Common Cold 0, Vitamin D 0 (Or Is It?)

Proof of association is not proof of causation.  Another way to state this is that correlation does not imply causation.  The example I've been telling my patients recently goes thusly: imagine that you see two men walking down a street, one very closely behind the other, the one behind with his hands together underneath a jacket, the one in front sweating & looking nervous.  Is it just coincidence that they're walking in such close proximity together on a hot Las Vegas afternoon?  Or does the guy behind have a firearm hidden under the jacket pointed in the back of the guy in front, thus causing him to be nervous?

So let's apply this scenario to vitamin D and the common cold.  Many observational studies have linked low levels of vitamin D to greater risk of upper respiratory infections, otherwise known as the common cold.  But we have yet to show that giving someone vitamin D actually decreases one's risk for catching a cold.

Thus, in a randomized, double-blind, placebo-controlled trial published last week in JAMA, the authors gave vitamin D 200,000 units orally once monthly for 2 months then 100,000 units monthly for the remaining time up to 18 months vs placebo throughout to 322 New Zealanders with an average baseline 25OH vitamin D of 29ng/mL or just under the usually accepted lower limit of normal of 30ng/mL.  Those who received vitamin D regularly saw an increase to at least 48ng/mL for the duration of the study.

However, by the end of the year and a half, there was no statistical difference in frequency of colds, severity of colds, duration of colds, and missed days of work.  In other words, vitamin D supplementation did not affect the common cold virus.  Or did it?  After all, everyone's vitamin D level was just shy of normal to begin with.  Perhaps that level was so close to normal that their immune systems weren't really affected negatively or positively.  Other studies have suggested that bolus dosing doesn't have quite the same effect as constant daily dosing.

What's this mean for you & me?  Should we stop taking vitamin D?  Of course not!  There are still benefits to be had from optimizing one's blood levels of vitamin D.  More importantly, this study didn't compare daily vitamin D supplementation in those who are truly deficient, say less than 20ng/mL, to those who are repleted to at least 50ng/mL.  Until then, read the editorial (which incorrectly noted the baseline 25OH vitamin D level as 25ng/mL rather than 29ng/mL).

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