Last year I was taking 1200 IU's of vitamin D in my vitamin supplement when my doctor decided to test all of his patients for vitamin D levels. He told me that I tested the second highest of his patients with 42 ngs, and that the highest was 52 ngs. I decided to add 1000 units of Vitamin D3 a day after reading research indicating its benefits. I haven't been tested to see if there is any difference in my ngs readings yet, but I can really tell the difference in how much better I feel and how much more upbeat my mood has been this winter. Beginning in late November through February, I'm usually very draggy, un-energetic, and down. I feel like I'm just holding on until spring comes. Since taking the additional vitamin D3, I've felt like I do in spring this whole winter.