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Vitamin D is for "doesn't do diddly" for the common cold (RCT)
Extra Vitamin D Doesn't Prevent Colds in Healthy Adults (JAMA)
It looks like you can add Vitamin D to list of supplements (echinacea, vitamin C, etc.) who've gone up against the common cold and lost. (Scorekeepers will note that zinc held its own, though, in a Cochrane analysis.)
Vitamin D plays an important role in immune responses, most significantly by inducing cathelicidins, specialized antimicrobial proteins made by white blood cells which are necessary for fighting infections. Although the cathelicidins are best-known for their antibacterial effects, they seem to help against viruses, too. Seeking to harness this natural antibiotic mechanism, investigators have been testing vitamin D supplementation in numerous infections. The results have mostly been disappointing, but a recent randomized trial showed vitamin D supplementation accelerates recovery from tuberculosis.
The existing trials testing vitamin D supplementation to prevent the common cold were relatively low quality and had conflicting results. David Murdoch, Sandy Slow, Robert Scragg et al fixed that with the publication of their VIDARIS trial in the October 3 JAMA, concluding that giving healthy adults with essentially normal vitamin D levels (mean 29 ng/mL) high doses of vitamin D for 18 months (400,000 IU orally over 2 months, then 100,000 IU / month) did not reduce the number or severity of upper respiratory tract infections (colds) compared to placebo.
The 322 participants in Christchurch, New Zealand got ~3.7 colds during the year-and-a-half follow-up period, regardless of treatment assignment. (The URIs were defined through symptoms, and nasopharyngeal swabs were collected and confirmatory in a minority of participants.)
People missed 0.76 days of work on average, had 12 days of symptoms, and equal severity of symptoms between groups, as well. The people with the lowest vitamin D levels at baseline did not gain a benefit from vitamin D supplements, when analyzed separately. There were no detected episodes of hypercalcemia or other adverse events.
Only 5 of their participants had baseline vitamin D levels less than 10 ng/mL, so it's still possible that vitamin D supplementation could prevent upper respiratory or other infections in people with true vitamin D deficiency.
This study tested vitamin D supplementation in healthy people with normal levels (depending on your definition) at baseline. Low baseline serum vitamin D or vitamin D deficiency has been linked to many chronic conditions, including chronic obstructive pulmonary disease (COPD), asthma, diabetes, hypertension, and numerous cancers. Vitamin D deficiency and critical illness are also associated.
It has never been clear, though, whether low vitamin D levels are a contributor to these illnesses and their severity, or if vitamin D levels just fall for some reason during chronic illness, without real clinical significance. To my knowledge, randomized trials have not yet shown any consistent benefit of vitamin D supplementation for people with vitamin D deficiency and asthma, COPD or other chronic illnesses, with the possible exception of osteoporosis.
David R. Murdoch et al. Effect of Vitamin D Supplementation on Upper Respiratory Tract Infections in Healthy Adults. The VIDARIS Randomized Controlled Trial. JAMA 2012; 308: 1333-1339.