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).