BBC News (among many other media outlets) reported about a study that “can find no convincing evidence to show that taking vitamin D supplements will fend off a cold”.
However, a UK cold expert they quoted said:
“There is sufficient information to indicate that vitamin D is a vital vitamin for the immune system.”
Unarguably, there’s not a mountain of evidence to support vitamin D for colds yet (much is anecdotal), but neither can we take this study as that valid.
The study itself is free to read here and in most respects it looks quite well done; but the 100,000 IU of D3 taken monthly for 18 months equates to less than 4000 IU a day. While that isn’t a bad average dose, it might not be enough for many people due to various variables. Furthermore, the average vitamin D level recorded was a not-bad 48 ng/mL – but current suggested figures suggest 60-80 ng/mL is better. The level is of more importance than the dose required to achieve it.
If I had the luxury of overseeing this study I would have observed what happens to people when they take 10,000 IU daily, or in some cases more, so that everyone at least reaches 60 ng/mL.
I don’t believe that vitamin D will totally eradicate the cold, but I do think it will make the frequency decrease statistically significant, and make it more manageable.