From what I understand, the “set point” for the hormonal triggers that decide whether someone stores fat or burns it are out of whack.

So someone might be so calorie-deficient that it becomes a serious issue, and yet their body is still trying to store fat instead of burning it.

This makes losing weight a lot harder and it’s a lot more miserable for them, as opposed to someone who can just eat a little less and their body will smoothly transition over to burning fat to make up the difference.