[Avodah] obligation? to take medicine or exercise?

[Harvey Benton]

RAF: Eating oneself into obesity is slowly becoming unacceptable, and dieting may 
hence become halakhicly obligatory for some.

HB: Is one halachically required to take medicine? Similar question here; can one be obligated halachically to DO something (as opposed to being forbidden to do something) like excercise? or for instance to extricate oneself from a dangerous situation  -- like a battlefield?  Not discussed previously are coal-miner or other industrial jobs that can expose one to carcinogens, and percentage wise RLexpectancy?  Are they considered socially acceptable?? 
 
Also, when would Shomer Petaim Hashem apply in these cases, a(nd when not)?    HB
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