What is more dangerous overeating or undereating?

IN THE SHORT RUN,: Undereating. In the long run, over eating. Both are bad for you either way.

Depends: I would say this depends on the severity and duration of eith extreme. In the short run, severe undereating (starvation) is probably a more significant issue than overeatimg. However, in the long run, chronic overeating (leading to obesity) is associated with more long-term complications. In the extreme, neither condition is healthy.