Vitamin D is an interesting cross between vitamin and hormone. Usually we metabolize the vitamin D that we need through exposure to the sun, but it is also available in capsule form and as a food additive. Vitamin D is hard for your body to absorb, so sun exposure is the most effective way to ensure that you have adequate levels.
I learned about vitamin D recently when I was diagnosed as having very low levels in my system. Since D is stored in fat cells it is slowly replenished in our bodies when taken orally. As a result I now take a prescription only dose of D, three times a week.
And the benefits of vitamin D? It is essential to good brain function, affecting memory, mood, depression and anxiety levels. Without good levels of D you can suffer from extreme fatigue (trust me). I have read of some people saying that restoring their vitamin D levels literally changed their lives for the better.
Several times a week, get a half hour of sun exposure on untreated skin before 10:00 am or after 3:00 pm. If you do suffer negative symptoms ask your doctor to test your vitamin D levels. Ensuring that you receive adequate doses of D is an easy fix for some frustrating physical problems.