A lack of vitamin D can have a huge effect on an active lifestyle. My doc recommended adding 1000 IU of vitamin D because I was having trouble recovering from workouts (dizziness, blurred vision, headaches). After a week, of Vitamin D I could increase my workouts from 20 minutes to 1 hour. I live in a more northern location and get a rash if I expose my skin to the spring, summer and fall sun for more than 5 or 10 minutes at a time without protection, so supplements are important.
I get my vit D from my calcium supplement, soy milk, fortified OJ and vit D supplements. If you google vitamin D, you'll find a lot of information on natural sources (including Wikipedia).