Like the video explained... about 50% of Americans are low.
In fact the NOW brand just came out with the 5000IU strength!
I prefer the gel caps, as I think they are absorbed better.

If you don't get outside, you are probably low. The higher north you are the worse it is.

You could start them at 2000IU for the winter, and less if they are outside in the sun as explained in the summer. If they are inside all the time...then 4000IU

This winter I upped from 2000IU to 4000IU and felt MUCH better.

Without testing, we are just shooting in the dark. I went by feeling for myself. In summer I am much less moody/tired, then as Oct approaches, I start to slide. This winter was better for me at the higher 4000 IUs. I might use 6000IU next winter.

I won't be taking any on vacation, as I am outside ALL THE TIME then.

I think we will see more and more research coming out on this subject in the near future. The breast cancer data was really compelling! Could explain why a friend of my husband's daughter died of breast cancer at 32. (a terribly tragic thing!)