And both are massively more dangerous than being fit.
Well, yes, by definition, since we're pretty much using underweight to mean 'having a weight low enough to damage your health' and overweight in a similar manner: I'm not sure what your comment is meant to add.
Related To: The Unfinished Mystery of the Shangri-La Diet and Missed Distinctions
Megan McArdles blogs an interview with Paul Campos, author of The Obesity Myth. I'll let anyone who is interest read the whole thing, but here's some interesting excerpts:
and