While taking supplements isn't paleo, one can argue for taking Vitamin D.
We just aren't getting the sun exposure that our paleo ancestors got, and
since most of us can't change our lifestyle, taking Vitamin D is the only
alternative.

This is a good two-part series on vitamin D research:
http://www.sciencenews.org/articles/20041009/bob8.asp
http://www.sciencenews.org/articles/20041016/bob9.asp

In the second one it includes:

"Hanley says that several studies indicate that most people need 75 to 80
nmol/l of 25-D in their blood to protect their bones."

"Calvo says that she prefers to use 80 nmol/l as the minimum adequate blood
concentration of 25-D."

Now I have been taking D for a few years. I had been taking 2000-3000 IU
per day. For the past year I have been taking 4000 IU a day. I have been
getting my 25-D tested. A couple weeks ago it was 69. Even with my 4000 a
day I haven't gotten it too high.

Don.