From what I know about DE, I certainly wouldn't take it long term. Yeah, there is foodgrade DE that is generally considered safe by the FDA, but I don't think it goes beyond that.

Foodgrade is mostly used as a de-wormer for livestock.

I was raised by a mother who did drug development in her early career and we were taught to treat the bug-killing powder with the utmost care.

http://en.wikipedia.org/wiki/Diatomaceous_earth

There was a huge controversial mess here a few years back when some developer decided he wanted to build houses on an abandoned diatomaceous earth mine.