Your body produces vitamin D, but only through sun exposure. I can't cite the source, but I've read that anywhere north of Atlanta and the sun angle and duration aren't sufficient for humans to produce the needed vitamin D in winter months. We are very far north of Atlanta...

I was diagnosed with a vitamin D deficiency a few years back after complaining at my annual check up about feeling low energy and fuzzy headed. It took a few weeks, but after vitamin D supplements I started feeling much better and clearer headed. Since then I supplement every fall through spring (as a runner, I get plenty of sun in the summer)