Question: Is a person sinful if they leave off a diet until he is harmed by that?
Answer: “It is not for a person to do something that leads to harm or increases his harm. As it is said: dieting is the best of cures. Such that a person takes a protection in the form of a diet. As it is well-known, the occurrence of harm is by the permission of Allah. There can be harm and there may not be harm if Allah wills. However, Allah decreed means and He has made some of these means and things a cause for the increase in sickness. So the least thing [that can be said], is that even though he is not sinning, he should not expose himself to what causes him harm”.
[Sharh Tirmidhi no. 222]
Faisal Ibn Abdul Qaadir Ibn Hassan