Our bodies make all the vitamin D we need from the sun, if you live somewhere like Utah there is a good part of the year when you don't see the sun for weeks on end. It's depressing for one, and it can also make us more prone to get the flu. Even the swine Flu. Dr Oz mentions this on his site,
"Vitamin D is produced in the body during exposure to sunlight. During the winter we tend to get less exposure so vitamin D supplements are recommended. People who take vitamin D supplements have better luck avoiding the seasonal flu; there is no reason to think that it won't do the same for H1N1 virus. Flu outbreaks tend to occur in places where solar radiation is low."
But, alas! Spring is coming! So get your self outside and take a walk, ride a bike, sit in the sun and make some vitamin D!