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Whether or not that is the correct source for the idea of avoiding danger (the Rambam cites hiShomer l’cha u’shmor nafshecha), everyone agree that there is a mitzvah to avoid danger.
However this mitzvah is not understood as a mitzvah to be fit and healthy in a general sense (or as you put “take care of our bodies”) but a mitzvah to avoid life-threatening danger. This means avoiding an activity or a situation where there is high chance of losing one’s life (i.e. >50%). Eating a hot dog, or smoking one cigarette are not activities that create a high chance of losing ones life and are therefore not prohibited. See Igros Moshe V.7 Choshen Mishpat 2:76.