Depends, I guess.

Sure, we can survive on less food and fasting occasionally is a useful tool for reducing the negative effects of our high calorie diets but there are people who simply shouldn't fast.

My grandma has a chronic illness and she would pass out if she didn't eat regular (healthy) meals because her blood sugar would go too low.

And even healthy adults shouldn't fast without asking a doctor for advice. The sudden shift from regular eating to very little or nothing at all puts strain on the body and can result in less physical and mental well-being for a very short term benefit, since the effects of fasting are very short term if it's not done correctly.

And I read your comment about fat people finding excuses not to fast: fat people are one of the groups who shouldn't just start fasting!!! They're used to even more food than regular people and thus are even more prone to the negative effects of fasting.