A new study in the Journal of the American Medical Association treated a population with vitamin D to see if vitamin D serum levels affect how often a person contracts a respiratory infection (Murdoch et al., 2012, free access). This was a randomized, double-blind study with a placebo group, so it's a high quality study and from my reading appears solid. 75 % of the study participants were female. The treatment method was to give participants an oral dose of 200,000 IU initially and again one month later. They then followed that up with 100,000 IU per month for the remainder of the study, which is about 3,300 IU per day. Personally I take 4,000 IU per day in the winter and none in the summer.
My first concern was whether or not the treatment method, large monthly oral dose vitamin D, especially because vitamin D is fat soluble, the vitamin D was given in tablet form, and there were no instructions to eat fat with the dose that I can find. However, as shown by Figure 2, they study had no problem creating a statistically significant difference between the treatment and placebo groups.
|Figure 2 (from Murdoch et al., 2012): Mean serum levels of vitamin D3 in the treatment and placebo group. Significantly different levels were achieved in only two months and maintained throughout the study period.|
The results show that there was no difference between the high and low serum vitamin D groups on the incidence of respiratory infections or days of work missed. So that's a big nada for vitamin D supplementation to improve immune system response against these particular viruses. Interestingly, while the common cold was not heavily affected by vitamin D status, more dangerous respiratory infections such as influenza were statistically significantly reduced,as shown in Table 2.
|Table 2 (from Murdoch et al., 2012): Infection rates by common cold associated viruses were unchanged with vitamin D status, but more dangerous flu-like viruses were significantly reduced.|
Of particular note, there were few cases of confirmed influenza infection among our partly vaccinated group of participants. Although adult data are unavailable, a randomized controlled trial in Japanese schoolchildren, set up to assess the effect of vitamin D supplementation on “doctor-diagnosed influenza,” did not report on that outcome but did report a statistically significant reduction in laboratory-confirmed influenza A infection (relative risk, 0.58; P = .04).There's still plenty of reason to supplement with vitamin D, cancer risk being a big one, but the common cold does not appear to be one of them.