There is a mitzvah in the Torah to guard your life.

If you don't there will be consequences. Hashem created the world in such a way that there can (and most likely will) be natural consequences. You have to realize that this is Hashem's imprint in reality.  Of course there are times when the consequences do not happen, to give us free choice.  There is no such thing as a consequence that is outside of Hashem's jurisdiction, but we have to see that normal consequences are Hashem's will.
from the Shiur on Naaleh:
Simcha and Bitachon Teacher: Rebbetzin Tziporah Heller Class: Bitachon: Meaning of Trust

Compulsive overeating is bad for our health, it is going against the normal way of Hashem's world.  300 pound people are not healthy.  We are risking our health by allowing ourselves to become obese.