Over 40% of us have a deficiency. I started taking this as a solution to try to help and clear up my skin via a post I read on body acne. Although it didn't work to clear my skin, the fatigue I was feeling on a daily basis completely disappeared. I know fatigue can be caused by a million things, but I had the kind where Id sit in my car for 30 minutes before getting the energy to walk into the gym sometimes. I was taking 10k iu a day for a few weeks (equal to 15 minutes in the summers sun) and then lowered it to 6k iu a day with breakfast. Bought 500 gel caps for 10 bux at Target.