I was working with a client on Skype the other night, and she asked about vitamin D. She wanted to know if she should be taking a supplement... Yes.
If you live in the northern hemisphere, I recommend taking a vitamin D supplement every day in the winter. The general rule I've read is if you're north of 37 degrees latitude (roughly Richmond, VA to San Francisco, CA). The further away you are from the equator, the more relevant.
You hear a lot about vitamin D, (it's actually a hormone...hello confusing health things!) because it's important for muscle strength, bone density, your immune system, and your mental health.
We rely on the sun for the majority of our vitamin D production - our body makes it in response to ultraviolet radiation exposure. (This does not mean the more exposure the better!)
Generally (this is a broad generalization - so many variables such as time of day, skin color, cloudiness, pollution) we make all we need from only 20 unprotected minutes in the spring/summer/fall sun. (Unfortunately being behind glass such as windows and windshields doesn't count.) But in the winter, the sun's angle is different and it's not as strong and worse, for those of us in the cooler temps, we don't spend as much time outside.
So, what to do? Take a vitamin D supplement. Pick your favorite brand, make sure it's D3 and take 1,000-2,000IU's daily. Bonus points awarded for getting your levels tested in November so you know much you need for the winter. (Ie. if you're deficient in you'll need more to build up, but if you're levels are adequate, then you just need to maintain.)
And of course, get outside whenever you can stand it in winter! (I'm a wimp in the cold, but I try to get some sun time when it's not too freezing or snowing.)