Check out the video above for a great explanation of Vitamin D. Basically, you want to get a good amount of natural sunlight everyday (20-40 minutes at least, but without burning) and if you cannot (especially during the winter months on the East Coast) then supplement with Vitamin D from a store. Eating meat will also provide a small amount of Vitamin D, but the best source hands down is the sun. Supplements are super cheap and depending on skin color, diet, etc. you could take anywhere from 4000-10,000 IU per day. By doing so, you bolster your immune system, retains calcium, reduces inflammation, and help a host of other processes in the body, including defenses against cancer and infections.
How much sun do you get per day? Do you notice your physical or mental health vary with the amount of sunlight you are exposed to?