Can vitamin d prevent the flu?

No: It doesn't prevent flu.

Not quite: Vitamin d doesn't exactly prevent the flu, but vitamin d may have a role in immunity responses. http://www.ncbi.nlm.nih.gov/pmc/articles/pmc3228300/ if your vitamin d levels are low, it might be helpful to supplement them -- it's a simple lab test to find out. But there's no evidence so far that high level vitamin d supplementation (100, 000 iu per day) in itself will prevent the flu.

Maybe: A study in a prison showed that inmates that were given vitamin d supplements has less flu than those that were given a flu shot. Also, several studies have shown that flu tends to occur in the winter because vitamin d levels are low and not from the cold weather. In the tropics people get flu all year around and they tend to have low vitamin d levels. Go to the vitamindcouncil.Org to see more.