If you want a ton of info, check out The Vitamin D Solution by Dr. Holick. I'm reading it now. I can tell you I was insufficient and I've been taking NOW brand chewables for about a month (4000iu). I also started hitting the tanning bed 2x/week. I'm feeling much better but it took a bit. The days I lay under the lights I'm much more energetic. The tan is just a bonus!