Lactose and casien (which are the carbohydrate and the protein in milk,
respectively) appear to be two things that human adults probably weren't
evolved to digest.  High-quality butter should have -very- little of
either, and should be all fat.  Clarified butter would probably take that a
step further and be still safer.

Although it's true that dairy in general probably is not healthy, I tend to
feel that a little butter now and then is far less disturbing or
threatening than a glass of milk and a box of cookies.  :-)