All the obese people you see walking around are not suffering from a lack of willpower to go on a fasting diet. All the thin people you see walking around are not meticulously tracking their calorie intake and burn.
There are people who eat more and still are thin but if you are not one of those people they should be none of your concern similarily to how people who inherited wealth - what they do - is meningless for person that was born poor, is poor and wants to get bit richer.
And fasting is conclusive anwer to weight loss if you can enforce it.
If you were in charge of getting inmate less fat that's exactly what you would order and he'd loose weight.
The problem with fasting in real world is that weight loss is not the only thing it affects end you can't ignore the side effects like you could do with an inmate.