. Thus the results of this study suggest vitamin D supplements may cut flu infections by more than a third, and asthma attacks possibly by a factor of 6.  


What’s fascinating about all this is how fast it happened.  People have known for thousands of years that the dark of winter is a time of greater illness.  That in the course of a few decades, a theory is proposed, elaborated and then a few years later proven, at least in a preliminary way, is remarkable.  That we now have a safe, simple and inexpensive means at our disposal to decrease influenza and asthma attacks is precious.


If this seems familiar it is because I 've written about it before:



Cannell JJ, Vieth R, Umhau JC, Holick MF, Grant WB, Madronich S, et al. Epidemic influenza and vitamin D. Epidemiology and Infection, Dec. 2006;134(6):1129-40.

Urashima M, Segawa T, Okazaki M, Kurihara M, Wada Y, Ida H. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren.Am J Clin Nutr. 2010 Mar 10.